Advice on Lipo cells, have I killed them?

[mattym0310]

I came to charging my saddle pack lipos, and somehow they are completely flat. They have no charge in them at all. I think it is because I left the car plugged into them since last use, although it was left switched off Because they are flat, none of my chargers will charge the battery, so Is there anything I can do to start them up again to use this weekend?

thanks a lot

[BagofSkill]

Matt, even with the switch off most Electronics have a small drain. In RC cars it's stuff like keeping some of the FETs awake and these days the capacitors charged.
You have to face it that your batteries are probably damaged.
If any of the cells are 'puffed' it's time for disposal.
I've once before revived a Li-Po pack (from a aeroplane that got stuck in a tree for 4 days). I used a Ni-Mh charge profile to get enough voltage in (at reasonable current, somehing like 1A I think) just to give enough voltage that Li-Po profile could recognise the cells.

Please do this outside, don't leave it un-attended and keep a bucket of saltwater an an extinguisher at the ready! (insert other disclaimers here)

For what it's worth the cells were revived but were never as good for runtime or voltage ever again, and had a much shorter life thereafter than an identical pack that didn't get stuck in a tree.

HTH,
Chris.

[Southwell]

It's not worth the risk of charging up completely flat lipos. Write them off as a 'learning experience'

[mark christopher]

they can safely be revived if there not swollen, do as post two. Use nimh charge mode when they get over six volts take then off and lipo mode charge them, they definatly are not throw away items.

[Southwell]

O right, i was alwasy told that if they drop down that low then charging can make them swell too?

[DCM]

all my cars now run small fans, not for cooling, but as a reminder, that the cells are still connected, and need unplugging.

[peetbee]

Originally Posted by Southwell

O right, i was alwasy told that if they drop down that low then charging can make them swell too?

I've had a pack that went below 6v and didn't swell at the time, nor when charged but a few days later it did swell up.
It's possible that you may be lucky but be prepared to keep checking the pack whilst stored in case it does swell up.

[BagofSkill]

Li-Po's are dangerous, but so are Ni-Mh, we've all just become comlacent with the familiar cells.
There's a good chance at least one of the cells is damaged, but they're not cheap so it's worth trying a salvage them if you can. As said, if they're puffed, don't even risk it. If they're not puffed, it's worth a go- using reasonable and proper safety precautions. When Li-Po are damaged their cell chemistry can lead to small explosions and self fueling fires which will burn for a couple of mins even if deprived of oxygen. That's why charge pouches are necessary- to contain the fire until it's finished.
Take care and things will be ok whichever way it goes.

Li-Po cells have the cell materials in a aluminium satchel, as opposed to a steel can that we're used to. These pouches are vacuum sealed to keep the cell materials pressed against each other. When cells are abused, either by drawing too much current from them (that's where 'C' ratings come in) overcharging them, overdischarging them, or any other abuse, sometimes gas can be given off. This gas is retained in the pouch, but then the pouch is no longer in vacuum, it's pressured by the gas, causing it to 'puff'. Once the cells are puffed the internalplates have much more freedom to move around (they are sealed in by vacuum remember) and this makes inter short circuits much more likely, and a serious internal short is what leads to the fire.

So because of the concequences it's a MUST that after any cells have had abuse, you monitor them closely during charging/use, and also keep an eye on them in storage, and store them somewhere safe, life in a li-po sack in a steel box with a secured lid but with a breath hole, on a concreate slab or other heat resitant area.

[mattym0310]

thanks for all the help everyone, I have managed to revive them and they seem to be working great. I'll keep an eye on them though to be safe