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Old 02-08-2009, 05:05 PM
RappyMan is offline
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Why do LiPo batteries die at low voltage?

Hi All
Does anyone know the chemistry behind why LiPo batteries will be permanently damaged if the volts per cell go too low? I've been searching around for an explanation & haven't found one yet.


Thanks
Terry
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Old 02-08-2009, 06:17 PM
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Re: Why do LiPo batteries die at low voltage?

It's really not that the batteries would be permanently damaged, but the charging circuits are designed to cut off, for safety reasons, if the voltage drops below a certain level. Some chargers (usually the older ones) will allow you to force a trickly charge into the battery, and doing that for a few minutes will sometimes get the voltage up high enough that the LiPo circuitry will work properly.

The battery and charger manufacturers take a necessary and very conservative position with respect to the ugly possibilities of LiPo battery charging... rather than run even the limited risk of overcharging a bad cell and causing an explosion, they by default program the logic to say that if the voltage drops belowa certain level (3 volts per cell)... shut the charger off and say the battery is bad. If somebody decides to override that by forcing a trickle charge or something, I guess the charger folks don't want to be blamed.
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Old 02-08-2009, 07:03 PM
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Re: Why do LiPo batteries die at low voltage?

My understanding is that lithium ions will revert to a solid state (clump to eachother) when nearly all electrons have been removed from the system, ie. overdischarge. Lithium must remain in its ionic, unimolecular form so it can cross from the cathode to the anode within the battery.

This almost doesnt make sense because we think of two positively charged Li+ ions repelling eachother, but elemental lithium exists as a diatomic and thus it would rather be Li2 than Li+ (bad for us).............i'm rambling now, sorry!
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Old 02-08-2009, 08:18 PM
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Re: Why do LiPo batteries die at low voltage?

uh..... yeah, what he said.
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Old 02-08-2009, 08:54 PM
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Re: Why do LiPo batteries die at low voltage?

Quote:
Originally Posted by gmorales83 View Post
My understanding is that lithium ions will revert to a solid state (clump to eachother) when nearly all electrons have been removed from the system, ie. overdischarge. Lithium must remain in its ionic, unimolecular form so it can cross from the cathode to the anode within the battery.

This almost doesnt make sense because we think of two positively charged Li+ ions repelling eachother, but elemental lithium exists as a diatomic and thus it would rather be Li2 than Li+ (bad for us).............i'm rambling now, sorry!
There you go!! That just about sums it up.
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Old 02-08-2009, 09:51 PM
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Re: Why do LiPo batteries die at low voltage?

Impressive! I was asked this by a freind & had to say "Good question".... so now I can tell him the ions run out of charge to hold them in place & clump togeter... which kills the battery. No wonder the answer just didn't pop up with a Google search.


Thanks
Terry
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Old 02-08-2009, 10:35 PM
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Re: Why do LiPo batteries die at low voltage?

I guess all that clumping together is what causes the bulge in one end of the battery.... and I always thought it was the crash that caused it!
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