Reviving overdischarged lithium phosphates

[Teh Stork]

I found this topic on another forum I often browse.

This is what the user said:

Hi all.

Made some progress today:-

1) Soldered to these cells properly using 30W temperature regulated iron, solder flux and indium.
Didn't damage them at all, as heat contact was less than 2 seconds.

2) Turns out that contrary to popular belief they can in fact be recovered from severe overdischarge.
The parallel string of 6 I had were all selected as they had the same low voltage i.e. 1.4V
left them outside charging on solar regulator that clamped at 3.7V but they didn't recover.
Maximum reading on them was 2.27V after half a day charging.

So.. left them out in the rain which required drying them out in a low oven (facepalm)
Would you believe it, heating them to 60C for an hour then putting them back on charge did something
intriguing, in that they then recovered to 3.2V.
Estimated charge time six hours at 350mA max.

Have left them outside in a sealed plastic box "just in case" but they seemed to hold a fair amount of
useful current judging by their performance into a 10A load.
Dropped down to 3.12V then slowly recovered to 3.20V after about five minutes.

(Do not advise trying this at home, cooking batteries is VERY VERY dangerous!!)

Any ideas if the capacity loss seen here is reversible or are they fried?

Due to not monitoring the charging I am not sure if they ever went near 3.65V so they may have only received a partial charge...

My back of the envelope calculations suggest that it took about 1.69Ah worth of charging to get them from 2.27V to 2.40V and 0.41Ah to get from 2.40V to 3.3? V which is a factor of 4.12
Just shows that when you overdischarge them the energy in the cells is 4* their rated Ah capacity!!

-A

I was intrigued by this, what mechanics kicked into action here? Electrolyte repair of some sort? And, his 4*capacity seems to be out of wack?

 

[Kin]

It's unclear to me how cold the rain was. I don't know if this would account for enough of the large jump in voltages, but the chemical potential of battery chemistry reactions are dependent on concentration, temperature, ehmmm, and not sure what else. But raising the temperature will elevate the voltage, so that might have just been the bounce back.

My guess would be there was no benefit to cooking them....Probably would recover as well with a slow recharge. I can only see things going wrong when cooking batteries.

 

[neptronix]

A capacity ( charge and discharge ) test would determine whether this actually worked or not.

Too bad the original person did not perform the test.

 

[whatever]

60degreesC is not very hot for iron phosphates, you could probably use hot water bath to get that temperature.
I forget exact temp but one study on a123 cells below about 60degree was no damage to longevity/capacity when discharging, above that they worked out
loss in capacity/longevity according to max temp reached, I posted link to the paper long time ago.
Worth a try i think.

 

[whatever]

guy i know tried this ( after I told him about this post), he tried on iron phosphates with no success.

 

[Kin]

Oh. So. I suppose this may have happened:

Low Voltage on the battery, BMS would not allow charger to work.
Elevated temperature, boosted the voltage by nature of temperature.
Immediately stuck it on the charger, so it was above the minimal threshold,
Soaked up enough energy before cooling down that it was still above the threshold.

So it revived the voltage enough to get the BMS out of it's limited state. That could have also been done by bypassing the BMS.

Cool story though. Maybe my explanation is hogwash. I don't have and LiFePO4, and I don't know much about the BMS. It would make sense that they have a low voltage cutoff as typical to have saftey parameters that trigger when you're far outside of the expected range for operation. (I mean the LVC of the BMS, not the obvious LVC where the BMS cuts to protect battery but where it should still allow recharge).

 

[whatever]

I have some cobalt lithiums ( lilo) one of the packs had all zero voltage, they have been sitting for probably 3 years or more at zero voltage,
I charged them on rc charger as nimh forming charge then switch to lilo charge once volts came up enough, to my surprise about 80% of the cells actually charged up and holding charge, haven't measure their capacity but will do one day, but a big surprise that they could even come back to life after sitting for so long at 0v.
The cells that didn't hold charge got warm during lilo charge, so had to dial back the amps to stop bad cells overheating.