Is It Safe?
Powering Your Calculator
Because I modify original battery packs and make adapters to allow you to easily replace the AA batteries in your vintage packs with any modern AA batteries, I'm often asked if it's safe to use batteries other than the original NiCads.
I am not an electrical engineer, but I've asked one. And I've asked other collectors. And I've read posts and comments on calculator collecting forums. My conclusion is that nobody knows for sure, and the answer may vary from calculator to calculator.
The issue is whether the higher voltages of modern batteries, 1.5v for alkalines and 1.3v for NiMHs, as vs. 1.2v for the original NiCads, will do any damage to the calculator circuits/chips.
The general consensus seems to be that the the difference between 1.3v and 1.2v is insignificant, but that 1.5v might be problematic - and why risk it since NiMHs are readily available?
(I'm not offering citations for this assertion. Please do your own research. I am not responsible for any conclusions you draw based on either what you read here or elsewhere. I'm only expressing my conclusions about what I've read and learned.)
This makes perfect sense to me, so if you want my recommendation, that's it.
That said, I can also tell you that I have run many calculators, even rare and valuable ones, on alkalines without a failure. The list includes all my HP Classics (two 35s, a 45, two 65s) and HP Woodstocks (21, 25), and all my TI's that take the BP1 and BP1A packs (including the really rare SR51 model 1). I don't personally consider it risky, but that falls short of making a recommendation to you.
I also offer, without attribution, this quote from the email I exchanged with the EE mentioned above. I asked about using 3 alkalines in an HP35:
"My take is that if such over-voltages didn't instantly kill the device, then heating would be a more real concern. By the numbers, an AAx3 pack delivers 25% more voltage than the HP expects for input. That suggests about 56% more power should be dissipated as heat inside the calculator. Normally, the electronics should last almost forever, overheating could lower expectations.
On the other side, the charger/wall-wart that comes with the HP-35 should deliver something like 4.5V 5.0V, anyways. That's because you need a higher voltage to trickle charge the NiCAD battery. So, it's quite possible that when you have the calculator plugged in, it's actually receiving the same voltage as with the AA pack anyways, and if so, would have to have been a design consideration. HP would have built it to withstand such operating conditions for extended periods.
All in, I do expect that the impact might be a slightly shortened lifespan, but doubt you'd see it. I don't suppose you operate them as frequently as HP intended them to be used."
I only assert that these were the EEs words; I do not assert that they are correct.
In sum, I believe it is safe to use either 1.5v alkalines or 1.3v NiMHs in my vintage calculators. You will have to decide for yourself what level of risk you're willing to take with yours.
Charging In The Calculator
A lot of people also ask if it is safe to use one of my HP or TI adapters/mods to put replaceable, rechargeable batteries in their calculator and then use the calculator's power supply to charge them. The answer is "yes" for NiCad and NiMH batteries, and "no" for Lithiums. (Obviously, it's also "no" for any non-rechargeable batteries.)
Note, however, that the "trickle charge" used by vintage calculators to replenish batteries takes time proportional to battery capacity. It took around 8 hours to charge the original ~700mAh batteries, and today you can get NiMHs with capacities of 3500mAh. Do you really want to take 40 hours to charge your batteries?
Instead, take them out the adapter and charge them in a modern recharger made for them. It will take 15 minutes to 2 hours depending on how "smart" it and the batteries are. Modern battery technology is WAY better than in the olden days!
Charging in External Chargers
Because it took 8 hours to trickle-charge batteries in the calculator, HP and TI used to sell external chargers that zapped their proprietary battery packs faster, with more power. I would strongly recommend you DO NOT load one of my adapters/mods with modern rechargeables, and then try to charge them in one of these external stations. You could damage the batteries and/or the charger.
Again, recharge modern batteries in a modern charger made for them.