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Cheap LiFePO4 BMS?


jetzi

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13 minutes ago, Richard10002 said:

 

You are, of course, correct :) 

 

However, whilst I have forgotten what Tony has, I have the 2nd hand Valence 12V batteries so, whilst I can see what individual cells are doing in the software on the attached laptop, I cant use the readings to turn stuff on and off. Because I am off grid, and only charge using the genny, I am fairly hands on when charging, so keep an eye on individual cells when getting above the "knee" - something I only do when doing a full 100% charge to reset the BMV monitor. 

 

I do, in fact, have one cell in one battery that is out of balance by quite a lot, at the top and bottom, but the balancing facility in them is shit. If there comes a point where the Ah I can use because of this cell fall too low, I can just take the battery out of the bank and live with 3 x 130Ah, instead of 4 x 130Ah.

 

I think peterboat has his set to charge to no more than about 13.9V, so he never gets into the knee, and I'm not sure he checks the balance, nor whether he has better luck with the balancing facility?

There is no BMS Peter. It can show you what is happening, but you cant use the info to automate anything.

It does Richard, when you plug in, the Battery balance boards become active, have a look, I have a video of it. The way I did it was to top balance all the batteries when I first purchased them with the puter connected, since then they have remained balanced, I did used to check but gave up when they were always balanced.

But you are correct I never go beyond 13.9 volts which is more than enough for my usage 

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21 minutes ago, Richard10002 said:

 

I dont think that would work at cell level. Here's a vid of someone fitting a real BMS to a valence battery:

 

 

 

Thanks Richard, thats really interesting. I think your more pragmatic approach will suit me- which is to say, avoid going into the 'knee' phase, so that even if one of the cells goes out of balance, it wont be harmed.

Its a challenge for me because my system has turned into a Frankensteins monster, with two MPPTs and four B2Bs.

And with so many devices charging at once, there is a tendency for a couple to pack in early and go into float mode, when there is more charging to do.  

So the temptation is to use a higher charge voltage to make sure they all carry on charging and dont go into float too early. 

 

My hope is that setting a disconnect at 85% SoC will stop the charging before any individual cell is damaged, but its been ages since I even looked at how each individual cell was doing. In fact I've changed laptops since, and I've no idea where the PC cable is.

Do you have a link for the one you have?

Cheers

 

 

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12 minutes ago, Tony1 said:

Do you have a link for the one you have?

 

Thats a long time ago :) 

 

I think the one I ended up with was from RS components. I'll see if I can find the order email around the end of 2019.

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3 minutes ago, Richard10002 said:

 

Thats a long time ago :) 

 

I think the one I ended up with was from RS components. I'll see if I can find the order email around the end of 2019.

 

Dont go digging for ages, I'm being lazy, it should be possible to find one using my old friend google

 

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9 minutes ago, Richard10002 said:

 

Thats a long time ago :) 

 

I think the one I ended up with was from RS components. I'll see if I can find the order email around the end of 2019.

 

Here you go.... you obviously have to fit a Tyco (??) connecter to the bare wires.

 

https://uk.rs-online.com/web/p/serial-converters-extenders/6877834/

 

 

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15 minutes ago, Richard10002 said:

 

Here you go.... you obviously have to fit a Tyco (??) connecter to the bare wires.

 

https://uk.rs-online.com/web/p/serial-converters-extenders/6877834/

 

 

 

Brill, thanks Richard. 

 

Btw, if you dont mind yet another question, what f anything do you have in place as a warning or emergency disconnect in case of high voltage events? 

 

 

 

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2 hours ago, MtB said:

 

I know it's a bit late in the day to question this, but surely you (everyone) should be monitoring individual cell voltages, not the whole bank voltage. It is perfectly feasible for one cell to get too low (or high) yet the whole bank voltage remains above the chosen emergency disconnect voltage.

It’s a valid point. My system has high and low voltage cutout at cell level. However the people who plumped for the Valence batteries don’t have that option. The valence batteries do top balancing at a limited current, but I suspect that a straight discharge from fully charged top balanced, to fully discharged, might result in significant imbalance if the cells are at all mismatched. That is why Tony is sensibly going for 12v discharge floor, rather than 2.5v/cell = 10v 

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1 hour ago, nicknorman said:

It’s a valid point. My system has high and low voltage cutout at cell level. However the people who plumped for the Valence batteries don’t have that option. The valence batteries do top balancing at a limited current, but I suspect that a straight discharge from fully charged top balanced, to fully discharged, might result in significant imbalance if the cells are at all mismatched. That is why Tony is sensibly going for 12v discharge floor, rather than 2.5v/cell = 10v 

 

The BMS board connected to my bare cell battery does the emergency disconnect on any cell falling or rising out of range. It also disconnects charging on temperature falling to 0C, or rising to 75C. Many BMS boards claim to balance the cells too but they never explain how they do this. Top or bottom? Never explained, or how in detail. All that is generally claimed is "active" or "passive" balancing. I've never seen either of these types of balancing coherently explained, it is always written about as if the meaning is obvious and anyone who doesn't understand must be an idiot!    

