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M. Guillot, I agree with Roxanne.
The better batteries have a microchip in them, to monitor the status of individual cells in the pack. Definitely not user-serviceable. Return to the supplier for testing and replacement.

 

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Ms. Guillot,
As a general rule, Ni-Cad batteries are good for about 300 charge cycles(completely flat to fully charged), provided one or more of the individual cells in the battery pack do not deteriorate more rapidly. It looks like you derived good longevity from your Bosch.
The Li-ion should do better than that, but as you say, it depends on the amount of charge in the battery when stored for long periods of disuse.
Have you considered converting your Bosch for use as a “corded” drill with a car battery? Open the battery case, remove all the cells (but take care to keep the contacts in the correct position - you may have to use an epoxy resin to do so) and drill a hole in the case for the two-core wire. Solder or otherwise attach the wire to the two contacts (there may be more than two - the others are for temperature regulation or monitoring), and re-assemble the case.
Put car battery terminals on the other end of the wires - remember to observe polarity - and you will be able to use the drill for many more years, even with a weak car battery. I used one for years, until it was stolen.
And it need not be a car battery either - a smaller, sealed 12V battery will do, and you can charge it with your car charger, or even asolar charger.

 

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I have tried that procedure with a NiCad battery, with limited success - it worked, but the battery capacity was significantly lower, and the effect only lasted for a couple of charges.
I used a car battery on a 12 volt battery pack. I also tried a circuit using mains electricity and a heavy duty rectifier and a current limiting resistor - cannot remember where I got the diagram. Same result as above.
I have read that the memory effect is due to the formation of dendrites (chemical filaments) in the battery electrolyte. Connecting up a charged battery theoretically “burns away” the dendrites. But after about 300 charge cycles, the individual cells deteriorate anyway. Time for a new battery.
I have not had the need, or the courage, to try it on a lithium battery. Flammability is said to be higher.
The question of why individual cells fail at different times, is a good one. Whenever I have had to replace a car battery only one of the lead-acid cells has been faulty. In the old days, it was possible to melt the compound holding the cells together, and replace an individual cell - they had metal connectors on top, connecting each cell in series with the next. But then at each service, the battery was drained, flushed and refilled. Now it is cheaper to replace the battery.