 The LED display version of the TI-57 had a rechargeable Nickel-Cadmium battery pack BP7 which contains two AA size batteries and electronics to raise the voltage to the 9V required by the calculator.A popular modification is to power it from a 9V battery and use the battery cover of a LED TI-30 or a part of the dismantled battery pack.This modification provides a better battery life than the original battery pack.