Over discharged my lipo pack how bad did I damage it?

[Offroader]

Not the biggest deal as I planed on a new build and these packs are coming up on 3 years old and have a new battery on the way to me, but anyway.

My 18s pack consists of 21 packs of Turnigy 6s 5000 MAH 20c.

I let the pack sit at around 3.65 to 3.70 volts with a very small parasitic load. The load consisted of a left on DC DC converter with no load.
I've been doing this since the summer without issue and it does drain the battery but very slowly.

I let the bike sit for a while in the ice cold garage and checking it only 5 days later my pack voltage was at around 3.1 volts per cell.

Some parallel groups of 7 packs had paralleled groups of cells as low as 1.8 volts. paralleled packs ranged from 1.8 volts to 3.xx volts.

I bulk charged for about 5 minutes at a low charge and all cells were back up already to around 3.5 volts. I bulk charged to 3.8 volts and all cells were all balanced within .02 (3.80 and 3.78) of each other, which is about normal as I hardly ever balance these cells.

I can't drive the bike to test it, but how bad did I screw up my pack by allowing this pack to very slowly drop to 3.1 volts average, with some packs falling as low as 1.8 volts? Did I possibly do less damage because of the winter and very slow discharge to get there?

 

[motomech]

Since the cells didn't stray all over the place when you charged them, I think it's likely they weren't harmed at all.

 

[dogman dan]

Above 2.8v per cell, no harm done. Unless they obviously puffed or something.

You did cut it down to the wire though.

 

[Drunkskunk]

1.8v basically destroys them. You may be able to get them to charge up again. they might even have most of their capacity still. It's doubtful they will have the same C rating, though. And they will probably have a much shorter cycle life. I'd suspect they will sag heavy under load. Discharging slowly to a low voltage is worse than discharging fast for Lithium.

 

[wesnewell]

Before being charged at the factory, what's the voltage of a newly assembled rc lipo pouch? 0V? They are generally charged for a 24-48 hour period the first time at a very low charge rate.

 

[Drunkskunk]

Before being charged at the factory, what's the voltage of a newly assembled rc lipo pouch? 0V? They are generally charged for a 24-48 hour period the first time at a very low charge rate.

That's not true. When you assemble a battery for the first time, it will have the voltage of it's fully charged state. The charge comes from the elements reacting to each other, which is how a battery works.

As for charging for 24-48 hours, that sounds highly doubtful, since charging lithium cells slowly causes dendrites to form between the layers.

 

[wesnewell]

Here's a short video of lipo manufacturing/
https://www.youtube.com/watch?v=IMOSletgaaY
And here's their first charge.
https://learn.sparkfun.com/tutorials/how-lithium-polymer-batteries-are-made#charging

 

[Offroader]

Thanks for the responses, I'll update this on how the pack is once the snow is melted and I can drive the bike again.

I estimate about 14 packs had voltage around 1.8 volts for cell 5.
The other cells ranged in the 2's and 3's.

I should have written down exactly what the voltages were before charging. I had 3 parallel groups of 7 packs in each group.

It is amazing how quickly these packs will fall off the cliff once you get around the 3.6 mark.

 

[dogman dan]

Yeah, I mis read again. I thought he wrote 2.8v. 1.8v is not good. Never bring inside again.

Below 3.5v, I never got further than about a half mile, and that is pedaling hard and using the battery only to cut down the cogging.

At 1.8v, did yours puff, or get all hot? I saw that big time with ones I took all the way to 0v, after a parallel connection came un plugged.

 

[Offroader]

My batteries are inside the bike metal case and a pain to get to. I don't think I'll bother pulling them to check, but I am also curious.

I'd be surprised if they got hot because the discharge was so slow.

I still got hope that they are OK. But I'll know because all my cells stayed used to stay in balance and I almost never balanced charge the pack. IF now the cell 5's start to give me trouble I know I screwed the pack up.

Most likely it will be only the cell 5's which I would have to replace as the other cells in the 6s packs stayed much higher.

 

[ecycler]

If it were me, I would pull them to see how puffy they get when fully charged.

 

[Offroader]

Just wanted to report back on my first ride with the batteries after I over discharged them.

There was no noticeable problems with the lipo packs. The cells discharged evenly, power seemed normal, range seemed normal also.

From what I've been seeing most people seem to not have issues with their lipo pack from failures like these. makes me think that lipo cells being produced these days are more robust then years ago and can take more abuse.

This is just my speculation after reading many people who over charged or over discharged their pack and all seemed fine with it over the last couple of years here. Where it seems that there are many that say the pack is ruined if you do this, which makes me believe that older lipo was easily damaged.

Could be also that I didn't over discharge them too low, didn't come close to 0 volts. I also had my packs connected in parallel which may have helped them and the temperature was close to freezing also. The discharge was also a very slow discharge.

 

[nutnspecial]

Here's a short video of lipo manufacturing/
https://www.youtube.com/watch?v=IMOSletgaaY
And here's their first charge.
https://learn.sparkfun.com/tutorials/how-lithium-polymer-batteries-are-made#charging

Good links wes, this was my impression as well. I have no idea what skunk's meaning was. . .

Offroad, glad your pack came back up ok. I had a handful drain rather low this past year myself. It's my impression a slow drain in no way harms them, it's charging them back up that 'cliff', so charge rate should be relational to their dod past the cliff. Lol, I think I brought mine back with .01% Crate to be safe. They seem fine, and I still treat them no different than the other cells, or the girls @ the bar- I protect myself cuz they all want to fock me.

 

[John in CR]

It sounds like you got lucky Offroader and just took them down to right at empty while cold. Had you drawn real current through those lowest voltage cells while riding I'm sure damage would have been done. Keep an extra eye on them, especially while charging. Personally I wouldn't charge them inside, because if an event occurs you'd have no time to move it outside...of course I always charge and park my bikes outside protected from the sun and weather anyway.