Cheap LiFePO4 BMS?

[MtB]

Just now, nicknorman said:

Moominpappa

 

YES!

 

 

Astonishing that none of the many lithium cell retailers or BMS sellers seem not to have done anything similar.

[Tony1]

2 minutes ago, MtB said:

 

YES!

 

 

Astonishing that none of the many lithium cell retailers or BMS sellers seem not to have done anything similar.

 

I think there's a catch 22 thing going on, in that more manufacturers wont start designing lithium BMS products until the user base is much bigger, but the user base wont grow very quickly because they perceive a lack of options and products to allow them to put together a BMS, and they aren't inclined to do lots of research to get a better understanding of what is needed, what products provide solutions, what products work with what other products, etc etc.

 

E.g. I was on the verge of buying a victron battery protect unit, but I couldnt find absolute confirmation about whether it could be used as a high voltage disconnect with a BMV712 )even on Victrons own discussion forum), so I looked further and found the 15 quid option which will do the job.

And not everyone has a knowledgeable source of info and advice about BMS's, such as Nick. 

 

 

[Tom and Bex]

14 hours ago, MtB said:

 

YES!

 

 

Astonishing that none of the many lithium cell retailers or BMS sellers seem not to have done anything similar.

There is a commercially available option here: https://www.solar4rvs.com.au/rec-bi-stable-relay-driver-bslrd-for-low-side-swit 

 

Only site I've been able to find it though. This is what I've used with the Tyco relays, not cheap for what it is though. There's also a circuit design on (I think) the Nordik design website. 

[nicknorman]

16 minutes ago, Tom and Bex said:

There is a commercially available option here: https://www.solar4rvs.com.au/rec-bi-stable-relay-driver-bslrd-for-low-side-swit 

 

Only site I've been able to find it though. This is what I've used with the Tyco relays, not cheap for what it is though. There's also a circuit design on (I think) the Nordik design website. 

That looks just the ticket!

One could try going direct to the manufacturer in Slovenia, rather than going via a retailer in Australia. They are only a little company, hopefully they’d sell to an individual or at least be able to advise of retailers in Europe.

Edited October 9, 2021 by nicknorman

[IanD]

6 minutes ago, nicknorman said:

That looks just the ticket!

One could try going direct to the manufacturer in Slovenia, rather than going via a retailer in Australia. They are only a little company, hopefully they’d sell to an individual or at least be able to advise of retailers in Europe.

For anythng to do with lithium batteries and the care and feeding thereof, sites/forums/blogs about RVs are much more likely to yield useful information because there are far more people using them (tens of thousands?) -- lumpy water boat sites come next (thousands?), inland waterways are a distant third (tens or hundreds?)...

[Richard10002]

18 minutes ago, IanD said:

For anythng to do with lithium batteries and the care and feeding thereof, sites/forums/blogs about RVs are much more likely to yield useful information because there are far more people using them (tens of thousands?) -- lumpy water boat sites come next (thousands?), inland waterways are a distant third (tens or hundreds?)...

 

I might have posted this video before, but its worth a watch as he does seem to cover all the requirements for keeping lithiums within their safe boundaries - other videos cover the fitting of the DC DC charger, inverter, and so on:

 

 

[jetzi]

3 hours ago, Tom and Bex said:

There is a commercially available option here: https://www.solar4rvs.com.au/rec-bi-stable-relay-driver-bslrd-for-low-side-swit 

 

Only site I've been able to find it though. This is what I've used with the Tyco relays, not cheap for what it is though. There's also a circuit design on (I think) the Nordik design website. 

That does look ideal for converting a standard "normally closed" relay output to drive Tycos. I see they have 2 German and a Norwegian distributors as well? I can't find that component in a search due to it not being in English probably, but someone more invested could see if they could find them.

 

My situation is different as I have a cell-level monitor board which can drive bistable relays, but Nick pointed out that unfortunately that the output of ~1A doesn't provide enough current for the ~3A Tyco coils.

 

I have been trying to learn how MOSFETs work and I think I know about enough to do some damage. I plan to get six of these N-Channel MOSFET, 30 A, 55 V, 3-Pin TO-220AB Infineon IRLZ34NPBF (data sheet). I will connect each MOSFET gate pin to a board output, the source to the battery positive. Then when the board attempts to pulse switch a relay, it will apply a voltage between the gate and the source and turn the MOSFET on. The drain pin of the MOSFET goes to the relevant relay coil with the other terminal being connected to the battery negative.

