The present invention discloses a method and system for charging rechargeable battery cells in series. When the voltage of a specific battery cell is too high, discharge the specific battery cell, and at the same time let other battery cells with lower voltage continue to charge, so that each battery cell in series can be charged to almost the same level. This invention designs in a “Total voltage follow-up charging method”, a Battery Manage System (BMS) detects total voltage of the series-connected battery in real-time and modifies an “equalizing trigger voltage”, as the total voltage drifts the equaling function still works well.