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9 hours ago, MtB said:

 

The BMS board connected to my bare cell battery does the emergency disconnect on any cell falling or rising out of range. It also disconnects charging on temperature falling to 0C, or rising to 75C. Many BMS boards claim to balance the cells too but they never explain how they do this. Top or bottom? Never explained, or how in detail. All that is generally claimed is "active" or "passive" balancing. I've never seen either of these types of balancing coherently explained, it is always written about as if the meaning is obvious and anyone who doesn't understand must be an idiot!    

I think all boards will do top balancing. This is because top balancing is generally considered the way to go, because one wants to be able to fully charge the battery unattended with a charger. On discharge, if a cell has lower capacity and falls down the knee early, the BMS will disconnect if necessary to protect it. It means the overall bank capacity is reduced, but tough!

 

Passive balancing happens near the end of a charge, where the top cell is going up the knee. A resistor is put across this cell to shunt away some of the current, ie partially bypassing the cell. Similar treatment for the other cells as they start to go up the knee, leaving just the lowest cell un bypassed. Depending on the exact algorithm, by looking at how the individual cell voltages behave, the BMS may take a guess at how many Ah need to be taken out of each cell and continue keeping the bypass resistors in circuit, thus discharging the cell a bit, for a while after the charger is disconnected. Physical complexity is fairly simple, just a mosfet and high power resistor for each cell, plus level changing system for each mosfet’s gate drive (very low power, MOSFET gates take virtually zero current) since each mosfet’s source voltage  is a cell voltage different from the next one.

 

Active balancing is similar in concept except that rather than just dumping the excess charge of the top cells as heat through a resistor, by electronic jiggery pokery the surplus power is level shifted and sent to the lowest cells, making the process much more efficient. Obviously this (level shifting the power) is a lot more complicated and so with the sort of BMSs we see on here, I’d be pretty confident that all use passive balancing.

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7 minutes ago, nicknorman said:

I think all boards will do top balancing. This is because top balancing is generally considered the way to go, because one wants to be able to fully charge the battery unattended with a charger. On discharge, if a cell has lower capacity and falls down the knee early, the BMS will disconnect if necessary to protect it. It means the overall bank capacity is reduced, but tough!

 

Passive balancing happens near the end of a charge, where the top cell is going up the knee. A resistor is put across this cell to shunt away some of the current, ie partially bypassing the cell. Similar treatment for the other cells as they start to go up the knee, leaving just the lowest cell un bypassed. Depending on the exact algorithm, by looking at how the individual cell voltages behave, the BMS may take a guess at how many Ah need to be taken out of each cell and continue keeping the bypass resistors in circuit, thus discharging the cell a bit, for a while after the charger is disconnected. Physical complexity is fairly simple, just a mosfet and high power resistor for each cell, plus level changing system for each mosfet’s gate drive (very low power, MOSFET gates take virtually zero current) since each mosfet’s source voltage  is a cell voltage different from the next one.

 

Active balancing is similar in concept except that rather than just dumping the excess charge of the top cells as heat through a resistor, by electronic jiggery pokery the surplus power is level shifted and sent to the lowest cells, making the process much more efficient. Obviously this (level shifting the power) is a lot more complicated and so with the sort of BMSs we see on here, I’d be pretty confident that all use passive balancing.

The new REC BMS uses active balance, Steve who was having all the issues contacted them, in the end proper top balancing sorted it.

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9 minutes ago, peterboat said:

The new REC BMS uses active balance, Steve who was having all the issues contacted them, in the end proper top balancing sorted it.

I think if you are starting out with reasonably well matched cells that are well top balanced, active balancing doesn’t bring much to the party. Yes it allows a modest increase in capacity by redistributing the high cells to the low cells at the end of discharge, but at only 2A (in the case of the REC active BMS) I can’t see it making a significant difference. Having a bit more capacity would be a lot easier!

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3 minutes ago, nicknorman said:

I think if you are starting out with reasonably well matched cells that are well top balanced, active balancing doesn’t bring much to the party. Yes it allows a modest increase in capacity by redistributing the high cells to the low cells at the end of discharge, but at only 2A (in the case of the REC active BMS) I can’t see it making a significant difference. Having a bit more capacity would be a lot easier!

I think it was a lesson for Steve, he insisted that top balance had been done by the seller, I thought it hadn't, so when he did top balancing the problem went away 

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On 12/10/2021 at 17:01, Tony1 said:

That plan leaves me short of a low voltage disconnect, so the other part of my cunning plan is to get a victron battery protect to do that job, as Richard suggested.

 

Fair warning, I remember while setting up my system I had the same cunning plan to use a Victron BatteryProtect, and I remember reading stuff that cautioned me off it. I recall two main reasons, 1) it doesn't liking shutting off during high current i.e. from inverters, which of course is exactly when you're likely to run out of juice; and 2) Victron say it shouldn't be used with reverse (charging) current "The BatteryProtect is not designed for reverse currents from charging sources"  https://www.victronenergy.com/battery_protect/battery-protect

 

 

On 12/10/2021 at 18:31, Tony1 said:

But 12v is about 10%, which is the point I'd want to disconnect, so maybe somewhere between 12 and 12.4v. 

 

The thought of 12V makes my palms sweaty. My emergency cut off when any cell drops below 3.0V, which means 12V if the cells are perfectly balanced, which they aren't, in practice I never let my battery drop below 12.6 or so.