 

MOSFETs actually seems just as straightforward as using a relay, which might be due to my cluelessness! But at just over a quid each and given that my cell performance monitor board isn't doing anything right now anyway, I can't see the harm in experimenting. I may also need to test and potentially replace the various components of the output stages of the board in case I fried something, but maybe the MOSFETs are all I need.

[nicknorman]

2 hours ago, jetzi said:

That does look ideal for converting a standard "normally closed" relay output to drive Tycos. I see they have 2 German and a Norwegian distributors as well? I can't find that component in a search due to it not being in English probably, but someone more invested could see if they could find them.

 

My situation is different as I have a cell-level monitor board which can drive bistable relays, but Nick pointed out that unfortunately that the output of ~1A doesn't provide enough current for the ~3A Tyco coils.

 

I have been trying to learn how MOSFETs work and I think I know about enough to do some damage. I plan to get six of these N-Channel MOSFET, 30 A, 55 V, 3-Pin TO-220AB Infineon IRLZ34NPBF (data sheet). I will connect each MOSFET gate pin to a board output, the source to the battery positive. Then when the board attempts to pulse switch a relay, it will apply a voltage between the gate and the source and turn the MOSFET on. The drain pin of the MOSFET goes to the relevant relay coil with the other terminal being connected to the battery negative.

 

MOSFETs actually seems just as straightforward as using a relay, which might be due to my cluelessness! But at just over a quid each and given that my cell performance monitor board isn't doing anything right now anyway, I can't see the harm in experimenting. I may also need to test and potentially replace the various components of the output stages of the board in case I fried something, but maybe the MOSFETs are all I need.

6 lovely melted MOSFETs, magic smoke all gone!

These are enhancement mode N channel MOSFETs, and do “low side switching”. You need to connect (fused) battery + to the appropriate relay coil terminal (remembering that the Tyco has freewheel diodes, so polarity is important), other relay coil terminal to the drain, and the source to 0v.

If you connect the source to positive as your proposal, you forward bias the body diode and it just conducts, almost like a short circuit.

 

This is a logic-level mosfet, which means it’s designed to operate on quite a low turn-on gate voltage, maybe 3.3v. Ok it doesn’t matter if the source - gate voltage (Vgs) is higher, up to its absolute maximum of 16v. You might say “well the maximum battery voltage is going to be less than 15v, so that’s not problem, there is over a volt of headroom”.

 

However a logic level mosfet gate insulation layer is only a few molecules thick, it’s capacitance is picofarads,  if the maximum Vgs is exceeded even for a few nanoseconds, it blows a hole in the gate insulation layer and damages the MOSFET. Most electrical system including boats, have voltage spikes (transients) generated by a number of thing such as water pumps, fridges switching off etc. Bye bye MOSFET.

 

So you could select a mosfet with higher Vgs, or drive it from a filtered battery supply so that spikes are eliminated. If it was me I’d do both.

 

The other thing is that a mosfet gate doesn’t take any current or conduct. At all. So you not only have to pull the gate up to some positive voltage to turn it on, you also have to pull it down to zero to turn it off. And fairly quickly, as a mosfet half on dissipates a lot of power.

Edited October 9, 2021 by nicknorman

[jetzi]

44 minutes ago, nicknorman said:

These are enhancement mode N channel MOSFETs, and do “low side switching”. You need to connect (fused) battery + to the appropriate relay coil terminal (remembering that the Tyco has freewheel diodes, so polarity is important), other relay coil terminal to the drain, and the source to 0v.

If you connect the source to positive as your proposal, you forward bias the body diode and it just conducts, almost like a short circuit.

 

Hmm, I don't see how this can be correct. Say I'm dealing with the "Reset" (disconnect) for overvoltage on the "charge" Tyco relay (terminal assignment diagram below, taken from the Tyco BDS-A data sheet)

 

The cell monitor board provides a pulse of GND (battery -ve) to switch the bistable relay (according to the manual, Umax output #2 provides a 150ms pulse). So with this output (-ve) connected the gate of the MOSFET, I figure that in order to create the voltage differential needed to switch the MOSFET, I would need to connect the source of the MOSFET to the battery +ve.

