A capacity detector for detecting capacity of an energy storage unit includes a first and a second buffers and a voltage divider. The first buffer is used for generating a first threshold reference according to a first adjustable reference which is predetermined based upon chemistry of the energy storage unit. The second buffer is used for generating a second threshold reference according to a second adjustable reference which is predetermined based upon chemistry of the energy storage unit. The voltage divider coupled to the first buffer and the second buffer is used for dividing a voltage across the first threshold reference and the second threshold reference and for generating capacity references indicating capacity percentages of the energy storage unit according to a voltage versus capacity characteristic of the energy storage unit.