LiFEPo4 pack - one cell different - high & low - help?

[Deepkimchi]

Got something strange with my 36V pack. One cell gets lower than the rest when I get to cutoff level - all were at 2.9V except one, which is at 2.75V

When i've been charging for a while (but not finished), all are at 3.37V except the same one, which is at 3.6V.

The one that discharges the fastest is the same one that charges the fastest.

Should I be worried??

 

[safe]

That's to be expected I think... cells always have slightly differing capacities, so when you cut off the pack because of the weakest cell then you simply lose the extra capacity.

It's a bummer, but that's the limitation of LVC and balancing...

"A pack is only as strong as it's weakest cell."

If you can identify the cell and remove and replace it then you could bring the whole pack up...

 

[fechter]

I'd expect that kind of behavior from the weakest cell. If its capacity is less than the others, it will discharge first, and finish charging first.

Is your total capacity dropping?

 

[Deepkimchi]

12 ah pack pooped out at 11.31 ah. That's after 27 km - 14 miles.

I'll keep an eye on it but I rarely use that much. Rode to the base, all over the base a few times, then back home (plus up and down the main drag).

DK

 

[slayer]

I killed a çell from a123 that behaved like yours but me it was some used a123 and 4 in series taking those that were the weakest.... one was dropping 2.5 volts after a couple charge discharge with 12 volts car halogen 110 wats ... i noticed that one cell going down further and up moreand stopped charging earlier until the last time it went over 4.2 volts while the others were much lower (i don t remember exactly) but with just a voltmeter cheking the overall voltage i just caught at the end that cell was going to be overvoltage and by the end voltage was rising very quick after that it never recovered ....it is now 0 volt when i put a charge it never goes more than 0.5 volts

 

[slayer]

what i mean to say is lifepo4 says the parameter not more than 4.2 volts and not less than 2.0 volts for a123 they mean it... so stay within those specs for each cells

 

[Ypedal]

Do you have any single cell chargers ? ( not sure . i can't remember if you ever got any or not ? )

These cells range from 3.6v (3.9v- 4v max on float charge ) down to 2v. but most of the discharge happens above 3.0v at 1C.

The 2.75v should be ok .. but i'd suggest charging this cell up to it's cousins level if you ever plan to drain it past 75 % DOD..

I"m cycling my packs as i type this, had the Red-Hornet out last night for a test ride.. damn that thing is some fast at 48v.. woooo.. but the controller sucks..

Anyways.. The single cell chargers i use show at least 1 cell per pack is taking over an extra hour to meet the others level.. this equates to a 2 ah unballance from just sitting for a few months.

 

[Ypedal]

Ok.. i re-read that and i missed an important part there..

It finishes charging before the others.. = capacity issue.. as Fechter pointed out. :|

Has the pack been used regularly ? or has it been sitting for a while ?

 

[Deepkimchi]

I used it recently about 4 times - didn't use it during the winter. As I pointed out to Andy before, it was the same one that arrived with the battery post loose, and I had to tighten it

DK

 

[Ypedal]

When i rebuilt the packs i have now ( same cells as yours ) the lugs on all the cells wiggle around, once you tighten the nuts the battery posts are pulled up right and stay solid.

Idealy, there whould be 2 nuts, one to hold the posts tight, then the copper tabs.. then another nut to lock together. However, with the lenght of the posts, and the diameter of the round copper, there is no room for a 2nd nut.

I used 10 gauge copper and hammered it flat, then tightened the nuts and hot-glued them in place so they don't ever vibrate loose....

A pack is only as good as it's weakest cell !

 

[Ypedal]

Well.. seems like i have a similar problem with one cell so far.

Packs 1,2,3,4 ( 24v, 12ah each.. )

Pack 1 and 2 were made of the best looking cells, purely on esthetics, used together so in 48v 12ah config, roughly 12 shallow cycles no more than 50 % ( 2 times tested on the kitchen table and LBD load to 10ah.. ) when on the bikes with 40 amp controllers, they remain fairly cool in a backpack at 40 amps.. so that's good.

Pack 3, cell no.7 self discharges faster than the others by a fair amount, up to 2 hrs with a 2 amp charger ie: 4ah within a few weeks. but does fully charge

Pack 4, tonight i just plugged the 48v pack to the 16 single cell chargers and one charger went green within 10 minutes !... all other 15 chargers are still charging 2 hrs later. ( pack was last used last friday for 5.5 ah on the e-BMX ). Strangely, also cell no.7

I checked with a volt meter to make sure i had a good connection, cell was at 3.88v, swapped chargers and same thing ( same thing ie: cell no.7 is charged and charger that was showing green light on a different cell goes Red and charges as expected.. ie: chargers are fine )

So.. this cell no good ?

Will report back once i cycle the pack on the LBD again..