I have 2 neighbors that support themselves metal recycling and working on cars, lawn equipment and off road motor bikes. If they find a battery powered tool or batteries and chargers they try to fix them and use them in their trade. I have a tab welder with built-in variable voltage charger so I get the problem cases.
The 20v and 40v Ryobi One Plus lithium packs are popular here, so I get to practice repairing those a lot. There seems to be a common mode of failure that points to something simple I haven't figured out yet. Most of the packs will appear to work in the manufacturers charger, but only half of the cells inside are actually charging. The charger shuts off long before the pack is full or properly balanced. If I open the pack and manually charge the depleted cells the pack will come to full voltage and work again - for one charge cycle. Then the pack fails to charge the same way.
If I open a pack, and hold it with the contacts facing me, the center cell always fails first, then the next cell back towards the contacts, then the last one at the edge of the pack. The two on the far side and both still fully charged. 1-2-3-4-5 cells in a 20v pack. 3 fails first, then 2 then 1. The plugin contacts are above 2 facing 1. That's the pattern for at least half the packs we get.
I have an SMD handheld resistor/diode/cap tester, but I don't know what to look for and probably couldn't fix the smd board inside a battery pack. They look like they must have been assembled by industrial robots. The cells are series tabbed but one end is wired through a triac circuit that turns on when the charger energizes the third external contact. The only way to brute force charge the string easily would be to open the pack and tab a connector to opposite ends on the battery string. Then you would need a variable voltage regulated charger that would trickle charge the string until it was full.
Does anyone have an idea what is the original point of failure?