I got 2 40v Ryobi batteries that were 10 years old for salvage. The riser tab labelled + is the true positive end of the pack, but the riser labelled - is not. The true negative end of the pack is on the circuit board. If you solder a wire to that spot on the board and run it out of the case you can force charge the pack without opening the case again. You can't use a Ryobi charger to do that, but for about $20 there are 42v power supplies that are simply off or on with no logic check.
The force charged battery did not work reliably in my tool. I checked the + and - riser pins and saw only 16v. If I grounded the true battery - to the heat sink (which is the true board ground) there was a brief mild spark and then the + and - risers showed 1/2 volt less than the true pack voltage.
1/2v is the common drop for a diode. Somewhere on the board is a failed diode. A 40v pack is two 20v packs tagged in series inside one case.
I added another wire behind the heat sink and ran it out of the pack just like the true battery ground. I put quick connects on each external end of the wires and when I want to use the tool, I connect the 2 wires so the tool sees the correct voltage. I don't have to open the pack anymore to either charge or use the pack.
When charging or storaging the two external wires are unplugged from each other. There is a 0.2ma parasitic drain otherwise.