 

To reset the Tyco, the relay terminals 1 (+ve) and 3 (-ve) need to be connected to the drain and battery. Since the drain, when the MOSFET is on, connects to the battery -ve via the source, this would go to terminal 3.  I would have to connect the other Tyco coil terminal 1 (+ve) to the battery +ve.

 

I have a suspicion the bit I'm not understanding is in "forward bias the body diode". I figure that all I need to do to cater for the diodes in the relay is to connect the coils the right way around (1 to +ve and 3 to -ve), but I fear there is more to it than that.

 

 

44 minutes ago, nicknorman said:

select a mosfet with higher Vgs

 

Yeah, I thought 16V would be plenty because I hadn't considered battery spikes - thanks for that. I was actually mainly looking at the drain/source max voltage (the relay circuit) which in this case was 55V, I will look for a MOSFET with a higher Vgs. edit: perhaps this one, which has a Vgs of +-30V.  N-Channel MOSFET, 8 A, 500 V, 3-Pin TO-220AB Vishay IRF840APBF


 

44 minutes ago, nicknorman said:

So you not only have to pull the gate up to some positive voltage to turn it on, you also have to pull it down to zero to turn it off. And fairly quickly

 

After 150ms, the pulse from the CPM would be over, and therefore send the voltage back down to zero? Or can I not rely on that?

 

 

 

I actually have also just noticed that the Tycos ask for an impulse length of 15 to 100ms, so I wonder if it might be asking too much of the Tycos to handle 150ms... I think maybe the arduino is really the way forward for me.

Edited October 9, 2021 by jetzi

[Tony1]

22 hours ago, nicknorman said:

You don’t need to invert the relay, although you could (but it just complicates things!). With the relay not inverted, you connect positive to the NC terminal, then it will open (and turn off the big relay) when the high voltage parameter triggers.

 

Well its nearly squeaky bum/testing time, but not just yet.

The relay arrived a bit late so I've only just finished cutting the new cables, fitting the lugs, soldering up the diode etc- and now its just a bit too late to actually install it all.

I'm fitting a new isolating switch as well, as the current one is getting a bit hot under full charging. The new switch will allow me to disconnect the loads, or the chargers, or both, so that will be a handy feature.

But I'm always reluctant to make any significant electrical changes once its past teatime, because if something goes wrong or I blow something up, its very late to try and fix, and I might be without power overnight. 

 

So the first test will be tomorrow, but I am concerned because the forecast is very sunny tomorrow, and in that weather , in early afternoon my two sets of panels will probably be producing about 20-30 amps each, which seems a lot to disconnect in an uncontrolled way.

I say uncontrolled because the BMS control wires switch the chargers off, whereas the relay just chops them off from their quite large current load- it seems like something that might damage them.

 

My ideal test scenario would be to disconnect everything charging at maximum, which is 90 amps from the two alternators and 70 amps from the solar, but now that the time is coming to try it, I must confess I am a little nervous about what will happen to the MPPTs and B2Bs when they are throwing out a combined 160 amps and I suddenly switch the relay and disconnect the lot instantly. 

 

I'm even thinking I might throw a tarp over part of the panels to reduce the current a bit, so its a bit less stressful for the MPPTs. 

Its one of those situations where the cost of replacing all of the chargers would not be that much cheaper than actually buying new batteries, so if the chargers are put at risk by abrupt disconnection, there is a question in my mind about how to push the test scenario, without wrecking my chargers in the process.

 

 

Edited October 9, 2021 by Tony1

[nicknorman]

You are right to be concerned. Many solar controller can be trashed by disconnecting them from the batteries when they are working hard. Realistically we are talking about one device out of the MPPTs and B2Bs going rogue and giving over-voltage. The other devices will already have shut off.
So in anger, if the disconnection of the relay damages one of the devices, and that device is already malfunctioning, it doesn’t matter. But you certainly don’t want to trash the devices just for the purposes of a test!

 

So yes, cover or disconnect the panels before interrupting the charge current by opening the relay and make sure the B2B2 are off /engine not running, for the purposes of the test.

