LiFePO4 cell to cell voltage difference allowable

[Gregte]

I just checked the individual cell voltages on my LiFePo4 battery (36v 16Ah). It has 40 cells and is comprised of paralleled groups or 4 cells per group connected in series.

The ten 4 cell paralleled groups are all welded so it is not possible to check the individual cells in a group of 4, just the voltage of the group as a unit.

I let the entire battery set for over an hour after having fully charged it then did the measurements.

The voltage levels I measured were:
4.18
4.20
4.19
4.18
3.95
4.1
4.2
4.2
4.2
4.1

The battery only has a few charge/discharge cycles, less than 5 or 6 I think and only one of them is more than a 90% discharged cycle.

Is the one that measured 3.95 bad or is this to be considered within allowable spec.?
TIA

 

[recumbent]

You should manually charge the lowest cell to meet the other cells, otherwise it will trip the LVC too early.

Your numbers seem high, 3.65 volts is the correct voltage limit for LIFEPO4 cells.
are you sure you posted the right chemistry?

 

[Takemehome]

I could be wrong, I'm no expert, but these are not LiFePO4.
LiFePO4 comes in 12S for 36V. packs. 3.2V. nominal and 3.7V. fully charged
Li-Mn comes in 10S for 36V. packs. 3.7V. nominal and 4.2V. fully charged
You could charge your low 3.95 with a 4.2V. charger to balance the pack.
Once balanced ( if Li-Mn ) it stays balanced.

Edit: I'm a slow typist, recumbent had the same view.

 

[Gregte]

I thank you both for the info. I though that I had bought a LiFePO4 battery from Golden Motor. The advertising on the website certainly implies it. But in light of what you have told me I think you must be correct, that it is a lithium manganese battery.

Tell me what you think the GM website is advertising and selling. It refers to it as "Lithium Iron Battery Pack with BMS/Charger". The word "Iron" is used, not Manganese.

Its model number is "LFP-3616S 36V/16AH". The LFP sure implies Lithium Iron Phosphate.

 

[Gregte]

You could charge your low 3.95 with a 4.2V. charger to balance the pack.
Once balanced ( if Li-Mn ) it stays balanced.
.

I have just be informed by the seller that it is indeed Manganese, not Iron.

I appreciate you guys straightening me out. I have a lot yet to learn about ebikes and related technologies.

I intend to charge the single cell up to 4.2v as you have suggested.

You say that it will then stay balanced. Does this mean that it does not require any BMS circuitry to keep the cells balanced?

 

[dnmun]

since it is 4P, that is suspect to have the one group so far off.

try using a wall wart or a cellphone charger to get it to 4.2V watch it like a hawk though, no need to overcharge it.

does the low group continue sinking and the others remain flat?

 

[liveforphysics]

LiMnCo will tolerate being charged to 4.2, or even 5v, however it greatly shortens the battery life. 4.1v/cell is the correct voltage for LiMnCo, and 4.2v/cell is the proper voltage for LiCo.

Do whatever it takes to get the pack in balance, then run it through a few cycles to break-in the cells, then re-balance if needed.

It's never a bad idea to re-balance the cells whenever you find more than 0.05v/cell difference, however, if you are only doing shallow discharge cycles, it shouldn't cause any risk.

Best Wishes,
-Luke

 

[Gregte]

Again thank you guys for the very useful info.

I just decided to go about the business of bringing up the voltage of the one low cell group ( it has been a few hours resting time now) and lo and behold, all 10 groups measured either 4.18 or 4.19 on my meter! That seems like I better leave it alone for sure. My meter might even read high for all I know so I will assume they are all likely to be correct.

I have learned that I can go 40 miles before my 3-LED fuel gage quits lighting the "full" LED. Most days I will not ride 40 miles, more like 20, so I probably will not deep-cycle it very often.

I still cannot get over how much fun this thing is. For the last 6 years I have been riding a plain bicycle. For 3 years prior to that I rode a moped. This ebike beats them both for fun and after the initial cost, the ebike costs less per mile to ride than the same bike without the "e" since food calories cost a lot more than electrical watts. I am already underweight so I don't need a great deal of workout, but do need some.

Are there any advantages of Lithium Manganese over Lithium Iron Phosphate? Cost, shelf life, charge cycles, safety, ease of maintenance etc.

Again, thanks to all of you for your advice. It is quite appreciated.

Gregte

 

[liveforphysics]

LiMnCo is considdered to be slightly safer than LiFePO4. It has a signifigant increase in energy storage density. It has a higher nominal voltage. If used correctly, both battery types will support >1000 cycles.

It is a great battery chemistry.