HP 35S battery drain

[cjsmithuk]

Hi all. Have a 35S here with the semi well known annoying battery drain issue. Lasts a few days only.

Very annoyed about this so thought hey let’s reverse engineer this so i took it to bits assuming it’s a bit of poor part selection somewhere. The power supply is basically a protection diode pair, an LDO, voltage detector and some capacitors for memory backup while changing batteries.

Well I pulled all of those parts off the board after buzzing them out with the 34401A (proper HP FTW not Kinpo garbage) and wired it up to a bench supply. At no point does the current drop below 0.5mA whatever you do to it and however long you leave it to drop into sleep. Thus the issue is actually likely a software or ASIC issue. I suspect software.

Circuit attached.

”miffed” comes to mind.