We presented a simple formula of V(subscript oc)≥1.36•N•1.17+0.7 (V) (N denotes the number of batteries in series) for determining the solar-cell's open-circuit voltage of a simple solar-cell-driven Ni-MH battery charger. Given a fixed collecting-area for a solar-cell module, there exits an optimal arrangement for the building blocks. Our predicted results will lead to the optimal design for transferring the maximum solar energy to the electrical energy deposited in the battery. We also experimentally verified this rule. Finally, we used an equatorial mount to fabricate a small tracking solar-cell battery charger to increase the collected solar energy.