The 1550 charging module appears to not notice the battery if the
voltage has dropped down a certain level.

Kick-start your battery by connecting it for a few min's (start with
30secs) to a normal car-battery charger. If at hand use a voltage
measuring tool to see if the voltage of the battery has increased (or
just put it back in the 1550).

If the voltage is high enough, the 1550 will 'see'the battery again
and will start charging.

Ronald