For the third time, I've had to replace the three 357 silver-oxide cells in my HP16C calculator. That's probably very close to 15 years on average between sets--not bad. What surprised me was that one day, the thing was working nicely and the next, it was dead--no display, nothing. A voltmeter confirmed the power failure. I suspect the cells get pretty near their shelf life in this application. Is it usual for them to reach the end of their shelf life so suddenly? --Chuck