 

 

On 12/10/2021 at 18:25, MtB said:

I know it's a bit late in the day to question this, but surely you (everyone) should be monitoring individual cell voltages, not the whole bank voltage. It is perfectly feasible for one cell to get too low (or high) yet the whole bank voltage remains above the chosen emergency disconnect voltage.

 

I think most of us are monitoring cell voltages for emergency shutoffs (at least I should be, but my system is not working at the moment - instead I'm relying on the overall voltage, very conservative battery use, and using the ISDT-BG8S for a low voltage alarm - it doesn't do high voltage unfortunately so I have to be extra careful while charging).

But the chargers (MPPT or smart alternator for those who have them) and in some cases the first load disconnect is handled by the overall battery voltage. Which is actually fine as long as the batteries are balanced, similar capacity and you don't set the cutoffs too low and too high. I am not sure how the valence batteries work, do they have an internal shutoff relay for the whole battery when one of the cells drops below a preset voltage (or rises above a preset voltage)? How much current can the internal relay handle?

 

On 09/10/2021 at 20:03, nicknorman said:

yes I hadn’t noticed that the BMS only had an option for a negative-going pulse. In that case you need to use a P channel mosfet and connect it as you proposed. It’s not clear whether when BMS’s  pulse to ground is not active, that output is floating or pulled up. In any case I’d add a pull up resistor from gate to battery +, so that the MOSFET gate returns to source voltage as soon as the pulse stops.

 

Tyco do specify a maximum pulse of 100mS but if it’s going to be infrequently operated than with a bit of luck it will survive 150mS. But you definitely want that pull-up resistor to ensure the pulse stops ASAP.

 

After some deliberation, I've decided not to continue to pursue the off the shelf cell performance monitor "BMS" approach. Now that I have some idea how MOSFETs work instead I am experimenting with an Arduino based system, using logic-level n-channel MOSFETs with pull-down resistors. I'm much more comfortable with software than hardware, and this will allow me to extend the BMS to include temperature sensors and a battery heater along with a relay for my dump load. I'm also buying a few small capacity LiFePO4 cells (14500 "AA" form factor) to test with. I'll get the parts I need in about a month. If I have any luck with this I'll post my solution for critique!

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55 minutes ago, jetzi said:

  

 

Fair warning, I remember while setting up my system I had the same cunning plan to use a Victron BatteryProtect, and I remember reading stuff that cautioned me off it. I recall two main reasons, 1) it doesn't liking shutting off during high current i.e. from inverters, which of course is exactly when you're likely to run out of juice; and 2) Victron say it shouldn't be used with reverse (charging) current "The BatteryProtect is not designed for reverse currents from charging sources"  https://www.victronenergy.com/battery_protect/battery-protect

 

The thought of 12V makes my palms sweaty. My emergency cut off when any cell drops below 3.0V, which means 12V if the cells are perfectly balanced, which they aren't, in practice I never let my battery drop below 12.6 or so.



But the chargers (MPPT or smart alternator for those who have them) and in some cases the first load disconnect is handled by the overall battery voltage. Which is actually fine as long as the batteries are balanced, similar capacity and you don't set the cutoffs too low and too high. I am not sure how the valence batteries work, do they have an internal shutoff relay for the whole battery when one of the cells drops below a preset voltage (or rises above a preset voltage)? How much current can the internal relay handle?

 

 

If a high current really does prevent the Victron BP from actually doing what its been purchased to do, that will be very disappointing and annoying...

 

But my level of concern on this depends upon what you consider to be a high current- or rather, what the BP considers to be a high current. 

 

My new kettle draws a horrific 80amps, but only for 3 minutes, whereas almost everything else draws 5 amps or less.

But if the kettle is on, then I'm there and able to react to the situation. 

Almost any current draw on the boat is well below what even the most feeble and cowardly BP unit would consider to be high, so I'm guardedly optimistic that the BP will not be challenged by a genuinely high current situation, and certainly not when I'm away from the boat.

I will also connect the remote switch on the BP to a BMV712, so that will act as a secondary instruction to disconnect the loads.

 

The low voltage protection is a thing that I need either when I'm away from the boat for a couple of weeks and the solar is very poor, or when I am aboard but for some reason I haven't noticed the falling voltage. 

I do take your point about 12v being a bit close to the edge, which is why I'm looking more at 12.5v myself (although in terms of SoC, those correspond to 9% and 14%, so not a huge difference.

In practice, I cant even remember the last time my voltage dropped below 12.9, other than a test I was doing a few months ago, so its a very uncommon scenario. 

That said, the voltage sag when I plug in the new kettle is huge, so I do need to consider that when setting a cut off. I dont want the loads to be disconnected because of the voltage sag if I use the kettle when the batteries are below 50% SoC, and the voltage is already around 13v.

 

I did originally set the low voltage alarm on the BMV at 12.8v, but in the early days I was woken a couple of times by the low voltage alarm in the early hours, so I backed off a bit, and set it to a more 'emergency' level. 

 

The valence batteries dont have any self-disconnection function. If you do feel it is necessary - and to be fair, there are long term users who have no disconnect and have had no problems), then you must set it up yourself.

