Abstract: A back-up electrical power supply module provides a back-up supply for a plug-in volatile electrical device such as a memory module. The memory module has a dual-in-line pins which engage socket openings of a standard dual-in-line circuit board connector. The back-up supply module is mounted on top of the memory module and has dual-in-line pins which contact corresponding ones of the pins of the memory module and corresponding socket openings. In this way the back-up supply module and the memory module can be unplugged as a unit and when unplugged, the back-up supply will maintain the contents of the volatile memory module intact.