Patent ID: 8179094

Claim:
A battery backup unit for a data storage device, wherein the data storage device includes a cache memory element configured to store data therein, the battery backup unit comprising: a plurality of battery cells configured as a first battery pack having a first charge capacity and at least one second battery pack having a second charge capacity; a first battery charge capacity gauge coupled to the first battery pack; a second battery charge capacity gauge coupled to the second battery pack; and a charging circuit coupled between the first battery pack and the at least one second battery pack, wherein the charging circuit includes a controller switch, wherein the controller switch is configured to couple a learn cycle load and the charging circuit between the first and second battery packs in such a manner that no more than one battery pack performs a battery learn cycle at a given time, wherein the battery packs and the charging circuit are configured within the battery backup unit in such a way that the overall available charge capacity of the battery backup unit at any given time is the combined capacity of the battery pack being discharged during the battery learn cycle and the full charge capacity of the battery pack not being discharged, wherein the first and second battery charge capacity gauges are configured in such a manner that, during a battery learn cycle performed by the corresponding battery pack, a depth of discharge level of the corresponding battery pack is such that at least a portion of the first charge capacity of the battery pack being discharged during the learn cycle combined with at least a portion of the second charge capacity of the battery pack not being discharged is sufficient to allow the data stored in the cache to be off-loaded to a physical data storage device when coupled to the data storage device.