Cheap LiFePO4 BMS?

[MoominPapa]

44 minutes ago, Dr Bob said:

That sounds really good. Well done!

How were you doing the manual interventions before?

The BMS has a function, you can just tell it to take xAh from cell y. This algorithm essentially automates what I was doing by hand.

 

MP.

 

[MoominPapa]

56 minutes ago, ivan&alice said:

 

 

This is very interesting and relevant to the conversation we were having at the top of this page. This surely only works if the voltage at that point is a good predictor of SoC for the cells, or if you are charging your cells to quite a high SoC. I find that the cell that climbs up to 3.5+V first is not the cell at the highest voltage in the mid range. It might just be that my cells have rather different capacities, though?

The cell voltage at  the knee is a pretty good predictor of how close to full charge a cell is, and as the aim of the balancing is to top balance, ie reach charge termination with all four cells at the _same_ voltage, it's pretty good for doing that. I know that my cell-groups have somewhat differing capacities, so they don't all hit discharged at the same point, and don't all have the same number of Ah remaining at all times; that's fine, I just want them to be at the same voltage at charge termination. 

 

Quote

I'm now fiddling with an arduino to try to get into electronics, hardware is not an area of expertise for me but I am a computer programmer so I have that going for me at least. My first aim is to create a basic data logger that I can log the voltages of each cell over a long period of time and hopefully draw some insights into the health of the cells. But eventually I'd love to get to the point that I can create a MoominBMS of my own..

 

Happy to share code and war-stories.

 

MP.

 

ETA. In case it wasn't clear, the balancing happens _after_ charge termination. The Arduino notes the cell voltages when it triggers charge termination, and then turns on the relevant balance resistors for the relevant amount of time determined by those voltages.

Edited September 5, 2020 by MoominPapa

[jetzi]

1 minute ago, MoominPapa said:

Happy to share code and war-stories.

Thank you, I will take you up on that one day! At the moment I'm just trying to get an LED to blink so I'm a way away from war stories

[MoominPapa]

Hit quote instead of edit and duplicated my post.

Edited September 5, 2020 by MoominPapa

[Murflynn]

I would love to replace my L/A leisure batteries that I use to propel my wee boat with used Valence batteries advertised on ebay, apparently containing all the necessary balance and protection systems, but all these high power technical discussions put me right off. 

 

I have 4x110Ah batteries, a multistage 20A mains charger used at overnight hook-ups and 300W nominal solar with a Victron 75/15 controller.  My motor draws about 50A max.  My bed is directly above the battery compartment and I don't want to be blown away in the middle of the night.

 

Is there a simple but safe solution?

[jetzi]

1 minute ago, Murflynn said:

I would love to replace my L/A leisure batteries that I use to propel my wee boat with used Valence batteries advertised on ebay, apparently containing all the necessary balance and protection systems, but all these high power technical discussions put me right off. 

 

I have 4x110Ah batteries, a multistage 20A mains charger used at overnight hook-ups and 300W nominal solar with a Victron 75/15 controller.  My motor draws about 50A max.  My bed is directly above the battery compartment and I don't want to be blown away in the middle of the night.

 

Is there a simple but safe solution?

When you say "propel" you mean your boat is driven by an electric motor drawing 50A?

With LiFePO4s being blown away is much less of a concern than other lithium technologies and arguably even less than with lead acids. There are videos of people chainsawing them in half on YouTube. The high-powered technical discussions we're having are a) really about trying to not damage the batteries rather than keeping them from blowing up, b) largely due to quite a variety of differing requirements between us with no standard equipment, and c) not actually as high-powered technical as they seem at first.

 

Everyone's situation is quite different. In your case your charging is lower current than even mine (and mine is probably the lowest among us) which does simplify things a bit. If you are all electric then you won't have an alternator and this simplifies things a lot.

 

The main concern is preventing your batteries from being over / under charged. I don't know if the Valence batteries have something built in that prevents that from happening, but this can be quite a bit more complicated than it seems. For solar charging, if you want a simple but not necessarily optimum solution it's just a case of setting a very low float charge (13.8V or less). If your mains charger can be set to terminate at a low voltage as well, then bobs your uncle there too. Then all you need is something that will terminate your load at low voltage. Perhaps the valences do that for you, but if not - if you're only drawing 50A - you can buy a cheap BMS board that can handle 50A currents.

 

This very simple system wouldn't tell you how much capacity you have left, it wouldn't have any extra layers of protection against damaging your batteries if something fails, there's no low temperature protection.

