LiFPo4 battery bank balancing - and limits

[mattonthesea]

I am about to be in the process of switching to a bank of two lithium 100 Ah batteries for the leisure side of a Stirling B2B system. The Renogy instructions say that the individual battery cells balance automatically, and I could have a bank of up to four batteries. What I can't work out, though, is the limit of difference in voltage between them when I connect them up. Would the cells from one battery try and balance themselves safely; or would they try to fast-charge, dangerously? Or, other words, do I need to make sure that both batteries have the same voltage? And, if so, how much can they differ by?

I have pondered that if I turn the B2B charger on almost immediately after connecting them then that voltage will be higher than the Lithium anyway so both will just charge.

Or am I over-thinking it?

TIA

M

PS the review scores of the BMS App for Renogy would be below 1 if that were possible so I'm tempted not to bother with that unless necessary. I've got a NASA BM1 which would provide the info I need

 

[Ian_Edwards]

I did this when I installed 4 Lifos 105 Amp hr LiFePO4 batteries in parallel for the first time.
I connected the negative cables to all 4 batteries, and then using a DVM on the mV scale to measure the difference between two positive terminals. Then by using a charger, and/or switching on some domestic load, I balanced the two batteries to within 5mV. A bit fiddly, but not hard.
I then connected batteries 1 and 2 together and repeated the process to balance 1+2 with 3, then again to balance 1+2+3 with 4.
It worked, as far as I could see, no sparks when I connected then, and nothing got hot.
If the batteries are too far apart in voltage, it's possible to generate some very high current, which are only limited by the internal impedance of the battery, which for LiFePO4 is very low.

 

[geem]

I am about to be in the process of switching to a bank of two lithium 100 Ah batteries for the leisure side of a Stirling B2B system. The Renogy instructions say that the individual battery cells balance automatically, and I could have a bank of up to four batteries. What I can't work out, though, is the limit of difference in voltage between them when I connect them up. Would the cells from one battery try and balance themselves safely; or would they try to fast-charge, dangerously? Or, other words, do I need to make sure that both batteries have the same voltage? And, if so, how much can they differ by?

I have pondered that if I turn the B2B charger on almost immediately after connecting them then that voltage will be higher than the Lithium anyway so both will just charge.

Or am I over-thinking it?

TIA

M

PS the review scores of the BMS App for Renogy would be below 1 if that were possible so I'm tempted not to bother with that unless necessary. I've got a NASA BM1 which would provide the info I need

How about connecting one battery first and fully charging it. Then do the same with the other one. Then connect them both in parallel. Job done

 

[yoda]

Because the battery voltage across the majority of the capacity range is pretty much the same, the relative state of charge between 2 batteries will only result in a fairly small voltage difference. Should be fine just to connect them together but worth a quick check with a volt meter to ensure they are a similar voltage.

 

[Pete7]

Matt, does the Renogy app give the cell voltages? If so its worth having it and checking what is going on at cell level every so often. However, the % or state of charge for most of the apps is generally inaccurate. We have the Sterling battery monitor rather than the NASA but that counts Ah in and out and gives us a better idea of how charged the bank is.

You might find with two batteries connected together, a very light load is only taken from one battery, but if you put a heavy load on then both will wake up and supply power then equalise out.

This is worth watching:

 

[gregcope]

They are highly likely to have been shipped at a standard shipping/storage voltage (13.3V?) that represents around 50% SOC.

 

[Poey50]

It doesn't sound like much of an issue to me. The voltage difference between the batteries is unlikely to be high enough to make any real difference. In parallel they will soon equalise. They will probably hit their BMS disconnect parameters at different times which could be an advantage as long as you have some means of knowing when disconnect has occurred. Not straightforward since it will be masked by the other functioning battery.

 

[mattonthesea]

Matt, does the Renogy app give the cell voltages? If so its worth having it and checking what is going on at cell level every so often. However, the % or state of charge for most of the apps is generally inaccurate. We have the Sterling battery monitor rather than the NASA but that counts Ah in and out and gives us a better idea of how charged the bank is.

You might find with two batteries connected together, a very light load is only taken from one battery, but if you put a heavy load on then both will wake up and supply power then equalise out.

This is worth watching:

Thanks all. I like this chap - wish I'd had him as a physics teacher! If I'd seen this before I would have got the 200Ah one. The reason I didn't was that I will, at some point, add in another 100Ah for when we move to electric cooking. I thought I would need similar batteries.

