A varying current charger for rechargeable Lithium-ion battery is implemented to maintain a predefined energy bound which is consumed by the overpotential equivalent resistance. For aged Lithium-ion batteries, the traditional CCCV charging method charges the battery with the same constant current no matter how the battery internal overpotential resistance increases. The varying current charger developed in this paper will follow the pre-defined energy bound, calculate the internal resistance from EMF and update the next charge current by matching the estimated time to consume the energy bound and the end of charge time. The hardware prototype is implemented with a FPGA and a current source board and the communicates with PC user interface through UART. The experiments show that the efficiency for the varying charge current method is hold for both a new battery and an aged battery although the charging time is compromised.