From Gumstix User Wiki
Batteries for coin cell backup battery holder
A rechargeable 6.8mm lithium coin cell battery, part #ML-621S/ZTN, is available from Digikey via this link.
What is the purpose of the backup battery? Is it only for keeping the clock in correct time, or are there other purposes? Does the Gumstix lose the clock immeadiately when AC-power is disconnected, thus making the backup battery practically a "must" accessory? Someone who knows, please replace these questions with the answer.