 

But it does make the point that the system needs to have good integrity, and not disconnect the charge randomly just because it’s in a bad mood.

[Tony1]

8 minutes ago, nicknorman said:

You are right to be concerned. Many solar controller can be trashed by disconnecting them from the batteries when they are working hard. Realistically we are talking about one device out of the MPPTs and B2Bs going rogue and giving over-voltage. The other devices will already have shut off.
So in anger, if the disconnection of the relay damages one of the devices, and that device is already malfunctioning, it doesn’t matter. But you certainly don’t want to trash the devices just for the purposes of a test!

 

So yes, cover or disconnect the panels before interrupting the charge current by opening the relay and make sure the B2B2 are off /engine not running, for the purposes of the test.

 

But it does make the point that the system needs to have good integrity, and not disconnect the charge randomly just because it’s in a bad mood.

 

Yes, this is not one of those parameters to set as a warning type alarm- this will be last-ditch, emergency only, something has gone wrong and its time disconnect immediately.

If the MPPT disconnect goes ok (and these are really nice Victron 50 amp units as well), I think l'll try the emergency disconnect with one B2B running on half power, just to get an inkling how they will behave. 

 

Knowing that the real life emergency disconnect would trash some of my chargers will certainly encourage me to set the cut off at a genuine emergency level, maybe 14.6 volts. 

 

With my crazy setup involving six different charging devices, I do have to play a slightly dangerous game, in that some of them do try to go into float earlier than I want them to as the battery voltage rises, so I have started to use profiles on the B2Bs that have higher charge voltages, to make sure they carry on charging - especially when I want to synchronise and get them up to 100%- a lot of times one or more of the chargers will drop out of the game if I use a lower voltage setting.

 

So my B2Bs and MPPTs are now set to bulk charge at 14.2v. I did originally user 13.9v, but I was finding they started to float too early, when the SoC was still around 75-80%, and there was more bulk charging for them to do.  

But I feel its a dangerous balancing act, and I'm trying to strike a balance between squeezing in as much charge as possible during the engine running time, but not being so aggressive that the battery voltage could creep up to a damaging level.  

 

Fortunately the victron MPPTs are very clever, and sometimes they will go into float anyway even if I try to drive them harder.

But as you say, if I can get the balance right, most of the chargers will already have started to float anyway by the time the voltage gets up to a dangerous level, so any damage will be limited to one unit.

 

 

[nicknorman]

1 hour ago, jetzi said:

 

Hmm, I don't see how this can be correct. Say I'm dealing with the "Reset" (disconnect) for overvoltage on the "charge" Tyco relay (terminal assignment diagram below, taken from the Tyco BDS-A data sheet)

 

The cell monitor board provides a pulse of GND (battery -ve) to switch the bistable relay (according to the manual, Umax output #2 provides a 150ms pulse). So with this output (-ve) connected the gate of the MOSFET, I figure that in order to create the voltage differential needed to switch the MOSFET, I would need to connect the source of the MOSFET to the battery +ve.

 

To reset the Tyco, the relay terminals 1 (+ve) and 3 (-ve) need to be connected to the drain and battery. Since the drain, when the MOSFET is on, connects to the battery -ve via the source, this would go to terminal 3.  I would have to connect the other Tyco coil terminal 1 (+ve) to the battery +ve.

 

I have a suspicion the bit I'm not understanding is in "forward bias the body diode". I figure that all I need to do to cater for the diodes in the relay is to connect the coils the right way around (1 to +ve and 3 to -ve), but I fear there is more to it than that.

 

 

 

Yeah, I thought 16V would be plenty because I hadn't considered battery spikes - thanks for that. I was actually mainly looking at the drain/source max voltage (the relay circuit) which in this case was 55V, I will look for a MOSFET with a higher Vgs. edit: perhaps this one, which has a Vgs of +-30V.  N-Channel MOSFET, 8 A, 500 V, 3-Pin TO-220AB Vishay IRF840APBF


 

 

After 150ms, the pulse from the CPM would be over, and therefore send the voltage back down to zero? Or can I not rely on that?

 

 

 

I actually have also just noticed that the Tycos ask for an impulse length of 15 to 100ms, so I wonder if it might be asking too much of the Tycos to handle 150ms... I think maybe the arduino is really the way forward for me.