After a few early near-misses during charging, I decided that I did need both high and low voltage disconnects, and initially I did both using a single BEP switch.

 

But recently I've separated out the high and low voltage disconnects, so that if there is a high voltage event, only the chargers are disconnected. And if there is a low voltage event only the loads are disconnected. Fingers crossed, the BP will do that disconnection job better than the cheap relay I tried out. 

 

I only have 400Ah so I dont have a lot of room for manoeuvre, but I cant monitor individual cells (at the moment), and I think the best thing is to avoid the 'knee' phase at both high and low SoC, so I tend to stay between 40-85% (unless synchronising).

 

 

 

Edited by Tony1
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39 minutes ago, jetzi said:

  

 

Fair warning, I remember while setting up my system I had the same cunning plan to use a Victron BatteryProtect, and I remember reading stuff that cautioned me off it. I recall two main reasons, 1) it doesn't liking shutting off during high current i.e. from inverters, which of course is exactly when you're likely to run out of juice; and 2) Victron say it shouldn't be used with reverse (charging) current "The BatteryProtect is not designed for reverse currents from charging sources"  https://www.victronenergy.com/battery_protect/battery-protect

 

 

 

The thought of 12V makes my palms sweaty. My emergency cut off when any cell drops below 3.0V, which means 12V if the cells are perfectly balanced, which they aren't, in practice I never let my battery drop below 12.6 or so.

 

 

 

I think most of us are monitoring cell voltages for emergency shutoffs (at least I should be, but my system is not working at the moment - instead I'm relying on the overall voltage, very conservative battery use, and using the ISDT-BG8S for a low voltage alarm - it doesn't do high voltage unfortunately so I have to be extra careful while charging).

But the chargers (MPPT or smart alternator for those who have them) and in some cases the first load disconnect is handled by the overall battery voltage. Which is actually fine as long as the batteries are balanced, similar capacity and you don't set the cutoffs too low and too high. I am not sure how the valence batteries work, do they have an internal shutoff relay for the whole battery when one of the cells drops below a preset voltage (or rises above a preset voltage)? How much current can the internal relay handle?

 

 

After some deliberation, I've decided not to continue to pursue the off the shelf cell performance monitor "BMS" approach. Now that I have some idea how MOSFETs work instead I am experimenting with an Arduino based system, using logic-level n-channel MOSFETs with pull-down resistors. I'm much more comfortable with software than hardware, and this will allow me to extend the BMS to include temperature sensors and a battery heater along with a relay for my dump load. I'm also buying a few small capacity LiFePO4 cells (14500 "AA" form factor) to test with. I'll get the parts I need in about a month. If I have any luck with this I'll post my solution for critique!

Valence internal BMS doesn’t do disconnect. You can allegedly get a separate bit that does the disconnect in communication with the internal BMSs, but it seems hard to come by.

 

when I was developing my BMS, I too got some AA lifepo4 cells. With an Arduino etc, you don’t need pull-down resistors, you just configure the outputs as normal outputs and they actively both pull up and pull down. If you are using a 3.3v Arduino and want to drive a MOSFET directly, you need to select carefully. Mosfets are a bit funny in that at a medium Vgs, they appear to have low on resistance but if the current gets too much, the resistance increases a lot. So if you are planning to switch fairly high currents, you need to check the data sheet carefully.

 

Looking forward to hearing your progress…

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2 hours ago, Tony1 said:

f a high current really does prevent the Victron BP from actually doing what its been purchased to do, that will be very disappointing and annoying...

Yeah, it seems weird to me too, but they do specifically say not to use it for inverters. It could just be Victron covering themselves, it seems very strange to indicate against the very thing the device is supposed to do.

 

The bit about it not being able to handle "reverse" i.e. charging current is more serious though IMO as this is an "everyday" situation rather than an emergency one. Unless you are only using the BP to terminate the load circuit, in which case I'd probably ignore it, especially if I already installed one.

 

2 hours ago, Tony1 said:

I do take your point about 12v being a bit close to the edge, which is why I'm looking more at 12.5v myself (although in terms of SoC, those correspond to 9% and 14%, so not a huge difference.

 

Yeah, many folks on here subscribe to the 80/20 SoC rule which is probably a bit conservative (considering that, with only 60% of your LiFePOs now usable, makes them roughly on a par with the 50/50 rule people follow with LAs).

 

I actually had the same problem with the voltage pulldown shutting off my system. I mostly solved it by adding more cells, but ocassionally I'll get an alarm when I try to do something high-current with a low SoC. Another point for a microcontroller system, you could add an ammeter and build in some allowance for a lower voltage when under a high current situation, at least for a few minutes.

 

2 hours ago, Tony1 said:

The valence batteries dont have any self-disconnection function. If you do feel it is necessary - and to be fair, there are long term users who have no disconnect and have had no problems), then you must set it up yourself.

After a few early near-misses during charging, I decided that I did need both high and low voltage disconnects, and initially I did both using a single BEP switch.

 

1 hour ago, nicknorman said:

Valence internal BMS doesn’t do disconnect. You can allegedly get a separate bit that does the disconnect in communication with the internal BMSs, but it seems hard to come by.


