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.

[Steve Simpkin]

Yes, it looks like you probably have a defective one. Most people seem to average about 18 months from a new set of batteries.

Regarding your reverse-engineered schematic. Most people who investigated the battery connections show the two cells as wired in parallel, not in series as you have drawn. Make sure yours are wired correctly as shown in this photo:
http://www.finetune.co.jp/~lyuka/interes...-35s-2.jpg

Here is some additional information about current drain.
https://www.hpmuseum.org/cgi-sys/cgiwrap...ead=164075

http://www.finetune.co.jp/~lyuka/interests/calc/hp35s/

[cjsmithuk]

Definitely wired in series here as measured. Also that’s a 5V LDO in there and a 5V voltage detector - I pulled the part numbers down and confirmed it out of circuit.

The inside of mine is slightly different. Not sure if it’s an earlier or later one this one. I’ve attached the photo from mine. Note the use of electrolytic rather than tantalum capacitors compared to the one you posted.

Anyway this is useless as it stands

[Steve Simpkin]

That is strange. I confirmed that my HP35s will run OK with one of the cells removed so they are definitely in parallel on mine (S/N CNA2440xxx). Either they radically changed the power supply setup on yours to run on 6V or there is something else going on here.

[cjsmithuk]

I may be wrong about the diode arrangement in that schematic as it's difficult to tell what it is in circuit. I would have thought that each battery would be bypassed by a single diode which would allow that behaviour.

It will run quite happily down at around 2 volts though. I think what they did here was power it with 5V from the LDO and use a voltage indicator for the battery light when that went below 5V. The series pair would have to discharge from 6V down to <5V for the battery light to come on.

Either way the circuit is a bit cruddy.