HP 35 charger/case questions


Hello, everyone! I recently became the proud owner of a Hewlett Packard 35 calculator (version 3). It came with several original accessories, including the hard plastic case, the leather soft case, a restuffed 82001A battery pack, and the manual. However, the power supply included with it was decidedly non-original: basically a 9V 200mA DC wall-wart connected to the charging pins of the original connector! For the hell of it, I measured the voltage at these pins without a load (I didn't try connecting the adapter to the calculator), and the meter immediately went to around the 20V mark! Even so, this worked to some degree, since the battery pack had a charge in it when I got it; of course, the battery understandably got hot during charging, according to the previous owner, so he didn't charge it often.

Just recently, I bought a genuine HP 82002A charger on CraigsList. When I got it, I decided to test the voltages from it, unloaded, using a Triplett 310 VOM. The side which powers the calculator during charging measures out to what seems like a proper 4.5V or so. The set of pins which charge the battery (marked with a '+'), however, seems to output closer to 16V! Is this normal for this portion of the charger while unloaded, or has something gone wrong inside the unit?

As an aside, how are the accessories supposed to be arranged within the hard case? I see a portion which is presumably meant for the power supply, but the 82002A I have doesn't have the same shape (nor does any other original supply I've seen pictures of). Is the leather soft case supposed to go in there as well, and if so, where? Anyway, thanks in advance!


The no load output from a typical 82002A adapter / charger is about 5 vdc on the adapter side and 16-17 vdc on the charger side, so your unit seems normal. Some older units output about 4 vdc on both sides, so charging took much longer.

The leather soft case must be tightly wrapped and placed inside a plastic ring to fit inside the case just behind the charger.


Thanks for the reply! Due to this apparently being a newer charger, should I charge the pack for less than the 14 hours recommended in the manual?

Also, could the apparent overcharging I mentioned earlier (due to the improper wall wart) have hurt the battery capacity? Both the battery pack and the transformer were apparently derived from a cordless phone of some sort.

Finally, what do you mean by 'behind the charger'? As it stands now, I have the charger near the front of the accessory compartment (sitting in the 'well' area), the charger cable in the middle (in front of the prongs), and the leather case rolled tightly against the back of the compartment. Doesn't quite close, but considering the lack of a plastic latch, I think it's doing fairly well. Anyway, once again, thanks in advance!


I don't think you can overcharge the battery. I would check it's voltage at its terminals, and if it's over 4vdc, then I'd stop further charging. As far as it's life being degraded due to the use of the wrong charger, I tend to think not. More important with NiCd batteries is the number charging cycles and it's better to go through a complete discharge / recharge cycle.

Yes, I meant that the soft leather case sits at the back of the hard case on the right side where the charger sits. Those latches are often broken off and missing, which is why cases with good latches command a premium.


Yet again, thanks! Is there a way to monitor the voltages from the adapter while connected to the calculator without crudely wrapping wires around the pins themselves? There are small holes behind each pin of the charger's connector, but I'd rather not stick my multimeter's leads in there if there's a chance it could cause a connection to break internally.

Glad to hear that the NiCads are somewhat forgiving of this sort of torture. I was worried when I discovered that 3.6V of NiCads had been charged with 9V (or more!) for some period of time, but since they're still holding a charge, the damage couldn't have been too bad. I am indeed waiting for the battery pack to discharge enough before I try charging it with the adapter.

Thanks for confirming the layout. Having everything together makes for a really neat package. I figured that latch would've been a weak point; this case looks like it was held shut using masking tape for some time, due to the amount of residue near the front of the case. While this looks rather crude, to me, it just makes the unit look well-used and well-loved, likely for a very long time after the unit was originally purchased. I may look for a new case someday, but it's not a huge priority.


I wouldn't recommend trying to insert probes in the back holes of the charger plug to monitor battery charging. While on the charger you will see a much higher voltage anyway, and have to unplug the charger for a minute to let the battery float to its no load voltage. I just leave mine on the charger for about 4-6 hours and then remove the battery to check its charge. It very rarely needs any additional charging.

As to the plastic latches, I open mine only enough to clear the case to minimize the fatigue on the hinge. Of course, this advice is a bit late in your case. I used to have a plastic case with a broken latch for an HP 45, which is similar in design to the HP-35 case. I just used a large and long rubber band to keep it closed.


I'm more interested in seeing whether or not the voltage from the charging section drops to something more acceptable than 16V when the charger is connected to the calculator. Since this battery pack has been overvoltaged at least once, I want to make sure that it being put on charge doesn't cause it to get heated up again.

I'll probably use a rubber band or two to keep the case closed in the long run. It's a shame that the latch turned out to be a weak link in an otherwise excellent design. Of course, the case does a pretty good job staying closed on it's own, and it likely isn't gonna be doing much traveling around these days.

Anyway, for your viewing pleasure, here's a photo of my HP 35, with the case and accessories:

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