A selective equalizer for NiMH batteries

A unique method has been developed to equalize nickel metal hydride (NiMH) battery packs using a new selective equalizer. This equalizer 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. The relay switching is controlled by a 32 bit microcontroller, and the boost current is supplied by a boost charger. Once a weak battery is detected, it is scheduled for a specific boost time by a special “Round Robin” (RR) algorithm. The equalizer was tested on a pack of 12 series connected 12 V 93 ampere hour (Ah) NiMH batteries. Test results show that the equalizer was able to re-balance an artificially unbalanced pack, and the capacity was increased by 27% within six charge–discharge cycles. Results indicate the number of cycles required to re-balance the pack was significantly reduced by using this technique.