Got some used 18650s, help me identify the bad ones

I took apart an old Powerbook battery that was only holding 5min worth of charge. There were 8 Panasonic CGR18650A cells inside. I took some voltage readings of the cells but I don't remember what the state (charged or not) of the whole pack was when I took it apart.

Here are the voltages of the cells (descending):

1)4.06V
2)4.06V
3)4.05V
4)4.05V
5)4.00V
6)4.00V
7)3.00V
8)1.7V decreasing*

The cell that was 1.7V decreased to 1.0V in about ~50sec just with the DMM as a load. I measured it again after a few minutes and it was back up to ~1.7V but did the same thing and decreased. I assume this one is gone but would it be safe to charge the 3.00V one?

Hello Shaocaholica,

If they were mine, I would toss the 1.7 and the 3.0 volt ones and keep the rest.

Tom

Would the 3V one be safe to recharge?

Also, what should the voltage be of a fully charged 18650 cell?

I would do as SilverFox recommended.

I would not attempt to recharge the 3.0V cell.

Fully loaded voltage depends on the charger, but 4.2 is considered fully charged.

Will

Hello Shaocaholica,

You would probably be OK in charging the 3.0 Volt cell, but a resting voltage of 3.0 volts is a bit low, and there has probably been damage done to it. Li-Ion chemistry can be a bit volatile, so I would just toss it and use the ones that you know are good.

The end of charge voltage depends on how many cycles you want to get from your cells. If you charge to 4.0 volts, you could end up with 3000 cycles from your cells. On the other hand if you charge up to the maximum voltage (4.2 volts) you may only get 300 cycles. You have to decide what performance you want. Charging to the maximum voltage gives you more capacity, at the cost of cycle life.

Tom

I'd say toss them all. If it's an old battery pack then the cells are probably 1300mah cells. If the laptop only was holding 5 minutes of charge then all the cells have probably degraded enough that it's not worth it.

Mac

cmacclel said:

I'd say toss them all. If it's an old battery pack then the cells are probably 1300mah cells. If the laptop only was holding 5 minutes of charge then all the cells have probably degraded enough that it's not worth it.

Mac

Click to expand...

Im pretty sure they are 2000mah cells. I was also under the impression that a single bad cell can ruin a laptop pack causing similar effects like rediculously low runtimes.

Also, I have a DSD charger coming in the mail. What kind of voltage will the DSD charger stop at?

What did the battery pack case say for MAH?? The specs should be there for the cells capacity. The last old laptop battery I opened was 3900mah 3s3p pack so the cells where 1300mah each.

Mac

Shaocaholica;

The Panasonic CGR18650A cells are 2000mah.

I have used many of these cells in the past and think the 3.0 V cell will be just fine.

The best way to test them is to charge and check run time with a fixed resistance.

The DSD I have charges to about 4.15 Volts.

Larry Cobb

Cool. I'll give the 3.0V one a try and see how well it does.

With the DSD charger, do I need to charge cells in pairs? Will a damaged cell harm a good cell if they are paired(excluding explosions)?

cmacclel said:

I'd say toss them all. ... If the laptop only was holding 5 minutes of charge then all the cells have probably degraded

Click to expand...

It's worth testing the cells, IMO. But then I have a fair number of uses for
cells that can put out Li-ion style current even at a fraction of their original
total capacity...

I recently found a Saphion laptop pack in our recycle bin. It was a bit
interesting that ALL the cells read 0V (it was a 3P 4S combination, I think.)
I don't think I've ever taken apart ANY battery pack that had all the cells
at 0V. But they seem to be charging up to 4ish Volts given enough time
on the CCCV supply, so perhaps I'll get some useful experiments out of them.