When the charger is connected to the battery.The battery charger needs a voltage back to operate a relay to start the charge process.If the battery voltage is too low , the voltage back is too low to bring in the relay.
If the battery is a standard 24 volt just use a standard car/lorry 24 volt charger to bring the voltage up.15 minutes should do.Then reconnect its own battery charger and it should be ok
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