What happens when a Li-ion is over-discharged?

I've heard the term used a lot, and know thats its not good if a li-ion overdischarges. What I dont know is what happens if it does. Does it explode? become unusable?

Thanks.

My understanding is that one of the odes (anode or cathode, don't remember which) actually starts to dissolve, and once it's dissolved, the cell is worthless, and you throw it away, and get another.

dendrils start forming

Hmmm, dendrils as well heh? /ubbthreads/images/graemlins/jpshakehead.gif

lol@bart i new ya was posting a gif.but i was thinking it was going to be a bomb gift

I never ceize to amaze... /ubbthreads/images/graemlins/tongue.gif



OK, I'll go hide now. /ubbthreads/images/graemlins/crazy.gif

i like em lways make me smile

Hello Mags,

The anode starts to disslove. This introduces a new chemical into the cell (copper) that can cause small shorts during the next recharge cycle.

Here is more information.

Tom

there is nothing that i can add ,i would have to take some down that far, and well that just doesnt sound smart /ubbthreads/images/graemlins/smile.gif
there is nothing LEFT in them at that point, so it also has little advantage to the user to go below, the current is gone.

when they are discharged to the Speced Cutoff, at that point they are weak to any reverse current, so in a PACK in series, when they are that low, they could EASILY be reverse charged.
so not only do you have the low voltage thing , that i aint about to test for. but you have the VERY LIKELY reverse blast from the others in the pack. and that will be really bad for them.
that is why i say in Series this low discharge could go bad fast, ya gotta remember that the other batteries in series might still be Pushing hard, completly unaware that thier teammate has fallen off the cliff /ubbthreads/images/graemlins/smile.gif /ubbthreads/images/graemlins/faint.gif

in the case of ni-mhys i can drop single cells well below .9V (slowly) and it doesnt kill them, but when that happens in a PACK, and one is way lower, the reverse charge degrades them very quickly.

so just to point out that , most of this stuff ends up IN packs, and in packs this stuff is WAY more critical to the operation and long life of the things.

[ QUOTE ]
SilverFox said:
Hello Mags,

The anode starts to disslove. This introduces a new chemical into the cell (copper) that can cause small shorts during the next recharge cycle.

Here is more information.

Tom

[/ QUOTE ]

Hmm interesting.

So with a 'dumb' charger, it's almost as if you inserted a copper rod in between the terminals, causing a dead short, which would explain what happened to one of mine a while back..

I'll spare you guys the gory details but recharging a dangerously overdischarged lithium ion is not a good idea.

Hello Kevin,

The thing that concerns me is that it appears that this effect is cumulative and the copper shunts are "available" for shorting whenever they migrate into place. This most often occurs during charging, but can also occur during discharging, or storing...

Tom

[ QUOTE ]
SilverFox said:
Hello Mags,

The anode starts to disslove. This introduces a new chemical into the cell (copper) that can cause small shorts during the next recharge cycle.

Here is more information.

Tom

[/ QUOTE ]
Tom:

In that link that you provided there is a statement that raising the LVC from 3.0V to 3.5V only causes a lose of 10mAh capacity. Look at any of your CBA graphs and tell me it's only 10mAh! /ubbthreads/images/graemlins/grin.gif Looks like around 300mAh to me. Where's this guy coming from? Different cells or what??

depends completly on the LOAD.
nasa recommendations, probably have enough money at nasa to not overdrive. wouldnt go to the moon with a 10 min runtime /ubbthreads/images/graemlins/smile.gif

that sheet was done in '95 and i wonder if anyone (other than sony and panasonic types) followed any of the recommendations of it to improve the cells?

always concider the source and the 'Type of use' that source would have.
when running leetel high capacity 123 or other tiny things hard (like 2c), things are going to change up fast.

if your running 2c then your going to get a lot faster voltage drop, long before the capacity is depleated, mabey a good time for a 2.4 low.
if your running .2c then the voltage is going to hang a lot better, then a 3.5 low is more appropriate.

what is the likelyhood that nasa is going to install with a 10X overrated load, on a cell that will last 50 cycles then.

Hello Bill,

I think VidPro hit the nail on the head. At low current draws, the difference would be minimal. At higher current draws, it could be quite significant.

They must have been referring to low current applications (less than 1C).

Tom

[ QUOTE ]
SilverFox said:
Hello Bill,

I think VidPro hit the nail on the head. At low current draws, the difference would be minimal. At higher current draws, it could be quite significant.

They must have been referring to low current applications (less than 1C).

Tom

[/ QUOTE ]
Tom:

The first graph I looked at was a RCR123 at 1A which was about 300mAh but a .5A load it was only about 60mAh difference. Thought I did some .01A tests but couldn't find one, so probably it would be lesser yet!

The basic care and feeding information for unprotected li ion cells is easy to remember: any substantial abuse will either render the cell useless or cause catastrophic failure.

Overcharge? Bang or vent.
Overdischarge? Bang.
Overtemperature? Bang or vent.
Freeze? Cell rupture.
Short circuit? Bang or vent.
Reverse charge? Bang.

The spurious cell problem has been solved since long, now. During a discharge - charge cycle, the lithium cobaltite (at the anode) passes from a crystalline state (charged) to an amorphous state (discharged), and it is unable to cycle back if the initial crystalline layer, which act as a "seed" to reconstitute the crystalline structure fully, is gone by over-discharge.

The bottom crystalline layer starts disappearing at 3.0 V, and is gone at 2.3 Volt. Li-Ion chargers starts with a trickle charge, trying to reconstitute the initial layer, when the discharged voltage is below 3.0 volt, starting the real charge at 3.2 - 3.3 Volts.

An overdischarged Li-Ion cell becomes essentially a paperweight. You can have an almost indefinite number of cycles if you set end-of-discharge at 3.2 Volt.

Hope this helps

Anthony