 

I'm very much learning though and I don't know anything about Valence batteries other than that they are apparently "smart" (which rouses suspicion, for me at least). I'm sure the more experienced members here will be able to give you better advice, particularly @peterboat who has experience with Valence batteries (and I think @Richard10002 if I recall - his system is also simplified by not alternator charging).

[peterboat]

Ivan google my big electric boat on here their is a thread on it, but the gist is it was diesel and I removed it. I fitted a 48 volt series DC forklift motor, I run it over voltage at 72 volts nominal via a curtis speed controller,  it's direct drive with a slightly cut down prop works well and cruising speed is 3.3 kWh 

[WotEver]

2 minutes ago, peterboat said:

cruising speed is 3.3 kWh 

<pedant mode.

 

It's either 3.3kW, or it's 3.3kWh/h

 

</pedant mode>

34 minutes ago, ivan&alice said:

I don't know if the Valence batteries have something built in that prevents that from happening, <snip> all you need is something that will terminate your load at low voltage. Perhaps the valences do that for you <snip> there's no low temperature protection.

Watch that vid I linked in #3313.  The Valences have all the above and more.

[Richard10002]

6 hours ago, ivan&alice said:

I don't know if the Valence batteries have something built in that prevents that from happening, but this can be quite a bit more complicated than it seems.

No... they dont. They have a monitor thing that you can connect to a PC, (with a bit of fiddling about), so you can see a variety of things. I watch individual cell voltages mostly.

 

If I were to set my Sterling Pro Charge Ultra 60A to charge up to 15.5V, (or whatever), the electronics in the batteries would not stop the charging at any voltage. I set it at 14.8V so that, due to voltage drop, it never gets higher than 14.6V at the batteries, and no cell gets higher than 14.8V. Because I charge with a generator, I am always on site when charging, regularly check how things are going, and only occasionally charge beyond about 90% SOC. If I were to leave the boat charging on shore power for a few days, I'd probably set the charger with a max of 13.8V, or less, (havent really needed to think about it).

 

As for discharge - I have a Victron protector which I have set to cut the power to the domestics if the voltage falls below about 12.4V.

 

I have a couple of relays that I havent fitted yet, one of which was to use the Victron BMV712 relay to cut the charging power when the voltage reaches a preset limit. Cant remember what the second one was for - possibly a low voltage limit?

 

500W of solar and a genny keep me topped up without needing to worry about alternator charging. If I were to cruise more, I would probably get a B to B gizmo.

 

[Richard10002]

5 hours ago, WotEver said:

Watch that vid I linked in #3313.  The Valences have all the above and more.

Mine dont.... there is a gizmo that they would have been connected to in their previous lives that took care of this, but it is either hugely expensive, or unavailable, so nobody seems to have one in their second hand systems.

[Dr Bob]

6 hours ago, Murflynn said:

I would love to replace my L/A leisure batteries that I use to propel my wee boat with used Valence batteries advertised on ebay, apparently containing all the necessary balance and protection systems, but all these high power technical discussions put me right off.

 

Is there a simple but safe solution?

If you understand Volts, Amps,  Amphours used and have kit to measure that lot and know how to use it on Lead Acids, then Lithiums are pretty simple.

The discussion on here spends an awful lot of time on things like balancing which I only do once per year and its pretty easy once you understand the rules for Lithiums. As long as you set up with safeguards over excessive charging etc then Li's become very simple. Dont let all this hi tech stuff put you off. You dont need to create computer based battery management systems and just buy off the shelf components and bolt them together.

You'll never again need to worry about having to charge to 100%.

It really is very easy for someone with a reasonable level of battery management knowledge.

[peterboat]

47 minutes ago, Richard10002 said:

No... they dont. They have a monitor thing that you can connect to a PC, (with a bit of fiddling about), so you can see a variety of things. I watch individual cell voltages mostly.

 

If I were to set my Sterling Pro Charge Ultra 60A to charge up to 15.5V, (or whatever), the electronics in the batteries would not stop the charging at any voltage. I set it at 14.8V so that, due to voltage drop, it never gets higher than 14.6V at the batteries, and no cell gets higher than 14.8V. Because I charge with a generator, I am always on site when charging, regularly check how things are going, and only occasionally charge beyond about 90% SOC. If I were to leave the boat charging on shore power for a few days, I'd probably set the charger with a max of 13.8V, or less, (havent really needed to think about it).

