Apple Inc.
Charging Technique for Series Cells

Abstract:

An electronic device has a battery system with control circuitry for use during charging and discharging. The battery pack has two battery cells coupled in series. The control circuitry includes battery charging circuitry that applies a charging current to the battery pack during charging. The control circuitry includes bleed resistors and switches that can be selectively activated to bleed charging current away from a selected cell during charging. This allows the control circuitry to balance charges stored on the cells and to balance associated cell voltages. The control circuitry is configured to maintain information on a difference between the charge stored on the first battery cell and the charge stored on the second battery cell. Information on this charge difference value is maintained during charging and discharging and is used in establishing a battery pack charging voltage target.