Ah I see, for me the emergency disconnect on per-cell over/undervoltage is the most important bit of the BMS. So really the internal BMS is actually just doing some on-the-fly balancing? I feel automatic balancing is a very optional extra, I am not going to bother with any of that, instead I just consider an annual manual top balance as part of boat maintenance.

 

On 12/10/2021 at 22:48, MtB said:

Many BMS boards claim to balance the cells too but they never explain how they do this. Top or bottom? Never explained, or how in detail. All that is generally claimed is "active" or "passive" balancing.

My understanding (probably wrong) that it's neither top nor bottom balancing, instead it's just on-the-fly balancing at whatever voltage the cells happen to be at. At least that's how I think the ISDT balancing works (my only experience of such a function), but this is triggered by a button so I suppose you could use it to top balance if you are very patient and don't mind holding your battery at a high SoC for the duration!
 

 

1 hour ago, nicknorman said:

when I was developing my BMS, I too got some AA lifepo4 cells. With an Arduino etc, you don’t need pull-down resistors, you just configure the outputs as normal outputs and they actively both pull up and pull down. If you are using a 3.3v Arduino and want to drive a MOSFET directly, you need to select carefully. Mosfets are a bit funny in that at a medium Vgs, they appear to have low on resistance but if the current gets too much, the resistance increases a lot. So if you are planning to switch fairly high currents, you need to check the data sheet carefully.

Alright, that's good to know. According to the docs the D-pins on the Arduino Uno are "on" at >2V (to a maximum of 5V) so I think it must be a 3.3V device. I'll only be switching Tyco BDS-A relays with the MOSFETs, so I can spec them all the same. According to the Tyco BDS-A data sheet the max coil power is 7W (~0.6A at 12V) and resistance is 47Ω, though you mentioned some time ago that the Tyco coils take a burst of say 10A at 12V (36W) - not sure if this is right but I will overspec the MOSFET to the higher figure just in case.
. image.png.8cc035c76c6dc677c1b01237a9a001ef.png

Still reading around what all the various MOSFET specs mean, but so far I think the one I linked to earlier would be suitable for this application. They can manage a pulse of 32A which is plenty to operate the coil. The Vgs threshold is 2-4V, so that seems correct for the Arduino outputs (would like to find one that was 2-5V to match the Arduino output exactly, but they don't seem to exist [edit: on second thoughts, I think I'm reading this wrong - I think 2-4V means that it requires a value of 4V to be fully on - so I think I need to look for a MOSFET in the range of 1-3V]). The gate-source maximum voltage is +-30V so that should be able to cope with the highs and lows of the system (though if the Arduino is outputting 30V, something has gone horribly wrong!)

 

Considering that the MOSFETs are so cheap, I see no reason not to overspec them for safety, but I am still trying to get to grips with how they work to make sure that it's not counterproductive.

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32 minutes ago, jetzi said:

Yeah, it seems weird to me too, but they do specifically say not to use it for inverters. It could just be Victron covering themselves, it seems very strange to indicate against the very thing the device is supposed to do.

 

The bit about it not being able to handle "reverse" i.e. charging current is more serious though IMO as this is an "everyday" situation rather than an emergency one. Unless you are only using the BP to terminate the load circuit, in which case I'd probably ignore it, especially if I already installed one.

 

 

Yeah, many folks on here subscribe to the 80/20 SoC rule which is probably a bit conservative (considering that, with only 60% of your LiFePOs now usable, makes them roughly on a par with the 50/50 rule people follow with LAs).

 

I actually had the same problem with the voltage pulldown shutting off my system. I mostly solved it by adding more cells, but ocassionally I'll get an alarm when I try to do something high-current with a low SoC. Another point for a microcontroller system, you could add an ammeter and build in some allowance for a lower voltage when under a high current situation, at least for a few minutes.

 

 

 

I'll certainly test the low voltage disconnect with the BP thoroughly, and that will include the inverter running. The inverter is a load after all, and in my system its only the loads that will get disconnected by the BP due to a low voltage event.

If victron are saying that the BP will not disconnect when the inverter is running, then it is not fit for purpose- not for most of the boaters and motorhomers who would use it. 

 

My charging current is handled by a separate high voltage disconnect device, so that bit is not a concern, at least in my case.

 

Re the width of the charging/discharging range, I'm ok with it being 50-60% of the total capacity. 

I'm sure you can push them harder (e.g., 10-95% each day) and you wont immediately damage them, but there is some evidence to suggest they wont last as long. 

I want them to last a decade, so I'm being conservative and generally going between 40-85%.

One of the reasons I dont go lower than 40% is that I dont quite trust the accuracy of the BMV712 after say a week or two since its last synchronisation.

If the BMV does stray in its accuracy, then when the discharge goers into the knee phase, a fairly small error could be amplified in terms of consequences.

Its still early days for me in terms of managing these batteries, so I'm being cautious.

 

But for me, the big benefit of these batteries is not about getting a bigger % of the total capacity than you can get with lead acid. 

It's how readily and quickly they will absorb charge compared to my old lead acids, especially solar charge. 

The guy on the boat behind me has about half the solar I have, so I know he will have collected about 550 Wh today,  but he had to run his engine for two and a half hours this afternoon, whereas I ran mine for 30 mins (and even that was mainly for a test).

