Simulate "load" (or whatever) of batteries in classics hooked to chargers?

Here's a question that's probably been asked before . . .

I'm not wanting to keep NiCds in my LED H-P's. I actually use them so infrequently, they just go bad on me. Besides, it's considered bad practice to keep batts in collectibles.

However, I would like to fire 'em up at a moment's notice to show 'em off.

Buuut, I've heard numerous times not to run (most? all?) H-P's on wall power without the batteries in place.

Soooo . . . Is there a way to fake the charging circuit into 'thinking' there are batteries installed, while running on wall power? (Some sort of r/c circuit in place of the batteries?)

It'd sure be cool to be able to flick a power strip switch and have the half-dozen LED calcs (in my little book shelf display) light up all at once!

Edited: 7 Jan 2010, 11:40 a.m.



The easiest and safest way would be to modify the charger in a way that it outputs only 2.5 Volts. This can be achieved with a Zener diode and a resistor.
But if I had to do this, I would use the power cord of the HP calculator and connect it to a dedicated, regulated power supply set for 2.5V.

As I have several calcultors from the '20 series, the only ones that you really can kill by connecting the charger without the battery in place, I keep one functional battery pack (always externally charged!) that I use with one calculator at a time.

Greetings, Max

Edited: 7 Jan 2010, 11:57 a.m.


It seems to be a common misconception that you shouldn't run Classics from the AC adapter without a battery inserted - from the schematics (available here on MoHPC) it is 100% clear that the battery is DISCONNECTED from the calc when you plug in the adapter - so there is absolutely no issue in leaving the battery out.

Caveat - the HP-65 and HP-67 require a good battery in place to provide the start-up current for the card reader motor.

Later series calcs (like the Woodstocks) absolutely DO require a good battery in place to avoid damage.

Also, the Classics run on 3x NiCd cells so nominally 3.6V, not 2.5V as above.... 2.5V is probably referring to Woodstocks and later 2 cell calcs....


2.5V is probably referring to Woodstocks and later 2 cell calcs....

Yes, of course. But I think I wrote somewhere that only the '20 series calculators (Woodstocks as you call them) really have this kind of problem.


I used the word "classics" perhaps too loosely. I meant "H-P LED models". I'm talking specifically of: an HP-45, HP-67 & 97, HP-21, HP-31E and 34C.

Is there something that might be installed in place of batteries to simulate whatever is required for safe charger-connected operation?

(Alternatively, if I just connected the appropriate DC supply voltage to each unit's battery connections, and left the chargers out of the picture, would that work?)


Alternatively, if I just connected the appropriate DC supply voltage to each unit's battery connections, and left the chargers out of the picture, would that work?

Of course! But be very careful, the polarity of the battery connectors is not marked very visibly, especially in Woodstocks!


I keep batteries in all the calculators in my collection so that I can "fire 'em up at moment's notice". My solution to the battery self-discharge and leakage/corrision issue was to use AA or AAA Energizer lithium cells.

My HP's classic calculators with AA nicad battery packs now have battery packs that use made out of AAA plastic battery holders modified to match the contact positions in the calculator battery compartment. The same is true for the Topcats. The Woodsock and Spice calculators make it easy change to lithium cells. All the non-HP calculators in my collect have had similar changes to accommodate Energizer lithium cells.

I've seen no ill effects from making these changes to over 200 calculators after several years.


"Wooden Socks" with batteries. ;)


You could use actual battery cells like the Sanyo low self discharge Nimh cells if you rebuild the battery packs. They do retain their charge for a year. I have them in the HP50s now. Sam


Caveat - the HP-65 and HP-67 require a good battery in place to provide the start-up current for the card reader motor.

According to Randy, you may actually damage the card reader circuitry, even if you don't operate the card reader, if you operate the calculator from the AC adapter without a battery pack installed. This is because the card reader circuit is connected directly to the battery terminals, and sees the 17vdc output from the charging side of the AC adapter. I simply connect a regulated DC power supply (3.75vdc) to the battery terminals when I use my HP 65/67.


"Wooden Socks" with batteries. ;)

It's a cure for cold feet!


The low discharge rate batteries from Sanyo are called "eneloop". There are other similar cells from other manufacturers too. These cells are supposed to keep 80% of charge after a year when not in use. The only downside is the maximum mAh I have found for these so far is 2000mAh where as "normal" NiMH cells can be up to 2500 mAh these days, maybe more.

As far as I know these are only available in AA size, I have not seen any other sizes.



... Energizer lithium cells ... over 200 calculators ...

Wow. According to my calculations, you have more invested in lithium cells alone than I have in all my calculators and peripherals! And my wife thinks I have expensive hobbies.

I guess that does add up. I've put them in most of my calculators. Fortunately, many only take two (Woodstocks), and some take 4 (Stings). I've found 8 packs of batteries for about 14 USD with coupons. That's about $1.75 per cell. At an average of 3 per calculator, and maybe 40 in my collection with these cells is $210.

I think NiMH cells aren't that much cheaper, though old NiCd packs for cordless phones are easily found in drug stores on clearance for $1. That's how I re-celled all my TI's, though those packs are now going on 10 years old already. I, too, like being able to "fire up" any calculator I pick up without messing with chargers and charging...

I still like Katie's approach. Lithium cells have at least a 15-year life, and I'll bet more... I still have some from 10 years ago showing full strength. But, with Katie hopefully living a long and healthy life, she just might have to re-cell them all once or twice...


My inclination would be to put a 1N4728A zener diode across the battery terminals. This should limit the voltage to 3.3V. That will still allow the NiCD batteries to charge properly, but when a charger is connected without batteries, the zener will limit the voltage and the zener and the existing series resistor will dissipate the excess.

I'd rather use a 2.8V-3.0V zener, but I can't find any in axial lead packages with suitable current ratings. I'm fairly certain that limiting the voltage to 3.3V would be sufficient to protect the chips. I haven't tested it, though.

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