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I am assuming that 18v is 5 cells in series, with a nominal voltage of 3.7 x 5 = 18.5 volts. At 15.22 each cell is around 3 volts. 3 volts is usually considered the absolute minimum you can discharge cells to. (3.3v is considered flat) If it won’t charge, it’s probably dead.

It might be possible to rig up a charger to very very slowly charge the battery up to about 3.3 volts, at which point the proper charger might agree to fully charge the battery, but with the cells already below 3.3v there’s a good chance it won’t work even if you did that. And it’s not without risks.

Thank you all for the responses. I ended up ordering a replacement battery, since i intend to sell the unit and don’t want to pass on a ‘pig in a poke’. Had the voltage been a little higher i would probably have attempted to recharge it.

In most cases of a dead battery it is just a lack of the circuit topology needed.

The thing is that the cells inside the pack have differences in resistance and this can drain the pack over time. That said, a cell can go bad and short circuit with a low resistance that will heat up massively and cause a fire if you are not careful.

If the ideal circuit topology were used, each cell would be individually controlled by the battery management chip. A less ideal situation is for each cell to have a thermistor, aka a low resolution temperature sensor to monitor each cell. Even less ideal, the batman needs at least one thermistor to monitor charging and ensure things do not get out of hand. The worst kinds of batman rely solely on the charge circuit topology to „safely” charge the pack.

The actual circuit topology for most lithium cells requires three different modes of operations. There is 1) initial trickle charging, 2) constant current mode, and 3) constant voltage mode.

If your battery does not charge, there are likely one of two reasons why. It could be well monitored and the batman has detected an over temperature anomaly in a cell or a short circuit in part of the pack. The second potential scenario is that the batman circuit is missing the trickle charge topology and there is a threshold pack voltage that must be met before the constant current mode can activate. If the cell has sat for a long period of time without charging, the second scenario is more likely, (but not guaranteed to the extent you should ever leave such a charging cell unattended for any amount of time).

I have a cheap power supply which can be limited in current and voltage and I've been able to revive many such batteries with it. I use it as a drip charger limiting current and voltage so neither gets too high until the regular charger picks up the battery again.