21700 batteries

[RetiredAutoTech]

I have been disassembling power tool batteries for a month or so now and charged and tested 250 of them. Almost all have had 21700's in them. The 20/60 volt ones usually have a few cells around 4 volts and a few below 1 volt. Today i had one cell volt test at 4.65 volts, one was 0 volts and the 13 others between 2.9 and 3.6. Is the 4.65 safe to discarge and use?

Second battery this morning was a 5 year old 9ah with 20700's in it. Pretty bad outer cover and when I opened it up the two lower rows had rust under the white rings and some surface rust. It seems the 20700s are thiner and easily damaged when removing the nickel strips. I'm guessing the proper thing to do is send it to the recycle bin and move on to the next one?

 

[OffGridInTheCity]

I have been disassembling power tool batteries for a month or so now and charged and tested 250 of them. Almost all have had 21700's in them. The 20/60 volt ones usually have a few cells around 4 volts and a few below 1 volt. Today i had one cell volt test at 4.65 volts, one was 0 volts and the 13 others between 2.9 and 3.6. Is the 4.65 safe to discarge and use?

It can be crazy. Last year, I pulled out an 18650 lithium-ion with negative voltage - couldn't believe it.

Second battery this morning was a 5 year old 9ah with 20700's in it. Pretty bad outer cover and when I opened it up the two lower rows had rust under the white rings and some surface rust. It seems the 20700s are thiner and easily damaged when removing the nickel strips. I'm guessing the proper thing to do is send it to the recycle bin and move on to the next one?

Some cells have thinner cans than others. In 18650 world, NCR18650As are that way. They're useable but you have to be extra careful processing them. For tabs, if you can't pull them off without damaging the cells - maybe cut around them and leave them attached?

 

[floydR]

I have used the twist method to remove spot welded nickel from a123 LiFePO4 26650 cells which are thinner than the average 18650 with sucess. What you do is with flush cut pliers you work under the nickel strip to the weld slowly twisting the weld back and forth till it comes loose. This may work.
later floyd

 

[paddy72]

Hmm, 4.65 V is very much over the top for a Li-ion cell and i am astonished that you found it with a voltage that high, sitting for some time already. In my experience if a cell is (slightly) overcharged, to say 4.3 or 4.4 V the voltage will drop rather quickly and in just a few hours the voltage will drop to around 4.2 or even lower. Did you charge the pack directly before measuring or did you find it like that somewhere?

 

[cak]

rust under the white rings and some surface rust

Never keep rusty cells as it is a disaster waiting to happen.

Is the 4.65 safe to discarge and use?

That is really high and means the BMS was not doing it's job properly. I would probably discharge ASAP and do some testing to see how they hold up with capacity and getting hot as they top off charging. If they haven't shorted already from that high charge they may be fine but it is also really hard on the internal cell structure and chemistry so hard to say what long term risks might be there from an overcharge like that. Any cells you can reuse are worth a pretty penny though I took apart some failed dewalt 9ah batteries and one had all 0v cells but everything else was in perfect condition but when I looked at the cost of purchasing new cells with similar specs to replace it would have cost at least half the cost of a new battery just for the cells. So now it sits and waits till I find other tool packs with bad bms but good cells. 21700 is were all the tool packs seem to be going which is great but it has still been hard for me to find cheap faulty packs to harvest from since they are relatively new compared to the 18650 versions that are everywhere. Tool packs are really hard on cells so usable rate has not been good in my experience but when they are good they are really high spec cells! Just wish "not working" packs on ebay weren't still selling for $2+/cell which adds up quick when it is a gamble what will be salvageable. So far I have just focused on the battery ecosystems I have tools for since then it is sort of worth it because of how expensive new batteries are but all the other brands of packs I have just sit waiting... I have DeWalt and Greenworks 80/82v(and all the lovely other brands that Greenworks makes that are electronically compatible)

 

[RetiredAutoTech]

Best that I can figure is someone tried to hot shot it with another battery to get it to start charging and got lucky and it didn't short out. I'll try and discharge it. Do a full cdc test also.

When I removed it from the tool battery the cell tested 0 volts and open on my ohm meter

 

[Overmind]

@cak what BMS? many mid to low brands dont't even use one.

The overcharged one can be discharged and cap-tested. If capacity is good, you can use it.