Very easy. The battery pack clips into the bottom of the machine, you just slide it slightly forwards (towards the '0 key end' of the machine and lift the rear edge to get it out. The battery pack contains 2 AA-size NICd cells connected in series, normally by a double-spring thingy (that;s a highly techncial term, of course). The housing is made in 2 parts which were glued together. You cna crack the glued joint, take it apart, and fit new cells. But actually, I don't recomend doing that. It's better to cut away the pack housing ad the front end, where the cells touch the contacts in the calculator. Then silde out the old cells and slide in the new ones -- taking care of the polarity (since HP calculators were designed for use with their own battery packs only, which only fitted the right way round, there is no reverse-polarity protection. The cells will be held in place when the pack is clipped into the calculator, and you can remove the cells easiy for charging in a normal charger, thus avoiding possible damage to -C models. -tony