Ran into a situation where the Festool 18V battery pack would not accept an charge from the official charger. The indicator light was blinking red. Placed the battery pack back onto the charger several times. Each time, the red blinking light indicate the same message.
The battery were stuck in a low-voltage state that the official charger could not recognize the battery. The official charger will not work with a low voltage battery pack. This typically happen to batteries that have been sitting idle for long periods of time or have been allowed to completely drain of they voltage.
To jump start the battery, I grabbed another 18V battery pack that was full charged. Placed both battery pack on the workbench. Also needed a set of alligator clip leads with one red wire and an black wire.
Jump start procedure
Locate the positive and negative terminal strip on each battery pack. The positive terminal strip have a plus sign next to it. Attached the red alligator clip to the positive terminal on the good battery. Attached the other end of the red alligator clip to the positive terminal on the dead battery. Attached the black alligator clip to the negative terminal on each battery.
Found out that this same method can be used on other brands of 18V to 20V battery pack systems. Told my friend to bring his brand of battery. The positive and negative terminal strips were in the same position. Followed the same steps to connect the alligator clips to each battery. This also worked fine for his brand of battery.
Left the batteries connected for around fifteen seconds. Did not need to leave the batteries connected for longer then fifteen seconds. Only needed enough voltage from the good battery to the dead battery to allow the charger to recognize the dead battery. The healthy battery did transfer some of its voltage to the dead battery through the alligator clips.
Removed the alligator clips from the battery packs after the fifteen second time period. Took the dead battery and placed it back onto the charger.
The charger recognized the battery pack. The blinking red light change to blinking green to indicate that the battery pack was now accepting the charge from the charger. This was a lot better response from the battery pack then the red blinking light that it exhibited for the past hour. Allowed the battery to fully charge to 100%.
Placed the now fully charged battery into the drill. The battery accepted the insertion into the drill. The drill start up without any issue. No more blinking red light on the battery pack. The battery pack was successfully jump started and is now functioning normal again. I was worried that the battery was dead and would of have to be replace.
