Sudden battery voltage drop, only when epever connected

[HRTKD]

I'm leaning towards a bad cell. Replacing a single cell, instead of the entire battery, requires a top balance. Do you have the equipment to do that?

However . . . I wouldn't rule out poor connections. Half the voltage issues seen on this forum are due to bad connections. Have you verified that all the fasteners are tight? If you were feeling adventurous I would say to pull the cells out and rearrange them. Mark the cells before you do this so you know the original order. If the voltage issue stays in the same position then we know you either have a bad balance lead connection or perhaps the BMS is bad. If the voltage issue follows the cell then it's safe to say that the cell is the problem.

My usual recommendation is to NOT put LiFePO4 batteries in series. If you had a 24 volt battery with a single BMS would that have worked around this issue? If the source of the problem is indeed a bad connection then the answer is no. If it's a bad cell the answer is also no. If it's a cell that needs a little more balancing than the other cells then the answer is probably.

 

[DaniSe]

I'm leaning towards a bad cell. Replacing a single cell, instead of the entire battery, requires a top balance. Do you have the equipment to do that?

Not at this moment, but I am looking into the equipment. Do you have any recommendations?

Also, for emergency backup / secondary power supply I am thinking of getting a small 12v inverter for the car - do you know off the top of your head what the typical wattage requirement for battery chargers is when balancing out the individual cells? (I suppose 3.65v and drawing 4.35 amps as per Wills balancing video -> 15.66 watts?)

However . . . I wouldn't rule out poor connections. Half the voltage issues seen on this forum are due to bad connections. Have you verified that all the fasteners are tight? If you were feeling adventurous I would say to pull the cells out and rearrange them. Mark the cells before you do this so you know the original order. If the voltage issue stays in the same position then we know you either have a bad balance lead connection or perhaps the BMS is bad. If the voltage issue follows the cell then it's safe to say that the cell is the problem.

I have not checked connections within the battery yet, to not risk voiding warranty while SOK is stringing me along, but it is a fair point. If it is a bad connection and not a bad cell, with the one cell being so far our it would still need top balancing to rectify?

My usual recommendation is to NOT put LiFePO4 batteries in series. If you had a 24 volt battery with a single BMS would that have worked around this issue? If the source of the problem is indeed a bad connection then the answer is no. If it's a bad cell the answer is also no. If it's a cell that needs a little more balancing than the other cells then the answer is probably.

Lessons learned. Would having a balancer for the 24v battery bank such as this make it less of an issue if we stay with 24v? This is what SOK recommended

https://www.amazon.com/Victron-BBA000100100-Battery-Balancer/dp/B01DH6B2D2/ref=asc_df_B01DH6B2D2/?tag=hyprod-20&linkCode=df0&hvadid=198107824285&hvpos=&hvnetw=g&hvrand=15914609246718302639&hvpone=&hvptwo=&hvqmt=&hvdev=m&hvdvcmdl=&hvlocint=&hvlocphy=9002827&hvtargid=pla-351132180708&psc=1

Or should I start looking for a 12v inverter to keep future frustrations at bay...

 

[HRTKD]

The power supply that I used to top balance my cells is rated for 10 amps. There are quite a few others that have used the same brand/model. Here is a link to it.

DC Power Supply Variable 30V 10A, 4-Digital LED Display, Precision Adjustable Switching Regulated Multifunctional Power Supply Digital with USB Interface, Disply with Output Power Lab Grade: Amazon.com: Industrial & Scientific

DC Power Supply Variable 30V 10A, 4-Digital LED Display, Precision Adjustable Switching Regulated Multifunctional Power Supply Digital with USB Interface, Disply with Output Power Lab Grade: Amazon.com: Industrial & Scientific

www.amazon.com

I would still check the connections.

I have no experience with battery balancers. It seems like a band-aid to me. Others are using them. I've seen threads where members were using them and subsequently removed them.

 

[DaniSe]

Got in touch, finally, with an SOK tech yesterday and they instructed me to test the cells under charge, and both batteries' cells were reading even across the board.

What I did notice while constantly monitoring voltages while charging in series was that the battery I have been worried about (on the - lead from controller) would show higher voltages than the other (on the + lead) while charging, and then drop way lower on the cutout. - 14v / 13.5v + to - 12.5 / 13.3v + Is this due to the nature of how electrons move when charging and discharging or is that abnormal? After watching the incoming voltage on the - lead battery peak suddenly above 16 after a sudden series of cutouts I shut everything down and the resting voltages were 13.23 with cells 3.31 on the -lead battery and 13.27 with cells 3.32 on the +

The thing I really don't understand in all this is how at first we were able to being the bank voltage up to 28.8 and float at 27.2, we might get a disconnect while it was trying to make that switch but that was all. Now the disconnects are rampant and even in float we get them.

I see they are still out of balance with each other and will hook back into parallel to charge for now - but a friend is lending us two 105aH AGMs so if I pick up a charger for the LiFePO4s I could get them both to full individually.

Side note - advice on charging parameters for the AGMs? I have read that their charge current shouldn't be greater than 20% AH capacity, and that they really need to get full after cycling to maintain their health.

 

[acdoctor]

Flooded lead acid max charge rate is what you stated. I believe AGM requires less. When you get them if it doesn’t tell on them look it up. Gross over charging could cause failure. But they do need long absorb time every few days minimum.
As to the SOK issue the real solution and the quickest way to see if the cell is “bad” mark them then remove the cell arrange in parallel and top balance. Reassemble in a different order ensure all connections are solid. Charge the other battery to the same voltage and run again. If the same cell goes rogue then you will know.
But that is why you bought batteries and not build your own I assume so you wouldn’t have to do that sort of stuff.
Alternative method is to charge battery to just before high cell disconnect then charge the “ bad “ cell while still connected in the battery with a separate power source. That would catch it up.
Third option would be to install a 4 cell balancer to consume power from to 3 good cells while allowing the slower 1 to catch up. I don’t think this would ever work to satisfactory end.
You will need to play the warranty game or fix it your self.