amber engine sign on display unit?

We are converting a DAF LF into a horsebox, when we tried to start it recently it wouldn't turn over. We charged the batteries and tried again but wouldn't start, however if you leave the battery charger connected we can get lights, radio etc but the amber engine sign appears on the display and it still wont turn over but if you disconnect the charger then there is nothing at all - We have turned over the engine regularly since doing the conversion without any problems - Any ideas please.