Cell balancing should be used in a design to maximum system lifetime when cells are connected in series. If standby or leakage current is not critical, as in a back-up power system connected to electrical mains, a passive approach is simple and most reliable. In a system where leakage current is critical, an active approach is typically used. Passive balancing utilizes fixed resistors in parallel with each cell. In a system with cells in series and in parallel, a common resistor can be used for the cells in parallel. Active balancing monitors the cell voltage and turns circuits on to drain energy from a higher voltage cell and reduce the voltage.