A selective boost equalizer for series connected NiMH battery packs

A unique selective equalization method has been developed to equalize nickel metal hydride (NiMH) battery packs. This technique uses a new selective boost equalizer that detects batteries either at a very low state of charge (SOC) or at an extremely high SOC. In this system a set of electromechanical relays is connected in a matrix to route boost current to the weaker batteries. A 32-bit microcontroller is used to control the relay switching, and the boost current is supplied by a separate boost charger. Once a weak battery is detected, it is scheduled for a specific boost time by a special "round robin" algorithm. The equalizer was tested on a pack of twelve series connected 12 V 93 ampere hour NiMH batteries. Test results show that the equalizer was able to rebalance an artificially unbalanced pack, and the capacity was increased by 27% within six charge-discharge cycles. The number of cycles required to rebalance the pack was significantly reduced by using the "round robin" algorithm.