Lithium (coin cell) backup battery

[rprosperi]

Among HP's graphing models, the first to use a coin cell backup battery were the 49g+ and 48gII in Oct 2003. The "HP-48gII" would appear to be a 48, but in fact it's really a 49g+(-) and not a 48 at all. The 39g+ followed a few months later and then the 39gs/40gs/50g a couple years later.

Among non-graphing models, many of the late, low-end models which use 2 x CR-2032 batteries can be considered to also use coin cells as a backup battery as they will actually run with only 1 battery installed, so the idea of having 2 is to have longer life between battery change, and then you install new batteries one at-a-time so the other battery is a backup for the one you're changing.