Very odd lifepo4 behaviour

[James28]

Hi i have been building this battery over the last few weeks. Its a 12s 36v 15ah lifepo4 and so far its been testing perfecly fine. I have been getting 560wh at a 10a load.

The bms is connected up properly now this is just an older photo. The over charge and balancing is working well.

I went to charge it last night and it wouldn't charge (through the bms), so i tested the voltage across the sets of 3 which should give me 9.6v each and they did apart from one. One set of three was only giving 6.4 which would indicate a cell at 0v.

By tapping the offending cell it would come back up to 3.2 volts again and the pack will charge and operate as normal and by tapping it or moving it, the voltage will gradually drop to 0v after a minute or so.
There is no sign of energy being used when this happens, no heat, sparks or any reaction of any sort.

The connections between the batteries are solid and i would expect to see open loop if something had been disconnected but my meter has not shown open loop in any case.

I know i will likely have to replace the cell, it would just be nice to hear other peiples thoughts on whats actually going on here.

Thanks

 

[matthewslack]

View attachment 46418

Hi i have been building this battery over the last few weeks. Its a 12s 36v 15ah lifepo4 and so far its been testing perfecly fine. I have been getting 560wh at a 10a load.

The bms is connected up properly now this is just an older photo. The over charge and balancing is working well.

I went to charge it last night and it wouldn't charge (through the bms), so i tested the voltage across the sets of 3 which should give me 9.6v each and they did apart from one. One set of three was only giving 6.4 which would indicate a cell at 0v.

By tapping the offending cell it would come back up to 3.2 volts again and the pack will charge and operate as normal and by tapping it or moving it, the voltage will gradually drop to 0v after a minute or so.
There is no sign of energy being used when this happens, no heat, sparks or any reaction of any sort.

The connections between the batteries are solid and i would expect to see open loop if something had been disconnected but my meter has not shown open loop in any case.

I know i will likely have to replace the cell, it would just be nice to hear other peiples thoughts on whats actually going on here.

Thanks

Are the cell to cell connections soldered, or by mechanical 'squeezed together' means?

 

[James28]

They are soldered with 10x0.5 copper plate, the terminals are copper aswell.

 

[matthewslack]

They are soldered with 10x0.5 copper plate, the terminals are copper aswell.

They are soldered with 10x0.5 copper plate, the terminals are copper aswell.

Spot weld or soldering iron? I can only think that perhaps prolonged exposure to heat during soldering has damaged a cell, unless there is something external to the cell not clearly visible.

Do you see the same voltage changing behaviour with the bms unplugged?

 

[Nealh]

I would also say that one of the copper series connections if giving a false reading via an intermittent contact.

 

[James28]

Spot weld or soldering iron? I can only think that perhaps prolonged exposure to heat during soldering has damaged a cell, unless there is something external to the cell not clearly visible.

Do you see the same voltage changing behaviour with the bms unplugged?

I see the same behaviour with and without the bms. Damage from the heat is possible, i was very careful about soldering these. I was able to fit a heatsink to the termianl while soldering and it was done very quickly, less than a second. I also used compressed air to help keep them cool. I can accept that my precautions weren't enough to protect the cell. I would just like to under stand the wierd behaviour of the voltages.

 

[James28]

I would also say that one of the copper series connections if giving a false reading via an intermittent contact.

This was my thought but i can get probes to touch directly on the ends of the battery bypassing any external connections to it.

Surely an intermittent contact would give open loop at times of disconnect rather than the intermittent 0v that i am getting?

I am always able to get continuity across the cell.

 

[matthewslack]

I see the same behaviour with and without the bms. Damage from the heat is possible, i was very careful about soldering these. I was able to fit a heatsink to the termianl while soldering and it was done very quickly, less than a second. I also used compressed air to help keep them cool. I can accept that my precautions weren't enough to protect the cell. I would just like to under stand the wierd behaviour of the voltages.

This might be one of those that doesn't easy get explained - at least not without opening up the cell, which is on my personal 'do not ever do this' list.

I think it is necessary to remove the cell, and then see if it behaves the same in isolation. If so, damaged cell. If not, need to look in detail at the remainder.

Perplexing, for sure.

 

[James28]

This might be one of those that doesn't easy get explained - at least not without opening up the cell, which is on my personal 'do not ever do this' list.

I think it is necessary to remove the cell, and then see if it behaves the same in isolation. If so, damaged cell. If not, need to look in detail at the remainder.

Perplexing, for sure.

That seems like the only way forward from here is to investigate further. Its a tad annoying because of how 'buried' in to the pack it is. I did design this so the top 6 can hinge open from the bottom 6 cells.

Im going to keep the cell in untill this weekend when the rest of the parts for my bike come so i can at least finally try and get the motor to turn and test my other components.

When i've replaced the cell (if required) if its any value to anyone on here i will test it to failure to see what happens when its properly abused.

The whole point of this battery was saftey, longevity, and i like the flat discharge lifepo4 delivers. I'm very happy with 550wh with a cell weight of 3.3kg and can deliver 15 amps without warming up at all. So i shall persevere.