LiFePO4 dead cell recovery

[Dave_H]

A couple of solar lights which use LiFePo4 14500 cells had been sitting around in the box for a while. When removed one read close to 0v, other about 1.6v. I have a proper charger for these cells (Rydelec) which seemed to not want to charge them; if at all, at extremely slow rate. Charger worked fine on 18650 LiFePo4.

I resorted to using a variable power supply, monitoring current carefully, to get them back up to around 3v, at which point the charger worked on them (red light on). Both cells seem to have recovered.

Not sure what to expect as far as reduced capacity/lifetime...any comments?


Dave

 

[parametrek]

The only way to answer those questions is to measure the cells. It can't be easily predicted because so much matters on the duration.

Do you have an analyzing charger that could do a capacity test? With that you'll be able to see exactly how much capacity was lost. And by knowing the lost capacity you can estimate the remaining life.

1 experiment showed that discharging LiFePO4 to 0V and then immediately recharging them did the same amount of damage as 10 normal full cycles.

 

[john61ct]

Not predictable.

Going to truly empty can render LFP cells scrap.

 

[Dave_H]

No capacity-measuring chargers/testers here but running cells down at known load and measuring run-time will suffice. Both 400Ah cells recovered OK. The really "dead" one ran 12 hours in the small spotlight at 35mA so it's good.

Rundown to that level is likely small leakage in the light as cell came installed. Perhaps a slow over-discharge is less likely to be damaging. Self-discharge is in there too.

A solar rope light here uses same cell type and I recall it initially needed a boost (light itself might have done that but I wasn't sure). It's run OK every night for a couple of years.

I was just musing about the charger's apparent lack of ability to recover cells in this state.


Dave

 

[snakebite]

a quality charger will trickle them till they come up to a safe voltage.then charge at normal rate.
you might need to wait a while.
if its a cheap n nasty charger it probably wont.
set power supply to 3.2v and current limit to 50 ma.
that way if you forget it wont overcharge.
if no improvement in a few hours recycle it.

 

[TinderBox (UK)]

I picked out one of my small LiFePO4 and it was dead, totally out of energy, Since LiFePO4 cells should not vent with fire, I took the chance and recharged it and it got too hot to touch so i trashed it.

 

[Dave_H]

Any cell which gets hot should definitely go, but not the case here. Suggested current limiting bring-up worked, cells recovered. The charger didn't do the trickle thing, now I know.

Lucked out today...snagged two 2-packs of 18500's for under $7 with tax, at a second-hand store, packages unopened! Cells OK, sitting around 3.3v, no recovery needed. Rydelec charger has sliding contacts which take various cell lengths; another bargain, $4 from Salvation Army store.

18500's should work in small 3xAAA lights which are too short to take regular 18650, albeit at slightly lower voltage.

A few years ago OTC LiFePO4 cells used to be 18650 and 14500 mainly, now they are shorter, 18500 and 14430.

Dave