Troubleshooting New Cell Break-in and LVC possible damage?

[rob123]

I recently replaced one cell in my LifePo4 20 amp hour pack as one cell had died after about 2 years. All the other cells maintain charge and balance at 3.4 vols storage or 3.6 volts straight off charger. I have used the pack about 600 charge cycles I would estimate.
I placed the new cell in the pack and checked the individual voltage which was 3.3 upon removing from the packaging. I charged to full and let the BMS balance.
I did about 3 rides of 5k or so and recharged after. Then left the pack alone and went on holidays. I rode the pack against after about 3 weeks idle.
The voltage was 52V. As I rode, for about 5 k, and up some hills, it started to drop rather quickly to 47volts then the LVC came in and shut the pack down.
I went back and pulled the battery apart and checked the individual new cell. The rest all were at 3.3V while the new cell was 2.5 v which is what triggered the BMS to shut off to protect the cell.

The question is, did this damage the cell at all going to 2.5V which is the LVC of the BMS?
Is this typical behaviour of a new cell that is not broken in? or is this the behaviour or a bad and faulty new cell?
The cell had not swelled and when I individually charged it, it went up very quickly to 3V before slowing down a lot as it started to take charge upward from there.

I have since individually charged this cell back to 3 volts, then placed pack back together and put back onto charger to fully charge and balance with BMS. I intend to do at least 10 more small cycles or dis-recharge only using battery for 5 k or so each time and hopefully get a proper balanced cycle out of the new cell where it performs like the other 15 do which last at least 50km before dropping below 2.8V.

 

[kdog]

I can't really gather from your post some specifics, but I think what has happened is that you have put a discharged cell in a full pack. Cells are shipped at ~<40% charge. Lifepo has a very flat discharge curve so 3.3v could represent a large range of SOC. that bms will takes weeks on continual balance to fix this discrepancy.
I suggest you charge the pack fully. Let sit for a little while. Then measure the v on the new cell. If it is lower than the rest take a 5v power source ( or the likes) and charge that cell till its at the same voltage as the rest. You will have to monitor it carefully to prevent over charge. Once you've topped it off, then the bms should be able to cope.
Good luck- hopefully this is the issue!

 

[kdog]

... And, 2.5v is low, but OK for lifepo. It should be fine, but don't ride till LVC kicks in as a habit. That will kill them early.

 

[rob123]

Ok,
Update. The charger finished charging the pack. I measured the cells individually. The front 3 nearest the black wire on the BMS were 3.66Volts. The New cell right next to them was 3.4 V, and all the other cells the other side of it were 3.45 V with no variation.

I hope they will balance in time. I am not sure if it is ok to place the 60V charger onto the individual cell with a volt meter, and charge till 3.5 volts then take it off. I did this to make it reach 3.2volts before I charged the whole pack as a whole.

It is a 5amp charger for 48v batteries.

 

[rob123]

I guess my last question adding to the reply above, would be whether it is fine to charge the new cell up to 3.6 volts with my 60V 5amp charger (it will be at full power all the way) and then shut if off or would this be too high charge rate at the top and hurt the cell. (I would have to sit and watch it over about 30 min I would think) (I know well that it cannot be unattended as fire would be the result of forgetting it)

If this is not an option, I will buy a proper single cell lifepo4 charger. If it is ok, It would be a quick fix to get the pack balanced to then help the BMS keep it that way.

 

[rob123]

Last update: I ended up charging the new cell individually with the pack charger to 3.6 volts, then shut it off. It took at least an hour.

Now all the cells read 3.6 as I charged the other ones up too. Now I hope that they will stay balanced and the new cell stay in balance with the bms.

And hopefully no damage to longevity done.

If anyone wants to add any information here about these problems I had, please do.

 

[kdog]

I can't recommend charging a single cell with a 60v charger! -most chargers won't even turn on under a predefined voltage so I'm surprised yours did.
Get one of the million 5v adapters In existence and charge it with that. Watch it carefully though! You might also want to use a small light ( of about 12v) to drain down the first three. Another option would be to charge all but the first three with your charger, but realise that it will over charge if you don't watch it like a hawk.
Aim to get all cells to equal V. Then charge full pack in normal fashion, and top balance, using same method, or just leave the bms to do it.
Just observe the rule never above 3.65v, and charging slower will be easier to manage V. Don't wreck anything!

 

[kdog]

Seems you've sorted it, the bms should be able to cope now.

 

[rob123]

Thanks for the tips. My charger works well with single cells it seems and I successfully got it up to 3.52 V yesterday. Today I did a ride on it for about 10 ks and checked voltage. All cells were even at 3.4 Volts, but the new one was still less at 3.35. But I just put them all on the charger to charge up and balance more with the BMS.

I have been researching what you are talking about overvoltage. So far I have found no definite answers on how harmful it is. Basically most (including mine) BMS will allow a cell or cells to reach at least 3.65volts before shutting off. Especially when balancing. I have seen up to 3.7 at times. There is no way to stop this unless careful shutting off manually when pack reaches a certain voltage. Without the BMS I have charged the cells to 60volts which meant each had at least 3.8V in them but not over 4. This only happened a few times when I had the BMS disconnected and only for a short period of time.

At one time, I was charging the pack as a 36volt trying to up the voltage on a slow cell. I made the mistake of placing the volt meter on the slow cell. The others for some reason had very quickly shot up to 4.4volts evenly. The time they were at this would have been less than 10 min as I quickly shut off, and went out for a ride and they came quickly back to 3.6volts.

What I have been trying to research but with no conclusive facts: Does the normal BMS of taking cells above 3.6 but not over 3.7 full dramatically hurt battery longevity, or just a small amount? (providing they don't go over 3.7 often however go often to above 3.6)

If you have any input on this, I would really appreciate it. The reason I ask, is because some "experts" say that you can take a lifepo4 cell even up to 10volts without fire, or real damage to it. Are they really that robust? My battery pack cost me a pretty penny so I would LOVE to see it last at least 2000 cycles before replacing it. So far, it has lasted 600 cycles or so, regularly going down to around 2.9v per cell per cycle then going back to 3.6 on charger with BMS attached. I rarely do shallow cycles since work is far enough away as to use most of the pack (45ks up big hills of around 700mt)

 

[rob123]

And I have one more technical question about my BMS that came pre-wired from china:

It is a typical one, with -B, CH, and P

The battery negative is soldered to the B,
The out negative that goes to the controller is wired to CH. Between the controller and negative is the charging plug.
P does not have anything attached to it.

What difference is it to use this configuration vs having P as the wire to the controller, and CH only for charging?

when on a ride, the BMS will get warm even though not charging.

that will be enough from me. Batteries wires and controllers and my bike sometimes not powering up is doing my head in with trying to learn about it!