So I have 4x 24V 100 AH Ampere Time batteries. I noticed that two of my batteries are at 27.2V and the other two are at 28.4V. I also saw that when on a load, the 27.2V batteries would drop their voltage to 26.8V while the other two are still at 28.2-28.4V. It seems it's only discharging from two batteries; I double-checked the wiring is correct. What should I look into? Before I put them together, the high voltage batteries were used for testing the solar system so that they might be a bit more charged, but that's only a few minutes of charging, and that's like a week ago. I thought BMS would balance them in series?
Do you need to balance Lifepo4 batteries in series?
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sunshine_eggo
Happy Breffast!
Yes.
They should be charged to full either individually or in parallel before they are wired in series. BMS can only balance the cells within its battery.
They should be charged to full either individually or in parallel before they are wired in series. BMS can only balance the cells within its battery.
John Frum
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How are the batteries combined?
Needless to say that is unusual.
4 24 volt batteries is series is 96 volts.
Needless to say that is unusual.
sunshine_eggo
Happy Breffast!
John Frum
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Might also be 4p.I'm hoping it's 2S2P...
Cajunwolf
Solar Enthusiast
How do you have them wired, and by that, I mean the load? Put another way, is the load across the terminals of one of the four batteries wired in parallel or from opposite ends of the bank? It would be best to have positive from one end and negative from the other to make current flow out and in from the whole bank instead of predominantly the first one or two batteries. First, you should fully charge all the batteries, let them sit for a good while to even out, and then carefully wire them in parallel.
RCinFLA
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The desired state is all of the parallel stacks contribute equally to the current the inverter is drawing. Even with same batteries bought at same time they will not perfectly match. You should have less than 20% current imbalance between the parallel stacks however.
Several reasons why they don't match.
1) battery not at same state of charge (not balanced) To your post title, it is important that batteries are balanced. Especially for series connected cells or battery packs.
If you charge them to 28.4v for several hours they should get balanced. If they are severely imbalanced it may take longer and you might get some BMS overvoltage shut downs along the way to balance. Failure to charge them to this level at least every few weeks will allow the batteries to get out of balance. This is a common issue for off grider's that only have limited PV power to charge batteries and seldom fully charge them. Most BMS's do not balance until a cell gets above 3.4v so continued partial charging will not get sufficient balancing time and only when cells get severely misbalanced.
When running separately you must make sure you have the same inverter load current when making comparison. The greater the load current the more the battery's terminal voltage will slump. You should not draw more than 50 amps from a single 100 AH series string
2) A bad battery connection on one of the series two batteries.
3) Longer battery cables on one string.
4) Differing aging conditions of batteries. One sting has batteries with higher resistance.
If you have multiple strings in parallel you should get a clip-on DC ampmeter so you can easily check each strings current contribution when they are running in parallel.
Several reasons why they don't match.
1) battery not at same state of charge (not balanced) To your post title, it is important that batteries are balanced. Especially for series connected cells or battery packs.
If you charge them to 28.4v for several hours they should get balanced. If they are severely imbalanced it may take longer and you might get some BMS overvoltage shut downs along the way to balance. Failure to charge them to this level at least every few weeks will allow the batteries to get out of balance. This is a common issue for off grider's that only have limited PV power to charge batteries and seldom fully charge them. Most BMS's do not balance until a cell gets above 3.4v so continued partial charging will not get sufficient balancing time and only when cells get severely misbalanced.
When running separately you must make sure you have the same inverter load current when making comparison. The greater the load current the more the battery's terminal voltage will slump. You should not draw more than 50 amps from a single 100 AH series string
2) A bad battery connection on one of the series two batteries.
3) Longer battery cables on one string.
4) Differing aging conditions of batteries. One sting has batteries with higher resistance.
If you have multiple strings in parallel you should get a clip-on DC ampmeter so you can easily check each strings current contribution when they are running in parallel.
sunshine_eggo
Happy Breffast!
