1 out 4 LiFePo4 cells is failing? Next steps?

[dmitrym]

Hey folks,

Was recently greeted with the following. Cell 4 is misbehaving somehow, well on it's way to discharge where's the other 3 cells are still fine. I've got a good hobby charger that I'm going to try and use to balance charge everything but to me it looks like it's maybe on the way out. Any suggested next steps?

Thanks,

Dmitry

 

[Will Prowse]

What cells are you using? Could you share more information about your battery?

What are your capacity test results when you do a full cycle?

 

[Gazoo]

Any suggested next steps?

I think you have 105ah cells based on your previous post? Did you parrallel top balance? What is your delta when the cells are fully charged?

 

[dmitrym]

I think you have 105ah cells based on your previous post? Did you parrallel top balance? What is your delta when the cells are fully charged?

What cells are you using? Could you share more information about your battery?

What are your capacity test results when you do a full cycle?

That's right, I have 4 x 105Ah cells. I'm not sure of the brand.

When I got them, I hooked them up to my imax b6, balance charged them; discharged them and balance charged them one more time. The capacity was pretty close to 105Ah if I remember correctly. Since then I've been using the BMS that I was supplied with.

 

[MisterSandals]

balance charged them

This could mean many things.

Were the cells kept in series but charged with a charger that also balances? What was the top battery voltage and individual cell voltages? Did the cells settle (with no load) to some lower voltage evenly after this charge?

 

[Bob B]

Those cells are REALLY far out of balance .... You should continue with your plan to top balance the cells and see what you've got after that. Leave them in parallel for 24 hrs after getting them to 3.65V.
If you are sure you had a good top balance in the beginning, it is possible you have bad connections on the bus bars causing them to get unbalanced. Do a thorough cleaning of them an assemble with noalox or similar .... a search will find you information about that.

 

[dmitrym]

@MisterSandals it was balanced serially with imax b6, if I recall the cells were all charged to 3.65, but Im not 100% sure.

Those cells are REALLY far out of balance .... You should continue with your plan to top balance the cells and see what you've got after that. Leave them in parallel for 24 hrs after getting them to 3.65V.
If you are sure you had a good top balance in the beginning, it is possible you have bad connections on the bus bars causing them to get unbalanced. Do a thorough cleaning of them an assemble with noalox or similar .... a search will find you information about that.

Should I top balance them individually or as part of the pack (ie how I originally did them)?

The BMS has an independent balancing feature, which may have had a hand in this as well. I've disabled it now.

I'll look up bus bar cleaning.

Dmitry

 

[MisterSandals]

Should I top balance them individually or as part of the pack (ie how I originally did them)?

Okay this is the right way to do it as far as i am concerned.

I was confused by your use of "balance charge" when you did what folks here normally call a "top balance".

Was there an event that might have affected your top balance or did it drift out of balance over time?

I'll look up bus bar cleaning.

If you disconnect them, be sure to label them (I recommend using letters A, B, C...) and make note of where each of the cells were and what position in your battery they were in.

Then reconnect them in a different order. From A, B, C, D to something like B, D, A, C.

 

[Horsefly]

I was confused by your use of "balance charge" when you did what folks here normally call a "top balance".

I'm pretty sure he is talking about using a balance charger, like the iSDT Q8 or equivalent. These are chargers that also have balance leads, and they attempt to balance the cells in place (in the serial pack) while charging. My experience is that these would not be a good way to top balance. Better to disassemble the pack and put them all in parallel.

 

[dmitrym]

I'm pretty sure he is talking about using a balance charger, like the iSDT Q8 or equivalent. These are chargers that also have balance leads, and they attempt to balance the cells in place (in the serial pack) while charging. My experience is that these would not be a good way to top balance. Better to disassemble the pack and put them all in parallel.

That's exactly it. Can I just charge each cell individually then?

Was there an event that might have affected your top balance or did it drift out of balance over time?

I can't think of anything specific. The battery only has maybe 3-4 cycles as a whole. It's been sitting idle since about Christmas and occasional check showed that the BMS was slowly discharging it by continuously trying to balance it. Why the BMS was doing that I don't know. I guess it was detecting some delta in the cells?

