Some nimh batteries decay in voltage in a very short time. If the voltage drops too low eg below .5 volt, the battery charger (some) cannot sense this low voltage to start the charge or pulse charge process.
This has happened to some of mine, they way around it, and it is dangerous is to use a 12 volt battery and just tap ths nimh battery for a millsecond. once usually does it, you may see a bit of a spark and the nimh battery will get a bit warm, but it will increase the residual voltage to a point that the charger will sense it and start charging.
I have used this method in the bush to fully charge a nimh battery from a vehicle battery.
nimh battey -ve to 12 volt battery -ve and, nimh battery +ve to 12 volt battery +ve.
Again it is dangerous, the nimh battery can get quite warm to hot and, possibly leak or explode.
This has happened to some of mine, they way around it, and it is dangerous is to use a 12 volt battery and just tap ths nimh battery for a millsecond. once usually does it, you may see a bit of a spark and the nimh battery will get a bit warm, but it will increase the residual voltage to a point that the charger will sense it and start charging.
I have used this method in the bush to fully charge a nimh battery from a vehicle battery.
nimh battey -ve to 12 volt battery -ve and, nimh battery +ve to 12 volt battery +ve.
Again it is dangerous, the nimh battery can get quite warm to hot and, possibly leak or explode.