Unless I'm missing something, that looks like 2P2S, i.e., you've paralleled 2X 24V and then placed them in series.
parallel connections are ellipses, series connection is a square.
The parallel 24V should always read the same. If they don't, you likely have a wiring problem (loose connection, bad crimp).
There may be a disparity between the upper pair and lower pair as depicted.
Yes, It is 2P2S. I am just copying the wiring on the internet without understanding what I am doing.
Would it be fine for me to just attach a 24V battery charger to the battery with a lower voltage and charge it without touching the wiring I have already done? It's scary because typically, when you connect positive to negative, you get a short circuit, but that's not happening when we are making a series connection of two batteries, but if by accident, somehow, two batteries are connected and become one, the series connection may become a short circuit.
Oh, I am a bit shamed by my electrical knowledge.
sunshine_eggo
Happy Breffast!
Yes, It is 2P2S. I am just copying the wiring on the internet without understanding what I am doing.
Would it be fine for me to just attach a 24V battery charger to the battery with a lower voltage and charge it without touching the wiring I have already done?
Yes; however, you should actually connect across both parallel batteries, one pair at a time as shown:
Charge the lower pair at the circled points and then the upper pair at the circled points. You don't need to disconnect anything.
You should also power down the inverter until both pairs of 24V have been fully charged. You don't want to input additional charge or extract any capacity from either set.
Cajunwolf
Solar Enthusiast
sunshine_eggo
Happy Breffast!
That's only in the context of DIY batteries and CELLS for top balance. The series portion (with BMS installed) is to allow for faster charging since you get more power at higher voltage rather than 3.4V * 10A = 34W.
For whole batteries, they can only be top balanced with each other either by individually charging to full or doing it in parallel.
In your configuration, you should charge each 12V separately to full or parallel them all as 12V and charge to full.
Cajunwolf
Solar Enthusiast
I've already charged them all, then put them all in parallel a while waiting on my inverter, then to series-parallel. They work out nice that way in my installation; being I can use all bus bars.
You can balance a whole battery bank with a balancer(s); I've seen Victron a lot, but you wire it in at the midpoint voltage, and if you have more than two battery pairs, you need a balancer for each set. I think that's how it is. I see it a lot in communications and server buildings with a generator and battery bank for backup power systems.
sunshine_eggo
Happy Breffast!
Yeah. I have and recommend 12V balancers:
These keep my 4S2P Trojan bank chugging along.
Affordable Electric Vehicle Batteries & Components from Electric Car Parts Company
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These keep my 4S2P Trojan bank chugging along.
codfish
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Kilovault makes a 48v balancer for 4S of 12v batteries. Unit works for both LFP 12v and lead acid 12v.
www.altestore.com
KiloVault Battery Balancers
KiloVault Battery Balancers help extend the life of a battery bank in your solar power system. 24V and 48V versions are used to maintain battery voltages across a series connection of batteries.
www.altestore.com
sunshine_eggo
Happy Breffast!
Kilovault makes a 48v balancer for 4S of 12v batteries. Unit works for both LFP 12v and lead acid 12v.
KiloVault Battery Balancers
KiloVault Battery Balancers help extend the life of a battery bank in your solar power system. 24V and 48V versions are used to maintain battery voltages across a series connection of batteries.
I had used the units I have for NMC balancing, so they were my go-to. The Kilovault is cheaper and easier. Would probably go that route if I had to do it again.
Cajunwolf
Solar Enthusiast
So, I need two balancers in my setup, one at the mid voltage point of each set of series batteries, right? This will keep each set of series 12 to 24-volt batteries in balance with each other.
sunshine_eggo
Happy Breffast!
If you're buying the one I linked, you'd need one for each 12V (4). If you get the Kilovalt, you'd need one for each string (2).
Cajunwolf
Solar Enthusiast
Yeah, I see; just connect the two midpoints to the balancer.
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