 

As for discharge - I have a Victron protector which I have set to cut the power to the domestics if the voltage falls below about 12.4V.

 

I have a couple of relays that I havent fitted yet, one of which was to use the Victron BMV712 relay to cut the charging power when the voltage reaches a preset limit. Cant remember what the second one was for - possibly a low voltage limit?

 

500W of solar and a genny keep me topped up without needing to worry about alternator charging. If I were to cruise more, I would probably get a B to B gizmo.

 

Richard when you plug in do you find your batteries out of balance? I think rather than being a BMS, the boards in the valence batteries are battery balance boards, the real BMS is a stand alone unit. Because I never go above 13.9 volts nothing is needed as the board balances the cells pretty much like the boards I have bought for my car 

[Richard10002]

5 minutes ago, peterboat said:

Richard when you plug in do you find your batteries out of balance? I think rather than being a BMS, the boards in the valence batteries are battery balance boards, the real BMS is a stand alone unit. Because I never go above 13.9 volts nothing is needed as the board balances the cells pretty much like the boards I have bought for my car 

One of the cells in one of my batteries is always out of balance by more than I feel comfortable with. In reality, it doesnt seem to matter much, and doesnt seem to be getting worse, so all is good, (although I'm only 9 months into ownership  ). If I only ever charged to 13.9V, I probably wouldn't even know about it. I mostly charge to a bit over 14V, (whatever 90%-95% SOC is).

 

My sense is that the actual balancing by the board is so feeble that it would take forever to balance the cell, if ever. Next time I charge to close to 100% I might take a note of when balancing starts and ends. Because I use a genny, rather than shore power, I'm not inclined to leave them charging on the genny and, like you, my solar is set at about 13.9V, to avoid overcharging when I'm not around, (or even when I am around).

 

The real BMS is definitely a stand alone unit which provides the protection.... I haven't seen anyone with 2nd hand Valences talking about the unit, other than that you cant get them and Valence wont sell them.

[Richard10002]

48 minutes ago, Dr Bob said:

You'll never again need to worry about having to charge to 100%.

I used to charge to a low tail current, for 3 or 4 hours every day, or every other day, in an attempt to maintain my LA batteries. Despite this I destroyed several sets, expensive and cheap, in fairly short order over the years. It was truly irritating to have to get the genny out so often.

 

Since getting the Li bank, I can go 6 or 7 days with no sun and no charging. When I charge from 20% to 95%, it takes about 6 hours, and I get 60Ah every single hour. With 500W of solar, since March, I've gone for one period of 38 days without getting the genny out, and mostly 12-15 days.

 

In addition, if pressed for time, I can charge for a couple of hours and get 120Ah into the bank, which beats the hell out of the last 20Ah into the LA bank.

 

Absolute no brainer, with hindsight - albeit a big decision without hindsight

 

 

48 minutes ago, Dr Bob said:

It really is very easy for someone with a reasonable level of battery management knowledge.

 

[peterboat]

30 minutes ago, Richard10002 said:

One of the cells in one of my batteries is always out of balance by more than I feel comfortable with. In reality, it doesnt seem to matter much, and doesnt seem to be getting worse, so all is good, (although I'm only 9 months into ownership  ). If I only ever charged to 13.9V, I probably wouldn't even know about it. I mostly charge to a bit over 14V, (whatever 90%-95% SOC is).

 

My sense is that the actual balancing by the board is so feeble that it would take forever to balance the cell, if ever. Next time I charge to close to 100% I might take a note of when balancing starts and ends. Because I use a genny, rather than shore power, I'm not inclined to leave them charging on the genny and, like you, my solar is set at about 13.9V, to avoid overcharging when I'm not around, (or even when I am around).

 

The real BMS is definitely a stand alone unit which provides the protection.... I haven't seen anyone with 2nd hand Valences talking about the unit, other than that you cant get them and Valence wont sell them.

I talked to a chap in NI who worked for Valence as was, he said the same buy new and the BMS is part of the package,  secondhand nowt available. 

[nicknorman]

8 hours ago, MoominPapa said:

The cell voltage at  the knee is a pretty good predictor of how close to full charge a cell is, and as the aim of the balancing is to top balance, ie reach charge termination with all four cells at the _same_ voltage, it's pretty good for doing that. I know that my cell-groups have somewhat differing capacities, so they don't all hit discharged at the same point, and don't all have the same number of Ah remaining at all times; that's fine, I just want them to be at the same voltage at charge termination. 

