In this paper, the authors report in-vitro tests of an ultrasound system and method of wirelessly transmitting significant amounts of energy into the human body for the purpose of recharging implantable batteries. We demonstrate the complete charging of Li-ion batteries with energy capacities of 35mAh, 200mAh, and 600mAh through several types of animal tissue while keeping the temperature rise of the tissue and piezoelectric receiver below the desired 2°C human safety limit recommended by FDA medical device guidelines. Given these encouraging results, it is logical to conclude that the rapidly growing neurostimulator market is a desirable and appropriate target for introduction of this novel recharging method.