A transport refrigeration system comprising: a refrigerant compression device, a refrigerant heat rejection heat exchanger, an expansion device, and a refrigerant heat absorption heat exchanger connected in a refrigerant flow communication; a battery; a supply line (L1, L2, L3) configured to power an electric device (E1, E2, E3); a switch (S1, S2, S3) connecting the battery to the supply line; a return line (R1, R2, R3) configured to provide a connection to the electric device; a controller configured to open or close the switch in response to at least one of battery voltage and current on at least one of the supply line and the return line; characterised in that the controller is configured to use the switch to disconnect the electric device from the battery to avoid under-voltage and over-current conditions; wherein the controller is configured to close the switch (S1, S2, S3), monitor the current on at least one of the supply line (L1, L2, L3) and the return line (R1, R2, R3) and keep the switch closed if the current is less than a first current threshold; wherein the controller is configured to open the switch (S1, S2, S3) after a first delay if the current on at least one of the supply line (L1, L2, L3) and the return line (R1, R2, R3) is greater than the first current threshold and less than a second current threshold; wherein the controller is configured to open the switch (S1, S2, S3) after a second delay if the current on at least one of the supply line (L1, L2, L3) and the return line (R1, R2, R3) is greater than the second current threshold and less than a third current threshold; and wherein the controller is configured to open the switch (S1, S2, S3) immediately if the current on at least one of the supply line (L1, L2, L3) and the return line (R1, R2, R3) is greater than the third current threshold. a refrigerant compression device, a refrigerant heat rejection heat exchanger, an expansion device, and a refrigerant heat absorption heat exchanger connected in a refrigerant flow communication; a battery; a supply line (L1, L2, L3) configured to power an electric device (E1, E2, E3); a switch (S1, S2, S3) connecting the battery to the supply line; a return line (R1, R2, R3) configured to provide a connection to the electric device; a controller configured to open or close the switch in response to at least one of battery voltage and current on at least one of the supply line and the return line; characterised in that the controller is configured to use the switch to disconnect the electric device from the battery to avoid under-voltage and over-current conditions; wherein the controller is configured to close the switch (S1, S2, S3), monitor the current on at least one of the supply line (L1, L2, L3) and the return line (R1, R2, R3) and keep the switch closed if the current is less than a first current threshold; wherein the controller is configured to open the switch (S1, S2, S3) after a first delay if the current on at least one of the supply line (L1, L2, L3) and the return line (R1, R2, R3) is greater than the first current threshold and less than a second current threshold; wherein the controller is configured to open the switch (S1, S2, S3) after a second delay if the current on at least one of the supply line (L1, L2, L3) and the return line (R1, R2, R3) is greater than the second current threshold and less than a third current threshold; and wherein the controller is configured to open the switch (S1, S2, S3) immediately if the current on at least one of the supply line (L1, L2, L3) and the return line (R1, R2, R3) is greater than the third current threshold.