The lead acids, for whatever reason, just dont seem to soak up the solar as well as lithiums, and for 8 months of the year that is a huge benefit of having the lithiums. 

 

 

 

 

 

 

 

 

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11 hours ago, jetzi said:

Yeah, it seems weird to me too, but they do specifically say not to use it for inverters. It could just be Victron covering themselves, it seems very strange to indicate against the very thing the device is supposed to do.

 

The bit about it not being able to handle "reverse" i.e. charging current is more serious though IMO as this is an "everyday" situation rather than an emergency one. Unless you are only using the BP to terminate the load circuit, in which case I'd probably ignore it, especially if I already installed one.

 

 

Yeah, many folks on here subscribe to the 80/20 SoC rule which is probably a bit conservative (considering that, with only 60% of your LiFePOs now usable, makes them roughly on a par with the 50/50 rule people follow with LAs).

 

I actually had the same problem with the voltage pulldown shutting off my system. I mostly solved it by adding more cells, but ocassionally I'll get an alarm when I try to do something high-current with a low SoC. Another point for a microcontroller system, you could add an ammeter and build in some allowance for a lower voltage when under a high current situation, at least for a few minutes.

 

 


Ah I see, for me the emergency disconnect on per-cell over/undervoltage is the most important bit of the BMS. So really the internal BMS is actually just doing some on-the-fly balancing? I feel automatic balancing is a very optional extra, I am not going to bother with any of that, instead I just consider an annual manual top balance as part of boat maintenance.

 

My understanding (probably wrong) that it's neither top nor bottom balancing, instead it's just on-the-fly balancing at whatever voltage the cells happen to be at. At least that's how I think the ISDT balancing works (my only experience of such a function), but this is triggered by a button so I suppose you could use it to top balance if you are very patient and don't mind holding your battery at a high SoC for the duration!
 

 

Alright, that's good to know. According to the docs the D-pins on the Arduino Uno are "on" at >2V (to a maximum of 5V) so I think it must be a 3.3V device. I'll only be switching Tyco BDS-A relays with the MOSFETs, so I can spec them all the same. According to the Tyco BDS-A data sheet the max coil power is 7W (~0.6A at 12V) and resistance is 47Ω, though you mentioned some time ago that the Tyco coils take a burst of say 10A at 12V (36W) - not sure if this is right but I will overspec the MOSFET to the higher figure just in case.
. image.png.8cc035c76c6dc677c1b01237a9a001ef.png

Still reading around what all the various MOSFET specs mean, but so far I think the one I linked to earlier would be suitable for this application. They can manage a pulse of 32A which is plenty to operate the coil. The Vgs threshold is 2-4V, so that seems correct for the Arduino outputs (would like to find one that was 2-5V to match the Arduino output exactly, but they don't seem to exist [edit: on second thoughts, I think I'm reading this wrong - I think 2-4V means that it requires a value of 4V to be fully on - so I think I need to look for a MOSFET in the range of 1-3V]). The gate-source maximum voltage is +-30V so that should be able to cope with the highs and lows of the system (though if the Arduino is outputting 30V, something has gone horribly wrong!)

 

Considering that the MOSFETs are so cheap, I see no reason not to overspec them for safety, but I am still trying to get to grips with how they work to make sure that it's not counterproductive.

I had a look at my own circuit for driving the Tyco and I did in fact use pull-down resistors (4.7k) , so I take back what I said earlier. The need for pull down resistors arises because when the microprocessor is unpowered, held in reset or first starting up, the IO pins are all configured as inputs by default, not pulled up or down. Only after the programme starts to run and the IO is configured (as an output, in this case) would the pins be pulled down. Likely only a very short time, but nevertheless… So I added pull down resistors just so the mosfet gates wouldn’t be left floating at all.

 

The Vgs threshold voltage is when the mosfet STARTS to conduct. You need to drive it with a higher voltage. Have a look at the data sheet graphs for drain current vs Vgs at different Vds. You will see that at low Vgs the Vds is low (resistive) at low current but hits a current plateau where the Vds increases rapidly with virtually no increase in current. You need Vgs to be high enough so you don’t hit this plateau at the current needed to drive the Tyco (about 3A). If it’s any help I used IRLL3303 MOSFETs, but these are surface mount of course.

 

May have been a typo from you earlier but Tyco coil resistance is 4.7, not 47

 

Oh and picking up on another of your points, a microprocessor generally has a cmos output, ie an IO a pin is pulled to either supply rail voltage provided the current demand is low. So you can plan on getting 0v and 3.3v at an output pin not subject to any significant current drain.

 

 

Edited by nicknorman
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On 12/10/2021 at 20:03, Tony1 said:

 

Brill, thanks Richard. 

 

Btw, if you dont mind yet another question, what f anything do you have in place as a warning or emergency disconnect in case of high voltage events? 

 

 

 

 

I have a 100A relay in a drawer, waiting to be fitted in the charging cable, and connected to the BMV (712S) :( 

 

Because I charge with a genny*, I am always around and hands on when charging, and the charger is set to max out at 14.6V, (with voltage drop, this means the batteries get to about 14.4V). So the chances of a high voltage event are slim.

