In this paper, a series-connected battery charging/discharging analyzer is developed to automatically test the batteries with a complete charging and discharging cycle. A micro-controller unit (MCU) is integrated to sample the battery voltage and current and calculate the charging efficiency. The charging and discharging processes are also controlled by the MCU according to the battery voltage and current. The charger provided by the battery manufacturer is used to charge the series-connected batteries. The voltage and current during testing procedure would be displayed on a LCD by the MCU. After the whole charging/discharging testing, the calculated charging efficiency would also be displayed on the LCD. From the experimental results, it is seen that the maximum voltage and current errors are about 0.23V and 0.1A respectively.