How much imbalance of Li-ion cells is needed to dramatically degrade a pack?

[Delta]

I've read recently about balancing Li-ion battery packs (or not) and how it affects capacity etc, and someone mentioned that re-balancing cells in a pack (that doesn't do it itself) can rejuvenate the pack.

The battery in my ancient Lenovo ThinkPad has recently gone really shit, so I opened it up.  I found a 2p3s configuration, and measured the voltages across each pair as
4.1v
3.95v
4.13v
So my question is; would this level of imbalance significantly affect the pack's capacity?

I've got a TP4056 charger module, so I'm tempted to charge each pair individually, then do a drawdown test to see how they perform.  Or is it more likely that a cell is completely kaput?

Cheers!

[KL27x]

First, take the pair of cells that measure 3.95V out of the circuit and measure them, individually. It is possible that one of that pair is failing and is the whole root of your problem. If this is the case, when you take them out of the circuit, they will have different float voltages. The one with the lower voltage would be the obviously bad cell, duh.

Then do w/e else you planned on doing.

[Delta]

First, take the pair of cells that measure 3.95V out of the circuit and measure them, individually. It is possible that one of that pair is failing and is the whole root of your problem. If this is the case, when you take them out of the circuit, they will have different float voltages. The one with the lower voltage would be the obviously bad cell, duh.

Then do w/e else you planned on doing.

Cheers for the info, I'll do just that tomorrow!

[KL27x]

Before you even do that, you might save some trouble with the soldering and repacking by testing them in pairs, in circuit.

The way I would do this is to set a pair at a time on a CV supply of 4.2V and connected to an ammeter. If you have a lab supply, just use that. If the current doesn't eventually trickle down 0mA to 1 significant digit, or at least under 10 mA, then at least one of the pair is bad. A bad 18650 cell will generally suck over 20mA, indefinitely, at 4.2V. Possibly a few hundred.

is it more likely that a cell is completely kaput?

This is highly likely the case. The weakest link is what kills the battery. Just a matter of what is the easiest way to verify that, for you, to do the simplest repair.

[Delta]

Cheers again, I wasn't looking forward to desoldering the cells, so I'll try that first. Should I charge each pair with the TP4056 first, before then putting 4.2v on them?

(Hmmmmm, maybe the TP4056 won't terminate the float charge if one of the cells is bad... kind of doing that test for me without me needing to bodge up a 4.2v supply)

[eas]

Why do you think that your ThinkPad pack doesn't balance charge?

That it is out of balance when you check it after charging seems another sign that it is in bad shape.

By all means, experiment with it, but things don't bode well.

[KL27x]

Cheers again, I wasn't looking forward to desoldering the cells, so I'll try that first. Should I charge each pair with the TP4056 first, before then putting 4.2v on them?

(Hmmmmm, maybe the TP4056 won't terminate the float charge if one of the cells is bad... kind of doing that test for me without me needing to bodge up a 4.2v supply)

Yeah, sure.

[djQUAN]

Cheers again, I wasn't looking forward to desoldering the cells, so I'll try that first. Should I charge each pair with the TP4056 first, before then putting 4.2v on them?

(Hmmmmm, maybe the TP4056 won't terminate the float charge if one of the cells is bad... kind of doing that test for me without me needing to bodge up a 4.2v supply)

The blue TP4056 boards available everywhere charge at 1A rate and terminate at about 100mA IIRC.

[amyk]

I've read recently about balancing Li-ion battery packs (or not) and how it affects capacity etc, and someone mentioned that re-balancing cells in a pack (that doesn't do it itself) can rejuvenate the pack.

The battery in my ancient Lenovo ThinkPad has recently gone really shit, so I opened it up.  I found a 2p3s configuration, and measured the voltages across each pair as
4.1v
3.95v
4.13v
So my question is; would this level of imbalance significantly affect the pack's capacity?

I've got a TP4056 charger module, so I'm tempted to charge each pair individually, then do a drawdown test to see how they perform.  Or is it more likely that a cell is completely kaput?

Cheers!

That 3.95V pair is definitely the culprit. Laptop packs usually do have balancing circuitry but the amount of bypass current they can pass is limited. Once the bottom pair reaches 4.2V the middle one may be only around 4.02V.