The initial problem
Everyone using battery tools may have been faced with the typical sad battery behavior bound to happen sooner or later. After a while the battery will reject to charge. This is very annoying and painful especially if the battery has not been used very frequently or is not very old. Additionally batteries normally are not covered by a warranty. They know best why to avoid providing warranty on batteries ...
The original charger rejected to charge - this was what happened to both of my batteries even if not used quite often, stored in a dry and well tempered area, charged to the advised level for occasional usage and not older than 2 years.
A conventional LiCoO2 battery cell provides a rated voltage of 3.6 volts, which is about three times as high as of a nickel-metal hydride rechargeable battery (NiMH rechargeable battery). The end-of-charge voltage is up to 4.3 volts. The end-of-discarge voltage is 2.5 volts. A deeper discharge leads to irreversible damage and capacity loss of the cell.
I have a four cells package which amounts to rated voltage of 4 times 3.6 volts equals 14.4 volts. End-of-charge voltage equals to 18.2 volts (4 times 4.3 volts). The limit for end-of-discharge voltage equals to 10 volts for the package.
So first, I have checked each cell in the battery pack for its current voltage. At least each cell responded even at a level below the end-of-discharge voltage.
Next I soldered a 5 pin balancer cable with standard connector (available for no money) to each cell port and connected it to my RC model lithium balancing charger "Turnigy Accu cell 6 80watts" (a great fully digital controlled charger with awesome fast charging capabilities and several programs . This enabled me to have full visible control on each of the four cells.
I tried to charge the cells with the LiPo program (lithium polymer batteries are just a derivate of a lithium ion battery) of the charger but didn't succeed. The charger signaled 'low voltage' and rejected to charge. That happened because the cell's current voltages were below the end-of-discharge voltage.
To force the cells to get charged I used the NiMh charging mode at a low charging current to avoid damage to the cells. This often is called jump starting the cells to get them out of dead lock voltage level.
When the cells reached the voltage level for the normal LiPo balanced charging I switched the charger to this program with the cycle option activated to get the cells refreshed.
All cells now can get almost charged (95% +) to their rated amount of 2000 mAh. That's awesome !!
Interesting thing is that the original charger still rejects to charge. I also have tried another original charger without success. Maybe the battery's onboard controller 'breaks' the batterie for safety reasons. But this is just a guess.
I have modified the battery package to permanently use my RC charger for charging with having full visible control of each cell of the pack. That's a great thing.
A closing word
I realize that this solution will not be a quick fix for all people, but for me as a DIY tech enthusiast it is great to overcome these challenges and I think it also can be a nice thing to share with other DIYers. Still it is a working permanent solution for this problem. :-)
For more details I highly recommend to watch my video above.
Apart from that, if you planning to purchase the mentioned charger (super versatile and powerful), I would be stoned if you use one of the amazon links below for your purchase. This doesn't effect you in any matter, but enables me to get a small compensation from amazon for your purchase and helps me to keep this site going.