 

* I have 500w of solar, but the controller is set to max out at 13.9V, and float at 13.3V, so, hopefully, no danger there either.

 

I'm fairly sure that someone, (Nick?), said that relays use some current, although I'm not sure how significant that is when you consider a lithium bank of 530Ah?

 

I suppose if I sold the boat, or someone else was using it for a while, they would have to be educated in the use, (or I would finally get around to fitting the relay ... or I might buy one of those expensive ones that dont use any current, latching relays???)

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53 minutes ago, Richard10002 said:

 

I have a 100A relay in a drawer, waiting to be fitted in the charging cable, and connected to the BMV (712S) :( 

 

Because I charge with a genny*, I am always around and hands on when charging, and the charger is set to max out at 14.6V, (with voltage drop, this means the batteries get to about 14.4V). So the chances of a high voltage event are slim.

 

* I have 500w of solar, but the controller is set to max out at 13.9V, and float at 13.3V, so, hopefully, no danger there either.

 

I'm fairly sure that someone, (Nick?), said that relays use some current, although I'm not sure how significant that is when you consider a lithium bank of 530Ah?

 

I suppose if I sold the boat, or someone else was using it for a while, they would have to be educated in the use, (or I would finally get around to fitting the relay ... or I might buy one of those expensive ones that dont use any current, latching relays???)

 

Not wanting to sound a negative note or anything, but my relay was rated at 200 amps (continuous), and its contacts became very very hot when it was handling a charge of 100-120 amps. That was a £12 unit from amazon, but it did have hundreds of very good reviews- so I'm a bit sceptical of the stated ratings, certainly on the cheaper units. 

Maybe dont make too many permanent or complex changes until you've installed and tested it out properly... 

 

But there are many lithium users who dont bother with high or low voltage disconnects, since they take other precautions that make the over-voltage event extremely unlikely- and we havent heard of any wrecked batteries from users like Peter and others, after several years of daily use. 

 

But I had a scare- a situation back when I first got the lithiums, and tried to charge them using my A2B charger. Despite the set charge profile being about 14v, when the SoC got up to a high level the voltage suddenly started to shoot upwards to 15v and beyond. 

I was seconds away from ruining my new £1500 batteries, and it was only that I was checking the voltage every few minutes that saved the day- thankfully they didnt seem to suffer any long term damage from that event.  

 

That experience encouraged me to take a very cautious approach- and I think even over-cautious, if I'm being honest.

The other thing is that, like you, I havent ruled out letting friends and family use the boat for a few days (particularly my brother who has a brand new burstner motorhome), and I know from hiring experience how easy it is for people to miss or ignore important instructions on electrical issues, whilst being given a full familiarisation tour of a boat.

There's so much to take in, they are bound to miss or forget some important information.

Plus they'll be using it it the summer probably, with lots of solar flooding in and potentially making it more difficult to understand whats happening. 

So I feel that automatic disconnects would be advisable.

 

Although I got rid of the A2B charger, my current setup is still somewhat risky, in that I have six charging devices in play (two MPPTs and four B2Bs) when doing a full-on charge. And to keep them all charging at once, I seem to have to set the charge voltages a bit on the high side (e.g. 14.2v).

If I set them all to 13.9v, some start going into float when the batteries are only 75% full.

Plus with six of them, there is the increased risk of malfunction in one. 

 

Its a proper Frankensteinian monster of a setup (and not one I would recommend at all), but it does work.

On a good day I'll be getting 60-70 amps from the solar, and maybe 90 amps from the alternators (via the B2Bs). 

I only switch on all four of B2Bs when I am static and doing an engine charge, running at 1400rpm so that the alternators stay cool.

When cruising (and thus when the rpm is often at tickover for long periods), I only use two of the B2Bs, and that reduces the load and keeps the alternators cool.

 

If anyone ever borrows my boat my instructions will be to pretend that the 'extra' two B2Bs dont exist.

The standard two B2Bs come on automatically anyway- and they still deliver about 60 amps between them. 

Its a lot of messing about for only four  months of the year, I must say, but its great on the really crappy solar days to see 100 amps going into the batteries, and getting charged up in less than an hour, when the boats around me are all too often running engines and gennies both morning and evening. 

 

ETA- I think I remember you saying that you are a CCer. Is it much of a hassle to transport petrol back to the boat? 

Would it be a pain without a car? 

I did try out a cheap genny to reduce the engine hours spent on charging, but after a couple of trips (on foot and bike) to nearby petrol station forecourts with my jerrycan, I went off the idea. 

But I can see that with a nice quiet genny, it makes sense to use that, and save loads of running hours on your expensive diesel engine. 

 

 

Edited by Tony1
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On 12/10/2021 at 19:48, Richard10002 said:

 

Here you go.... you obviously have to fit a Tyco (??) connecter to the bare wires.

 

https://uk.rs-online.com/web/p/serial-converters-extenders/6877834/

 

 

 

Hi Richard, apologies for pestering you with questions but I've just got my new victron battery protect, planning to install it tomorrow, and just noticed something that Jetzi also mentioned, which is a statement that the BP should not be connected to devices with capacitors on their inputs, e.g. an inverter. 

They suggest that in these cases you use the BPs relay function to switch off the inverter- but what about the 12v loads? Its hardly a complete solution. 

