A stimulator includes an implantable pulse generator comprising circuit elements, a first power source, such as an ultracapacitor, that provides operating power for the circuit elements of the pulse generator. The pulse generator can also have a memory associated therewith, such as a volatile memory for storing programming data. A second power source that has higher voltage retention than the first power source can also be included. The second power source can be dedicated to the volatile memory and can provide operating power for the volatile memory.