A system comprising a main battery and an auxiliary battery charging apparatus for charging an auxiliary battery from the main battery , the main battery having at least a first module (5-1) comprising a plurality of first cells (6-1) and a second module (5-2) comprising a plurality of second cells (6-2), the main battery further comprising a first module controller (7-1) for monitoring said first cells (6-1) and a second module controller (7-2) for monitoring said second cells (6-2), the auxiliary battery charging apparatus configured to receive data from the first module controller (7-1) and the second module controller (7-2), the apparatus comprising: an intermediate storage device for connection to the auxiliary battery ; a first switch unit for selectively connecting one or more of the first cells (6-1) to the intermediate storage device and a second switch unit for selectively connecting one or more of said second cells (6-2) to the intermediate storage device ; a charging control unit having an electronic processor configured to receive a first balance control signal (BAL1) for identifying one or more of said first cells (6-1) having a relatively low depth-of-discharge from the first module controller (7-1) and a second balance control signal (BAL2) for identifying one or more of said second cells (6-2) of the second module (5-2) having a relatively low depth-of-discharge from the second module controller (7-2); wherein, in dependence on the first balance control signal (BAL1), the electronic processor is configured to output a first switch unit control signal (SW1) to control said first switch unit to connect the identified one or more first cell (6-1) to the intermediate storage device to charge the auxiliary battery and in dependence on the second balance control signal (BAL2), the electronic processor is configured to output a second switch unit control signal (SW2) to control said second switch unit to connect the identified one or more second cell (6-2) to the intermediate storage device . an intermediate storage device for connection to the auxiliary battery ; a first switch unit for selectively connecting one or more of the first cells (6-1) to the intermediate storage device and a second switch unit for selectively connecting one or more of said second cells (6-2) to the intermediate storage device ; a charging control unit having an electronic processor configured to receive a first balance control signal (BAL1) for identifying one or more of said first cells (6-1) having a relatively low depth-of-discharge from the first module controller (7-1) and a second balance control signal (BAL2) for identifying one or more of said second cells (6-2) of the second module (5-2) having a relatively low depth-of-discharge from the second module controller (7-2); wherein, in dependence on the first balance control signal (BAL1), the electronic processor is configured to output a first switch unit control signal (SW1) to control said first switch unit to connect the identified one or more first cell (6-1) to the intermediate storage device to charge the auxiliary battery and in dependence on the second balance control signal (BAL2), the electronic processor is configured to output a second switch unit control signal (SW2) to control said second switch unit to connect the identified one or more second cell (6-2) to the intermediate storage device . <EOS>