 

 

Happy to share code and war-stories.

 

MP.

 

ETA. In case it wasn't clear, the balancing happens _after_ charge termination. The Arduino notes the cell voltages when it triggers charge termination, and then turns on the relevant balance resistors for the relevant amount of time determined by those voltages.

What is the equation that relates voltage difference at charge termination, to how many Ah to extract from a cell please?
 

When my cells arrive, which will hopefully be in the next few weeks, I will need to code my BMS to drive the balance FETs/resistors. Too much “gain” would result in chasing one’s tail, too little would mean several bites at the cherry.

[Craig Shelley]

Hello,

 

Apologies in advance if this question has already been discussed in this thread.

I've just been trying to determine the "number of cycles" we're putting on our battery pack per year. I have doubts that this data has much meaning because it doesn't factor in depth of discharge, but was trying to get a rough idea of usage/aging.

The values i'm getting are in the region of 30 - 40 cycles per year, that's on a ~600Ah pack.

Just wondering if these values seem sensible.

 

The method used to estimate this, which is perhaps a little debatable, is to sum the absolute value of the charge for all samples to find the total charge which has passed through the shunts (in+out). Then divide by 2, and divide by the capacity. I also subtracted the measurement drift error which accounts for 3 cycles or so.

I imagine there will be other factors which affect cell ageing before the cycles do.

 

--

Craig

 

[peterboat]

18 minutes ago, Craig Shelley said:

Hello,

 

Apologies in advance if this question has already been discussed in this thread.

I've just been trying to determine the "number of cycles" we're putting on our battery pack per year. I have doubts that this data has much meaning because it doesn't factor in depth of discharge, but was trying to get a rough idea of usage/aging.

The values i'm getting are in the region of 30 - 40 cycles per year, that's on a ~600Ah pack.

Just wondering if these values seem sensible.

 

The method used to estimate this, which is perhaps a little debatable, is to sum the absolute value of the charge for all samples to find the total charge which has passed through the shunts (in+out). Then divide by 2, and divide by the capacity. I also subtracted the measurement drift error which accounts for 3 cycles or so.

I imagine there will be other factors which affect cell ageing before the cycles do.

 

--

Craig

 

Honestly I don't even try to work out such things, my batteries had done one hundred cycles, 3 years on if I plug them in they will tell me how many cycles they have done,  but I ain't bothered as at 62 they will outlive me by many years

[Dr Bob]

26 minutes ago, Craig Shelley said:

Hello,

 

Apologies in advance if this question has already been discussed in this thread.

I've just been trying to determine the "number of cycles" we're putting on our battery pack per year. I have doubts that this data has much meaning because it doesn't factor in depth of discharge, but was trying to get a rough idea of usage/aging.

The values i'm getting are in the region of 30 - 40 cycles per year, that's on a ~600Ah pack.

Just wondering if these values seem sensible.

 

The method used to estimate this, which is perhaps a little debatable, is to sum the absolute value of the charge for all samples to find the total charge which has passed through the shunts (in+out). Then divide by 2, and divide by the capacity. I also subtracted the measurement drift error which accounts for 3 cycles or so.

I imagine there will be other factors which affect cell ageing before the cycles do.

 

--

Craig

 

You need also to record how many days the wind blows from the north and multiply this by 2.3 times the maximum discharge and add to the final number.

?............but the answer in advance like Peter said is that Li's just keep working. 

Edited September 5, 2020 by Dr Bob

[MoominPapa]

1 hour ago, nicknorman said:

What is the equation that relates voltage difference at charge termination, to how many Ah to extract from a cell please?
 

When my cells arrive, which will hopefully be in the next few weeks, I will need to code my BMS to drive the balance FETs/resistors. Too much “gain” would result in chasing one’s tail, too little would mean several bites at the cherry.

 

0.25Ah per 10mV voltage difference, capped at 2.5Ah total. ie cap voltage difference to  100mV. That's for a 480Ah battery.

 

Looking through the logs, the cap has never activated, when I added the function, the spread was up to 90mV, it's reduced to 30mV last time they were fully charged. It's probably worth having the cap as a sanity check.

 

MP.

 

[jetzi]

Thanks for the info Richard. I think your situation perfectly illustrates the differing requirements because you are able to get all of the benefits that apply to you with very little of the faff.

