Woodstock - Battery Charging Indecation


Is there any way to know when/if the batery charger is actually charging when connected to an HP21 or HP25? I know don't do it - but it has taken me a while to get the batteries to work reliably and now I don't want to remove them unless I have to!

Mike T.


I can't answer your exact question but there is a way to tell if the battery charger is not charging when it is hooked up to a Woodstock. You will notice a little mushroom cloud forming over your formerly working calculator.


Hi Mike,

There is a big resistor in the rectifier circuit that gets quite warm while the calculator is charging. You can feel it through the back of the case on the enter key side (I think). I recall this from the old days, when I used to connect my 29C to the AC adapter (which of course I don't do anymore). You may be safe connecting a non-continous model to the AC adapter, as long as you don't turn it on while connected (If I am wrong on this, someone please correct me). On the continous memory machines, the CMOS circuitry is always connected to the battery, so the danger of overvoltage if you loose battery continuity is always there.

Good luck, and best regards, Hal


Now I'm more confused than ever. Is it OK to use my battery charger that came with my HP-25C 28 years ago? Everything seems to work real well when charging, and of course the calc always did get warm while doing so, and the Owner's Handbook said that this was normal.



Well Trent,

The way the woodstocks were designed, the battery pack acts as the voltage regulator for the charging circuit, imposing sufficient load to bring the voltage output of the internal rectifier down to about 2.5 to 3VDC. The kicker is that the no load (i.e., battery removed) output of the rectifier is in the area of 16VDC, which can damage the calculator circuitry. If your battery pack develops a bad contact, or has a bad (open) cell, or loses continuity in any other way while your calc is connected to the AC adapter, you now have a no load situation, and 16VDC is being applied to the calculator, which will almost certainly damage it. In my humble opinion, betting that your battery pack is always going to have good contact is not worth the risk. That's why I charge externally.

Best regards, Hal

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