External battery pack for camera flash provides substantial power for a long period of time, thus it's essential to maintain & replace the seal-lead acid batteries once awhile. Every 3-5 years, I would like to replace the battery, just to keep the pack fresh & ready whenever I need them. The Battery One pack uses Hawker Cyclon battery pack, like this: http://www.batteryuniverse.com/Sealed-Lead-Acid/EnerSys-Cyclon-Monobloc/0800-0102/0800-0102_EnerSys-Cyclon-Monobloc_0800-0102_Sealed-Lead-Acid-Battery