A battery pack includes an arrangement of battery cells organized in battery groups connected in series, with each group having one or more battery units connected in parallel, and each battery unit comprising one or more series-connected battery cells connected to a battery switch. Charging of the battery pack uses pulse charging. The charging pulses provided to the battery units can be determined based on one or more measured characteristics of battery cells comprising the battery unit so that charging of the battery units can be optimized according to those characteristics. The charging pulses provided to each battery group are timed so that there is an uninterrupted flow of charging current through all the battery groups at all times.