Embodiments of the invention include a device and method for improved battery learn cycles for battery backup units within data storage devices. The backup unit includes a first battery pack, a corresponding charge capacity gauge, one or more second battery packs, a corresponding charge capacity gauge, and a controller switch configured to select only one battery pack for a learn cycle at any given time. The charge capacity gauges are such that, at the end of the learn cycle discharge phase, the depth of discharge of the learn cycle battery pack is such that the charge capacity of the learn cycle battery pack combined with the full charge capacity of the remaining battery packs is sufficient for the device cached data to be off-loaded to a physical data storage device, and the data storage device does not have to switch from a write-back cache mode to a write-through cache mode.