More than one brand of inverter/charger is programmed to prevent charging
into a dead battery
. This is done so that the charge function will not turn on without a battery or so that a battery with a shorted cell will not be charged which results in excessive gassing of the other cells.
The usual voltage sensed by the inverter charger
is near or below 10.5Volts when the charge function is disabled. It is necessary to "jump-start" the charge function when attempting to charge a dead battery by connecting in parallel another battery that is over 11Volts or so until the charge function comes on.