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Lifepo4 voltage and capacity question

JoeyJibJab

New Member
Joined
Nov 13, 2020
Messages
19
Location
Tasmania, Australia
Hi guys. I just recently built my 2p8s 400ah 24v battery bank, I built it without doing an initial top balance because I wanted to see how it went before forking out the money for a power supply. All the cells were at 3.31-3.32v before assembling. Everything looked okay and the BMS read all voltage had very little cell differential. That is until I put a load on the pack.

If I pull a 1500w load (50ish amps), the cell drift can be quite extreme. The worst cell for example can read 3.31v, and during a 1500w load can drop to 3.1v. These cells were advertised as grade A but obviously aren't. I'm hoping that the large cell drift is somewhat due to me not doing an initial top balance and that some of the cells charge levels differ quite a bit, which might account for the large cell drift. But was wondering, if all the cells read 3.31-3.32v before a load is placed, can that still indicate that some of the cells capacitys differ?

Hope that makes sense ?
 
You can have cells with same Voltage but SOC can be different, you need to perform top balancing.
 
Okay cool, have purchased a power supply so will get onto it. Do you think that could account for the large cell drift when a load is placed on the battery?
 
When I see cell drift under load, I start thinking bad connections.

The standard answer is to disassemble, mark cells so you know which ones were dropping under load, clean bus bars and cell pad, apply thin coat of noalox ..... re-assemble with the cells in a different position .... re-test and see what you've got.
 
When I see cell drift under load, I start thinking bad connections.

The standard answer is to disassemble, mark cells so you know which ones were dropping under load, clean bus bars and cell pad, apply thin coat of noalox ..... re-assemble with the cells in a different position .... re-test and see what you've got.
Yes been meaning to do this, especially as I'm made some bus bars and I didn't give them as good a polish up as I could have. In saying that though, if I read the cell voltages directly on the battery terminals (under the bus bars), the cell readings still drift quite a bit. Does that mean it's not the bus bars and connections?
 
With a bad connections it's hard to know for sure .... the bad connection could be on one of the other cells.

If the problem is one or more weak cells, when you re-assemble them, the problem will stay with the cell .... not the cell position .... Oh, and move the cells around but try to keep the bus bars in the same position so only 1 thing is being changed.
 
Okay cool, have purchased a power supply so will get onto it. Do you think that could account for the large cell drift when a load is placed on the battery?
Get the balance right first. If too many wheels are turning it could be hard to know. Be methodical and deliberate with making good connections when you reassemble the cells.
 
BTW, when you get the power supply, make sure to use 10Awg wires and good terminals, not the one supplied with the power supply.
 
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