 

In my case, I have a fall back plan, which is to use the BP as my high voltage disconnect, placed between chargers and batteries, and control it with a BMV712.  

 

But before I start doing any work tomorrow I wanted to ask whether you had connected your BP to an inverter (as I would expect), and if so, whether you had tested it out, whether you had noticed any potential problems? 

 

If it genuinely canty be connected directly to an inverter, I'd rather make a call on which way to use it before I spend two hours configuring the cables, lugs etc. 

 

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Bit of code tweaking today. Everything has been working fine with the alternator controller, but I needed a few changes to the BMS to make it control the Combi when on shore power. So settings of the switches on the Alternator regulator (3 position for desired state of charge - 50, 80, 100% - and rate switch (slow and fast) are passed to the BMS over CANBUS and the BMS uses them to control the Combi over Masterbus. So when the rate switch is set to slow, the Combi is always in float mode with max current 25A (which is plenty to keep the batteries steady). The float voltage depends on the setting of the other (SoC) switch, either 13.15,  13.3 or 13.4v.
If the rate switch is moved to fast, Combi voltage is set to  14.2v and max current to 90A, until the target SoC is reached, then it goes permanently to float at max 25A until and unless the switch is set back to slow and then to fast again. In other words, it’s a one-time charge pending further user intervention.

 

So the general idea is to leave it on slow and 50% SoC when in the Marina, either unoccupied or with us on the boat. If we are going to go cruising, we can set the switch to fast a few hours before and maybe to 80% or 100% SoC so that we set out with well charged batteries. Also of course if in the Marina for a long time, can charge to 100% to synchronise the BMV712 and Mastershunt as a one-off event, and after fully charging the system will automatically revert to float rather than holding at 100%.

Edited by nicknorman
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8 minutes ago, nicknorman said:

So the general idea is to leave it on slow and 50% SoC when in the Marina, either unoccupied or with us on the boat. If we are going to go cruising, we can set the switch to fast a few hours before and maybe to 80% or 100% SoC so that we set out with well charged batteries.

 

Sounds great. Any desire to connect the BMS to the internet to allow you to set the combi to fast charge if you are not on the boat? Can imagine that would be useful so that the battery is full by the time you arrive ready for your cruise. Perhaps would be nice to be able to monitor the SoC remotely as well.

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On 13/10/2021 at 21:33, Tony1 said:

 

If a high current really does prevent the Victron BP from actually doing what its been purchased to do, that will be very disappointing and annoying...

 

But my level of concern on this depends upon what you consider to be a high current- or rather, what the BP considers to be a high current. 

 

I haven't read anything like this. My assumption is that, if you have the 200A BP, it can cope with up to 200A, similar with the 100A version.

 

Apropos something I saw you post earlier, (later), my inverter, (victron), is powered direct from the batteries, so doesnt disconnect if the BP cuts off the domestics due to low voltage - I cant recall why I've done it this way, but there is/was a good reason, (it's not the amps as it is 1600w, thus 133A +/-). I think it was Jen in Wellies who gave me some advice about connetcing the remote on the BP to the remote on the inverter, so the inverter would cut off if the BP had a low voltage cut off.

 

Hope that makes sense - it did at the time, although I havent connected the remotes yet :( 

 

 

On 13/10/2021 at 21:33, Tony1 said:

But recently I've separated out the high and low voltage disconnects, so that if there is a high voltage event, only the chargers are disconnected. And if there is a low voltage event only the loads are disconnected. Fingers crossed, the BP will do that disconnection job better than the cheap relay I tried out. 

 

 

My BP has shut things down a few times, (usually not long after going to bed, watching TV :( usually after deciding not to top upwith the genny earlier in the day :( ).

 

Does the job seemingly well, and powers back up when I change the parameters, or charge up the batteries the following day.

 

As a matter of interest, I have retained my Lead Acid bank, and have the ability to switch to using it to supply the domestics, (and also to switch the solar and shore power/genny charging to it), so, if the lithium cuts out due to the BP, I just switch to the LA bank, and all is good for the night, (or whatever). The BP doesn't protect the LA bank. 

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24 minutes ago, nicknorman said:

 

So the general idea is to leave it on slow and 50% SoC when in the Marina, either unoccupied or with us on the boat. If we are going to go cruising, we can set the switch to fast a few hours before and maybe to 80% or 100% SoC so that we set out with well charged batteries. Also of course if in the Marina for a long time, can charge to 100% to synchronise the BMV712 and Mastershunt as a one-off event, and after fully charging the system will automatically revert to float rather than holding at 100%.

 

 

But if you're going out cruising, there's no point wasting charge beforehand getting the batteries up to 80% or 100%? 

Won't the cruising get them up to full charge?

 

I never have shore power these days (although I remember it being a lot cheaper than diesel-generated electricity), so maybe my approach is different as a CCer.

But if I know I'm going cruising the next day, I wont worry if the batteries go a bit low the day before, because I know they'll be getting a good long charge once I set off. 

 

I would never dare charge to 100% remotely in order to synchronise, if I was not there in person. Is it worth it?  

That said, I'm not an electrical wizard, so I would have an innate hesitance about pushing the batteries to extremes when I'm not around.  

 

 

 

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