A safety circuit for use in low-voltage systems improves safety of and additional features to low-voltage connections. When incorporated into a battery, the circuit leaves the battery disconnected from the low-voltage system until it determines that it is safe to make a connection. When the safety circuit determines that no unsafe conditions exist and that it is safe to connect the battery or low-voltage systems, the safety circuit may make the connection by way of a soft start that provides a connection over a period of time that reduces or prevents inductive voltage spikes. A method is used for detection of proper polarity of the connections between the battery and the low-voltage system. When incorporated into a jumper cable, the safety circuit provides communication abilities, can provide test abilities, and improves connection safety and functionality, such as allowing transfer of power between low-voltage systems having different voltages.