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Theo[_3_]

wrote:
Hi all,

I have a few 18v Graco (same as DeWalt lithium ion) batteries for my paint
sprayer which seem to charge well but then die very quickly. Once charged
they show just over 20v on the multi meter. I have been doing some
searching and come across people saying
1. Zap them with 36v - although this seems to be if they are not charging at all
2. Slowly drain them until tool doesn't work any more. Leave them and drain again. Repeat until completely dead and then charge.

Neither.

Charge them up, then take them apart and measure the voltages across each
cell as the battery is discharged (the car bulb suggestion is a good one).
You'll probably find one or more of the cells drops rapidly, which causes
the whole pack to cut out. A healthy Li-ion cell should be 4.2V when fully
charged, dropping to about 2.5V when empty (and the protection circuit
should cut in).

You can try replacing the dead 18650 cells (requires a tab welder or cells
with pre-welded tabs - or using a pack for donor cells). But it might be
easier to buy a new battery, which will last longer.

Theo