In one aspect, an embodiment of this invention comprises an apparatus for updating an active fuel gauge of a battery cell. The apparatus comprises a memory, a sensor, and a processor. The memory stores data regarding an active fuel gauge of the battery cell based on a first voltage and state of charge (SOC) relationship. The processor receives first second, and third resting voltage measurements at different times. The processor calculates differences in capacity between pairs of the resting voltage measurements based on the first voltage and state of charge (SOC) relationship. When the differences are equal the reference capacity- difference, the processor identifies an estimate of a remaining capacity of the battery cell corresponding with the active fuel gauge. When the differences are not equal to the reference capacity difference, the processor updates the active fuel gauge based on an identified second voltage and SOC relationship.