an ultracapacitor energy storage cell pack includes an ultracapacitor assembly having a plurality of series connected ultracapacitors and balancing resistors , each balancing resistor connected in parallel with each ultracapacitor to automatically balance each ultracapacitor over time , thereby automatically over time discharging the ultracapacitors of the ultracapacitor assembly ; an enclosure to enclose and protect the ultracapacitor assembly ; a controller for the ultracapacitor assembly ; and one or more temperature sensors to monitor temperature of the ultracapacitor assembly and coupled to the controller .