Battery packs of multi-batteries supply high voltage when batteries are connected in series and high capacity when connected in parallel, which provides a new substitution of power supply for portable electronic device. A simulation method is, therefore, proposed to simulate the discharge behaviors of battery system with parallel and/or series connection. Using the simulation proposed, voltage, discharging capacity and residual capacity of the pack and individual battery at every time unit may be calculated at a given discharge current. When one of the batteries in the series-connection has a distinctly lower capacity, this battery reaches its fully discharged state sooner than the others during discharge. The simulated current distribution of a battery pack of two different-capacity batteries in parallel shows that although the pack is discharged at constant current, current through each battery is neither a constant nor proportional to its capacity. A battery of higher capacity carries a larger current. A pronounced change in current is observed near the end of the discharge process where voltage is near the cut-off value. We believe the proposed simulation method may be extended to a battery pack of multiple batteries of any capacity in any number and connections.