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.

[Tony1]

On 09/10/2021 at 19:00, nicknorman said:

You are right to be concerned. Many solar controller can be trashed by disconnecting them from the batteries when they are working hard. Realistically we are talking about one device out of the MPPTs and B2Bs going rogue and giving over-voltage. The other devices will already have shut off.
So in anger, if the disconnection of the relay damages one of the devices, and that device is already malfunctioning, it doesn’t matter. But you certainly don’t want to trash the devices just for the purposes of a test!

 

So yes, cover or disconnect the panels before interrupting the charge current by opening the relay and make sure the B2B2 are off /engine not running, for the purposes of the test.

 

But it does make the point that the system needs to have good integrity, and not disconnect the charge randomly just because it’s in a bad mood.

 

Hi Nick- since you've been so kind with advice and info about all these BMS/charging issues, I just wanted to let know how the cheap relay turned out- which was not very well. 

It certainly works, in that it disconnects the chargers in response to the BMV signal, but the terminals, and the lugs attached to it, are getting very hot to the touch. The other cables and connectors before and after the relay are all fine, but the relay is not. 

 

Part of its problem is it has small M6 terminal studs, so I did try using large flat washers to get a lot of contact area, but its not resolved the heat problem. 

 

My next cunning plan is to use my BEP switch (currently being used as my low voltage disconnect) instead of the relay- that is rated at 275 amps, and it has giant M12 studs so its bound to stay cool. 

 

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. They do a model rated at 220 amps, and even in high summer the highest charging that could ever happen will be about 160 amps, so it should be fine. 

It doesnt need a BMV either. Rather disappointingly, its not truly programmable, but allows you to select from a range of presets for your low voltage disconnect, and the highest voltage preset available is 12.0v, which as I understand it is too low, and by the time the battery gets to 12.0v I'm assuming its already at risk of being damaged. So its not ideal, but it is a good quality reliable unit.

 

What I would like is a low voltage cutoff at about about 12.6v, but I'm hopeful I can link the BP unit to a BMV712, and control it remotely. 

 

So sadly the cheap relay didnt do the job (safely), and I'll have to use the BEP instead for my high voltage disconnect. But the end of the BMS saga is definitely very close. 

 

 

Edited October 12, 2021 by Tony1

[Richard10002]

1 hour ago, Tony1 said:

 

Rather disappointingly, its not truly programmable, but allows you to select from a range of presets for your low voltage disconnect, and the highest voltage preset available is 12.0v, which as I understand it is too low, and by the time the battery gets to 12.0v I'm assuming its already at risk of being damaged. So its not ideal, but it is a good quality reliable unit.

 

 

 

I've got the Smart Battery Protect 220A version. It's programmable via an app on a smartphone, and I'm certain I've had it set above 12V because it has woken me up unnecessarily. Having said that, it is currently set to shutdown at 12.0V and restart at 12.1V. I often read that lithium can go down to 10.0V without damage, although I have never tested that, nor do I see a need to. I rarely go below 13.0V, (maybe 12.8V from time to time), and the Battery Protect is truly a last resort protection.

 

Dont forget current can only flow one way through it. I have mine in the cable that goes to the domestic draw, so there is no chance of current flowing the wrong way.

 

https://www.victronenergy.com/upload/documents/Datasheet-Smart-Battery-Protect-65-A--100-A--220-A-EN-.pdf

 

https://www.victronenergy.com/battery_protect/smart-battery-protect

 

 

 

 

[MtB]

3 minutes ago, Richard10002 said:

Having said that, it is currently set to shutdown at 12.0V and restart at 12.1V.

 

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.

[peterboat]

1 minute 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, notwot 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.

That's wot the BMS is for 

[MtB]

1 minute ago, peterboat said:

That's wot the BMS is for 

 

What BMS? 

[Tony1]

5 minutes ago, Richard10002 said:

 

I've got the Smart Battery Protect 220A version. It's programmable via an app on a smartphone, and I'm certain I've had it set above 12V because it has woken me up unnecessarily. Having said that, it is currently set to shutdown at 12.0V and restart at 12.1V. I often read that lithium can go down to 10.0V without damage, although I have never tested that, nor do I see a need to. I rarely go below 13.0V, (maybe 12.8V from time to time), and the Battery Protect is truly a last resort protection.