Assuming then, that there is less than a 0.2V difference the wire and fuse capacity suggested by Sterling will cover the quick current surge (his is 0.1V difference and 2A) it should all cope. If higher then I will do some charging/discharging to manage.

Poey, I think I am covered for disconnect in that the charger has a max voltage limiter and temperature limiters, and the BM1 will monitor voltage too. When away from the boat I will charge to 80% and turn them off. Yes your thread was what convinced me to go down this road

Again, a good example of how supportive this forum is. Thanks

 

[Poey50]

Poey, I think I am covered for disconnect in that the charger has a max voltage limiter and temperature limiters, and the BM1 will monitor voltage too. When away from the boat I will charge to 80% and turn them off.

One reason why you shouldn't completely rely on chargers switching themselves off in time prior to the target voltage of the chargers is that voltage alone is not a totally reliable guide. If for any reason the cells become unbalanced then, while the pack voltage may look fine it only takes one cell to exceed the high voltage disconnect figure for the BMS to shut down. For example, with the charger set to switch off at 14.4 volts (or + a short absorption period) a balanced set of cells might read 3.6, 3.6, 3.6, 3.6. If the high voltage disconnect is set to 13.65 volts (a common setting for safety) then no cell will reach disconnect before charging ceases. But an unbalanced battery might read 3.57, 3.57, 3.55, 3.70 so cell 4 would trigger high voltage disconnect before charging stops.

In addition there are ten reasons why a FET-based BMS might trigger a disconnect. 1. Discharge current too high, 2. Short circuit, 3. BMS chip fault, 4. Charge current too high, 5. Cell voltage too low, 6. FET temperature too high, 7. Cell voltage too high, 8. Cell temperature too low, 9. Cell temperature too high, and 10. Voltage differences between cells too high.

So in my opinion a safe and seaworthy system should always plan some kind of safety net to cover a BMS disconnect.

 

[mattonthesea]

One reason why you shouldn't completely rely on chargers switching themselves off in time prior to the high voltage disconnect figure of the chargers is that voltage alone is not a totally reliable guide. If for any reason the cells become unbalanced then, while the pack voltage may look fine it only takes one cell to exceed the high voltage disconnect figure for the BMS to shut down. For example, with the charger set to switch off at 14.4 volts (or + a short absorption period) a balanced set of cells might read 3.6, 3.6, 3.6, 3.6. If the high voltage disconnect is set to 13.65 volts (a common setting for safety) then no cell will reach disconnect before charging ceases. But an unbalanced battery might read 3.57, 3.57, 3.55, 3.70 so cell 4 would trigger high voltage disconnect before charging stops.

In addition there are ten reasons why a FET-based BMS might trigger a disconnect. 1. Discharge current too high, 2. Short circuit, 3. BMS chip fault, 4. Charge current too high, 5. Cell voltage too low, 6. FET temperature too high, 7. Cell voltage too high, 8. Cell temperature too low, 9. Cell temperature too high, and 10. Voltage differences between cells too high.

So in my opinion a safe and seaworthy system should always plan some kind of safety net to cover a BMS disconnect.

Ah, I interpreted your post as just knowledge of disconnect. Yes I have a safety net/plan if it goes AOT. I've been reading your thread and the links Can't remember where I read but one incident was a disconnect at night in a shipping lane - that's enough of a cautionary tale for me!

 

[Poey50]

Ah, I interpreted your post as just knowledge of disconnect. Yes I have a safety net/plan if it goes AOT. I've been reading your thread and the links Can't remember where I read but one incident was a disconnect at night in a shipping lane - that's enough of a cautionary tale for me!

Good news!

 

[Refueler]

I am about to be in the process of switching to a bank of two lithium 100 Ah batteries for the leisure side of a Stirling B2B system. The Renogy instructions say that the individual battery cells balance automatically, and I could have a bank of up to four batteries. What I can't work out, though, is the limit of difference in voltage between them when I connect them up. Would the cells from one battery try and balance themselves safely; or would they try to fast-charge, dangerously? Or, other words, do I need to make sure that both batteries have the same voltage? And, if so, how much can they differ by?

I have pondered that if I turn the B2B charger on almost immediately after connecting them then that voltage will be higher than the Lithium anyway so both will just charge.

Or am I over-thinking it?

TIA

M

PS the review scores of the BMS App for Renogy would be below 1 if that were possible so I'm tempted not to bother with that unless necessary. I've got a NASA BM1 which would provide the info I need

You have just highlighted one of the worst features of ALL Lithium based batterys.

I have been a user of Li based battery's for decades an I have to say I have never considered swapping over to Li tech. for the boat ...