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I just noticed under the battery cover on the 35s it says: "Please replace batteries one at a time".

Why would that be? If they are wired in series, then removing one would disconnect both, so I don't see the reason for that. But if they are wired in parallel, then I would be putting a fresh battery in parallel with a dead battery (even if only momentarily) which would start to discharge my fresh battery.

Or am I missing something?

Parallel? Series? Neither!

If you take a look at the insides:

photo from http://www.voidware.com/index.php?option=com_content&task=view&id=37&Itemid=2

You'll see that each battery is connected to the board separately.


Edited: 21 Dec 2007, 10:28 a.m.

This will lead you to a discussion (among other things) of some battery current measurements which several of us made on the HP35S. (Go towards the middle of the thread.)

I was impressed with how similar the current delivered by each of the batteries is. As close as I could measure it, it is identical.

As you both note, the batteries are certainly not in series. I suspect they are in some sort of parallel, using a circuit which isolates the batteries from each other but balances the load.

I just tried my calculator with one battery at a time - and that works, briefly! I get a couple of seconds on either one until it shuts itself off. Whether it shuts off after sensing that a battery is low or missing or whether it is running on residual charge I don't know, except that I was able to turn it back on (in the single battery mode) several times.