 

16 hours ago, Richard10002 said:

I have a couple of relays that I havent fitted yet, one of which was to use the Victron BMV712 relay to cut the charging power when the voltage reaches a preset limit. Cant remember what the second one was for - possibly a low voltage limit?

May I ask what relays these are? I want to fit one to my BMV712 as a backup but I don't think it can handle a bistable Tyco relay (of which I have several spare).

 

14 hours ago, Craig Shelley said:

I've just been trying to determine the "number of cycles" we're putting on our battery pack per year. I have doubts that this data has much meaning because it doesn't factor in depth of discharge, but was trying to get a rough idea of usage/aging.

The values i'm getting are in the region of 30 - 40 cycles per year, that's on a ~600Ah pack.

Just wondering if these values seem sensible.

 

The method used to estimate this, which is perhaps a little debatable, is to sum the absolute value of the charge for all samples to find the total charge which has passed through the shunts (in+out). Then divide by 2, and divide by the capacity. I also subtracted the measurement drift error which accounts for 3 cycles or so.

I imagine there will be other factors which affect cell ageing before the cycles do.

 

Someone please correct me if I am wrong in my definition of a "cycle": one iteration of discharging the battery to your set floor and then charging it up to your set ceiling?

 

I ignored that calculation. I can't see it being a useful measurement for most boaters' situations, where one is using the batteries at the same time as charging them. This is in contrast to an EV say which (ignoring regen braking) are generally discharged while moving and then recharged while stationary.

 

Depending on the sun / how much I cruise I may discharge the battery anywhere between 10% and 60% of their capacity per day. My hunch is that different portions of the cells would be cycled many more times than other parts, so an overall "average" is probably meaningless.

 

Most of us will be charging and discharging these cells at a far slower rate than the manufacturers generally expect. This is going to be much kinder to the cells than EVs (say) are. So I tend to agree with Dr Bob and Peterboat that we should forget the "batteries are consumables" notion that makes sense for LAs. I'll capacity test mine once a year say and see how they deteriorate. LiFePOs should be considered a permanent piece of equipment. As long as you have a solid BMS setup, they are most likely to be destroyed by some outside influence, or outlive you / the boat / your ownership of the boat.

[cuthound]

1 hour ago, ivan&alice said:

Someone please correct me if I am wrong in my definition of a "cycle": one iteration of discharging the battery to your set floor and then charging it up to your set ceiling?

 

Certainly with commercial lead acid batteries a cycle is from 100% to 0%  and back to 100% SoC, so taking a lead acid battery from the usual 100% to 50% and back to 100% is half a cycle. I cannot see any it should be any different for a Lithium Ion battery. However many liesure batteries state cycles to a higher figure than 0% in an attempt to flatter the performance.

 

So 80% to 20% and back to 80% SoC would 0.6 of a cycle.

Edited September 6, 2020 by cuthound
Clarification

[Richard10002]

3 hours ago, ivan&alice said:

May I ask what relays these are? I want to fit one to my BMV712 as a backup but I don't think it can handle a bistable Tyco relay (of which I have several spare).

Durite 0-727-10 12V/100A

 

https://www.ebay.co.uk/itm/Durite-0-727-10-12V-Extra-Heavy-Duty-Make-Break-Relay-100A-/253937713202?hash=item3b1fde0032

 

But bear in mind that I'm flying by the seat of my pants here, have always been wary of relays, and haven't got around to testing out my theory that they will work as I expect

 

Having said that, the theory isn't complicated, so no reason why they shouldnt. Need to get a round tuit

 

 

 

[nicknorman]

17 hours ago, MoominPapa said:

 

0.25Ah per 10mV voltage difference, capped at 2.5Ah total. ie cap voltage difference to  100mV. That's for a 480Ah battery.

 

Looking through the logs, the cap has never activated, when I added the function, the spread was up to 90mV, it's reduced to 30mV last time they were fully charged. It's probably worth having the cap as a sanity check.

 

MP.

 

Thanks. And do you find an element of “chasing your tail” (sawing the legs of the table etc) ie on consecutive cycles, Ah being taken out of a cell that was previously the lowest, or not?

[MoominPapa]

8 minutes ago, nicknorman said:

Thanks. And do you find an element of “chasing your tail” (sawing the legs of the table etc) ie on consecutive cycles, Ah being taken out of a cell that was previously the lowest, or not?

No, The pattern in remarkably consistent. There's one cell that always has the most charge removed, another which has somewhat less, and the the other too take the place of the lowest cell more-or-less at random.

 

MP.