 

Dont forget current can only flow one way through it. I have mine in the cable that goes to the domestic draw, so there is no chance of current flowing the wrong way.

 

https://www.victronenergy.com/upload/documents/Datasheet-Smart-Battery-Protect-65-A--100-A--220-A-EN-.pdf

 

https://www.victronenergy.com/battery_protect/smart-battery-protect

 

 

 

 

 

Thanks Richard- and hopefully Mr Victron wont sue me for defamation- I had another look at the instructional video and on the drop-down list of presets there is one for user defined values. 

 

Just on that issue of what is the best emergency cut off, I do think I was being a bit conservative. According to the rough guide I've been using, 10v is effectively zero SoC, so it wont be easy to recover from that sort of event. 

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

 

I do recall that the voltage will read lower if the battery is under load than if it was resting, and in these situations it will be under some sort of load, so it will reach the cutoff voltage at a higher SoC than you would think, if you were using the resting voltage vs SoC graphs. 

All of which I think means you have a margin of error sort of built in. 

 

 

 

 

 

 

[peterboat]

4 minutes ago, MtB said:

 

What BMS? 

They all have valence batteries with built in BMS Tony and Richard 

[Richard10002]

1 minute 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.

 

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?

8 minutes ago, peterboat said:

That's wot the BMS is for 

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

[Tony1]

6 minutes ago, MtB said:

 

What BMS? 

 

Mine have a BMS cable connecting the batteries together, so although it doesnt act as a full BMS (which is an external module that nobody can get hold of), I believe it does help to keep the cells in balance. 

 

 

[peterboat]

17 minutes ago, Richard10002 said:

 

I've got the Smart Battery Protect 220A version. It's programmable via an app on a smartphone, and I'm certain I've had it set above 12V because it has woken me up unnecessarily. Having said that, it is currently set to shutdown at 12.0V and restart at 12.1V. I often read that lithium can go down to 10.0V without damage, although I have never tested that, nor do I see a need to. I rarely go below 13.0V, (maybe 12.8V from time to time), and the Battery Protect is truly a last resort protection.

 

Dont forget current can only flow one way through it. I have mine in the cable that goes to the domestic draw, so there is no chance of current flowing the wrong way.

 

https://www.victronenergy.com/upload/documents/Datasheet-Smart-Battery-Protect-65-A--100-A--220-A-EN-.pdf

 

https://www.victronenergy.com/battery_protect/smart-battery-protect

 

 

 

 

I have taken the batteries down to 10 volts and they just charged back up again with no apparent problems. My 36 volts valence batteries in some cases came flat! Again they charged up the BMS was reset to clear red light and 3 years later still working great 

[Tony1]

10 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.

 

If the BMS cable can read data from individual cells, maybe you could get one of those victron battery balancers, which might help to actively keep the cells in balance?

If its do-able, it would be 60 quid well spent.

 

I'm gutted I didnt get a fourth valence battery when I had the chance, as Peter advised- it would give me more flexibility. I have three 135Ah units, but a fourth would have been great. 

 

https://www.amazon.co.uk/Victron-Energy-BBA000100100-Battery-Balancer/dp/B01DH6B2D2/ref=asc_df_B01DH6B2D2/?tag=googshopuk-21&linkCode=df0&hvadid=310746095651&hvpos=&hvnetw=g&hvrand=2569593360304888800&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=1006940&hvtargid=pla-351132180708&psc=1&th=1&psc=1

 

 

 

 

Edited October 12, 2021 by Tony1

[Richard10002]

8 minutes ago, Tony1 said:

 

If the BMS cable can read data from individual cells, maybe you could get one of those victron battery balancers, which might help to actively keep the cells in balance?

If its do-able, it would 60 quid well spent.

 

https://www.amazon.co.uk/Victron-Energy-BBA000100100-Battery-Balancer/dp/B01DH6B2D2/ref=asc_df_B01DH6B2D2/?tag=googshopuk-21&linkCode=df0&hvadid=310746095651&hvpos=&hvnetw=g&hvrand=2569593360304888800&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=1006940&hvtargid=pla-351132180708&psc=1&th=1&psc=1

 

 

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