The Bosch 12V battery stopped charging some weeks ago and the charger showed permanent green light and simply did not work. I found that permanent green light on this model of Bosch charger means one of two things: either the battery is fully charged, or it does not charge because of a fault in the battery itself.
The problem did not show right away, because the battery looked fully normal from the outside. No visible damage, bump or fault on the contacts. Even so, when I laid it on the charger, it only showed me permanent green LED that seemed to trick me.
The voltage was so low that the charger did not even try to start the charging process.
How to Jump‑Start a Bosch 12V Battery That Won’t Charge
Basically, everything depends on the voltage, when one checks it outside. Those lithium batteries have safety circuits that stop everything, if the voltage sinks too much, and later the charger can not find the battery any more. Most commonly the battery not truly dies, it simply needs a tiny push to raise the voltage to a level, where the charger can spot its presence.
Here one uses the method of jump-starting. For the rescue job you need a second Bosch 12V battery that still works well and holds charge.
I took my other battery from the workshop that still worked normally, and placed both on the table.
First one finds the positive and negative poles on both batteries. If you look closely at the plug part, the poles on those Bosch batteries are clearly marked. Placing both batteries and checking that their positive and negative spots match, took only one minute.
Later connect the wires. One connects positive to positive with the red and negative to negative with the black. That forms a direct path for the flow of electricity from the good battery to the dead.
Placing the cables needed a bit of care, because the poles are covered in the body of the battery. I had to make sure that the metal rings make firm contact with both poles, and leave the link tight. When everything was in place, I pressed the cables strongly in position on both batteries.
One leaves the batteries connected for around fifteen seconds. In that time flows electricity from the charged battery into the dead one, slowly raising its voltage. I kept close watch on the hole setup during the whole time.
The cables did not warm and there was no smoke or burning smells, what already quite a lot calmed me. When the time was up, I carefully removed first the red cable, later pulled the black free from both batteries.
The jump-start ended, and now came the real test. I took the before dead battery and laid it on the base of the Bosch charger. The LED of the charger changed from permanent to blinking green light.
It worked.
Later the charger took over the task, handling the charging normally and bringing the battery to full power in two hours. Do try that before dumping a battery that looks fully dead, because commonly those units are actually saveable. Everything ended well, saving the cost of a new battery, and now both are again in use in my workshop.