 

[MisterSandals]

It's been sitting idle since about Christmas and occasional check showed that the BMS was slowly discharging it by continuously trying to balance it. Why the BMS was doing that I don't know. I guess it was detecting some delta in the cells?

Okay this makes sense! The BMS pulls power from somewhere and its one of the cells and in your case it must be cell #4.

What happens when you charge them up from here with your balance charger? If they behave at the top, you're probably good to go as is. If they're out of whack near the top, then this gradual drain on one cell might have undone any top balancing that you had.

 

[dmitrym]

Okay this makes sense! The BMS pulls power from somewhere and its one of the cells and in your case it must be cell #4.

What happens when you charge them up from here with your balance charger? If they behave at the top, you're probably good to go as is. If they're out of whack near the top, then this gradual drain on one cell might have undone any top balancing that you had.

I will charge the pack as is tonight and report. It might take a little a while since imax b6 only charges at 50 watts.

 

[Horsefly]

Like I said, to do a true top balance, you need to break up the pack and put them all in parallel. Then charge them at 3.65V (or 3.6V, or 3.55V if you wish), and continue until the current drops to near zero (maybe 1A in your case).

A balance charger will try to get all the cells to the voltage you have set, but most balance chargers will simply shut down once they get to that voltage. It is highly likely that your cell #4 is at a lower SoC than the others at whatever top voltage you have set. Until and unless you fully charge all four cells - as indicated by the current dropping to near zero - you will not be top balanced.

Balance chargers (also known as hobby chargers) are really not intended for high Ah LiFePO4 cells.

 

[Bob B]

@MisterSandals it was balanced serially with imax b6, if I recall the cells were all charged to 3.65, but Im not 100% sure.



Should I top balance them individually or as part of the pack (ie how I originally did them)?

The BMS has an independent balancing feature, which may have had a hand in this as well. I've disabled it now.

I'll look up bus bar cleaning.

Dmitry

I think MisterSandals and Horsefly are both giving you good advice .... so I will just 2nd their input.

 

[dmitrym]

Got it. I'll disassemble the pack, reassemble it in parallel and charge it.

 

[dmitrym]

Wow, this took way longer than expected (I was away for 2 weeks as well).

After 2 weeks of top charge, current stats:

- Cell 1: 3.42
- Cell 2: 3.41
- Cell 3: 3.41
- Cell 4: 3.42

Charged with the LifePo4 profile on SkyRC mini B6, in series, to a 3.6V. Is this sufficient or should the cells be charged to "true" 3.6V?

 

[Horsefly]

I can't tell for sure, but it sounds like you didn't put them in parallel, and so you didn't charge them in parallel until the current dropped down to almost nothing. You are still using a hobby charger with the cells in series. Your last post seemed to indicate that you understood what we all were saying, but maybe you don't.

Read this: https://diysolarforum.com/resources...ls-using-a-low-cost-benchtop-power-supply.65/

How were the cells configured and on what charger for those two weeks?

 

[dmitrym]

I can't tell for sure, but it sounds like you didn't put them in parallel, and so you didn't charge them in parallel until the current dropped down to almost nothing. You are still using a hobby charger with the cells in series. Your last post seemed to indicate that you understood what we all were saying, but maybe you don't.

Read this: https://diysolarforum.com/resources...ls-using-a-low-cost-benchtop-power-supply.65/

How were the cells configured and on what charger for those two weeks?

@Horsefly I definitely reconfigured the pack to be in parallel. I use a LifePo4 profile, for 1 cell *not* a 4S balance charge.




I read through the PDF during the charging process. I followed it (and instructions above) other than using the hobby charger (it is the only charger I currently have).

Charging at as a single cell with the pack in parallel is this equivalent to a "bench top power supply"?

 

[Horsefly]

Ok then. This is what had me confused:

Charged with the LifePo4 profile on SkyRC mini B6, in series, to a 3.6V. Is this sufficient or should the cells be charged to "true" 3.6V?

You specifically said "in series" which is not "in parallel" so I believed you.

Did you leave the four cells charging in parallel until the current dropped down to about 1A?

 

[dmitrym]

Ok then. This is what had me confused:

You specifically said "in series" which is not "in parallel" so I believed you.

Apologies, it was parallel.

Did you leave the four cells charging in parallel until the current dropped down to about 1A?

It was barely above 1A when the B6 stopped charging.