Abstract: In accordance with the present invention there is provided an engine (100), the engine (100) having a first mode and a second mode, the engine (100) comprising: a first battery (101); a second battery (102); a first charge level monitoring device (106) attached to the first battery (101); and a control unit (105). The control unit (105) is arranged to: measure the charge level of the first battery (101) using the first charge level monitoring device (106) when the engine (100) is in the second mode; and cause the second battery (102) to charge the first battery (101) if the charge level of the first battery (101) is beneath a first threshold value.