Resuscitating new Vtc6a from complete discharge

Batteries, Chargers, and Battery Management Systems.
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electric_nz   10 mW

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Resuscitating new Vtc6a from complete discharge

Post by electric_nz » Nov 29 2019 3:11pm

So I lucked out and bought a bulk lot of scrapped battery packs containing US21700VTC6A for approx $2 per cell. They are date code 2019 and cells measure consistently on each pack, suspect the battery packs were scrapped due to faulty BMS rather than cells.

Most of the packs measure around 3-4V per cell, with several packs around the 1V range and one pack that is dead flat. Like 0.04V per cell. Maybe the BMS ran it flat?

The data sheet for the 18650 version says that cells below 1V should not be recharged, however I’m wondering if anyone has any experience with dead flat sony cells in particular?

I’m thinking of slowly recharging the dead flat ones with a 1A charger and just seeing if it doesn’t heat or self discharge- if they come back up then Carefuly use them. Use case is a 1-2C ebike pack (batteries are apparently rated 30A!)

Any suggestions welcome.
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john61ct   100 MW

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Re: Resuscitating new Vtc6a from complete discharge

Post by john61ct » Nov 29 2019 3:57pm

Segregate the actual good ones for important use case.

Really should just recycle those gone near flat, but if not, treat as a science experiment, far from human habitation.

If you can get them to hold voltage, then do proper load tests. If over 80% SoH, maybe use for unimportant, low c-rate usage, or honestly sell on for $1 each or something.

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amberwolf   100 GW

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Re: Resuscitating new Vtc6a from complete discharge

Post by amberwolf » Nov 30 2019 2:16am

I'd strongly recommend following that spec sheet warning, at minimum.

Most of the cells I've seen spec sheets for call the minimum voltage for discharge to be 2.8v or so, but they dont' usually explain the reasons. If the reason happens to be that damage can occur inside the cell below that point, then while you may be able to recover cells that went below 2.8v, I would consider them unsafe to use, as it's possible for physical damage to the cell internals to have occured that makes them a fire risk.

If the reason is just because there's no energy left at that point, then they'd still be safe to use down to whatever the actual unsafe point is.

Unfortunatley you cannot know if such damage has occured, unless / until a failure occurs that shows you it was there. Little late to find out at that point.


YOu'll also find that some people will call it 2.0v for that unsafe point, and others will say that it's always safe.


I prefer the "abundance of caution" approach, given the potential for destruction and death that results when something does dramatically go wrong.

leffex   100 W

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Re: Resuscitating new Vtc6a from complete discharge

Post by leffex » Nov 30 2019 11:09am

electric_nz wrote:
Nov 29 2019 3:11pm
...
Most of the packs measure around 3-4V per cell, with several packs around the 1V range and one pack that is dead flat. Like 0.04V per cell. Maybe the BMS ran it flat?
...
Any suggestions welcome.
The 3-4 volt ones are OK, good.

But do really let go of the bad cells or use them as "learning" for soldering or putting packs together. Recycle or leave them at dangerous goods service department or other.
Don't try to charge them as 0.04v per cell means they are broken or shorted (inside fuse I guess, if reset it won't blow again)
Don't use them for home / car or ebike use as it is better to be safe than sorry with known good or tested cells than crappy shit that you just experiment and blindly believe in.

The BMS functionality is to usually shutdown charge/discharge if one cell group is outside the voltage range. In this case 0,04v that is way off. Also if one group is lower than the other groups for example 3v instead of 4,15v then the BMS might cut power during high discharge as one group goes below the lowest set allowable voltage or total voltage programmed in the bms.
Salestech in an ebike store

999zip999   100 GW

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Re: Resuscitating new Vtc6a from complete discharge

Post by 999zip999 » Nov 30 2019 1:52pm

I bought use high grade makia packs that where open up and recharged and sold as 3.0 v
and above 350 cells. Yes they came that way but we're just juiced up. Money does weird things to people. I can tell as he took the white plug out of most. He was looking for the newer pack with split tab welded. Funny thing he got the packs for free.
So as I took them apart things started to show them selfs some would just shoot fire directly out of the top. I got rid of the bad ones ? I left some bad ones in the back of the garage. Moved for recycling and that night the box caught fire put it in the garage on fire. Do not try to recharge under 3.0v recycle the rest or test charge 2.8v be and store in BBQ. Yes they are a great cell if good. Good luck be careful. Fire no good.

HK12K   100 W

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Re: Resuscitating new Vtc6a from complete discharge

Post by HK12K » Nov 30 2019 3:03pm

Very good cells if in good health.

You knew enough to check the spec sheet and note the exceedingly generous 1V bottom limit. You know how to attempt to bring them back and do some testing to see how they fare. I'm pretty sure that you also know doing so is potentially dangerous and can result in a fireball at literally any time in the future if you go for it.

Could you attempt it and find yourself with that many more working cells? Absolutely. Is it worth the potential risks should things go awry during charging, testing, or even long after you've finished testing and called them good? Probably not.

Keep in mind too, if they're going to go into thermal runaway after suffering internal damage it'll more than likely happen either during charging or while being used while under load. Under load you might just watch your bike burn down and maybe some of the forest if that's where your riding. If while charging, well...

Lot of energy potential there, especially when in pack form, and when one in the pack goes thermal the whole pack goes with it.

electric_nz   10 mW

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Re: Resuscitating new Vtc6a from complete discharge

Post by electric_nz » Dec 01 2019 2:11am

Thanks for the responses, I think I’ll follow the spec sheet. I have easily enough cells measuring 3-4v for a 14s4p pack, and will keep any of the rest above 1V for spares. Any below 1V will be binned. A low risk maybe but high consequences as many have pointed out. Thanks

999zip999   100 GW

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Re: Resuscitating new Vtc6a from complete discharge

Post by 999zip999 » Dec 01 2019 11:34am

1v ? As low I would temp is 2.8v that's empty. 3.0v is more safe charge to 4.2v let rest and see if they hold voltage

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