 Reviving overdischarged lithium phosphates
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Conundrum
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Thu Aug 04 2011, 07:35PM
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Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062
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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
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