Using High-capacity NiCad/NiMH cells in Vintage Calculators



I bought some AAA-size NiCad and NiMH cells and I intend to use them in vintage calculators. I remember reading that the original NiCad cells were 250 mAh, 1mA max. recommend load current. The new AAA NicCad are 600 mAh (the lowest capacity I found) to 900 mAh, 300 mA to .5A, max. recommended load current. The recommended load values were suggested by the seller.

Well, I would like to know if vintage owners/users have changed their original battery cells for new ones. If so, did you also change the charging procedure? I'm using external charger to charge the batteries I'm using in an HP31E because I do not want to use it's internal charger with batteries with a capacity that is higher than the original ones. I do not know how does the Spices' internal charger work, so I am not sure about how long will the new batteries need to be in "active cycle" for their full charge (and this way avoiding batteries' charging known problems) and what consequences should be expected in charger circuitry if we are using batteries with different internal resistance. I did not even dare charging two NiMH, as I see some 3-hours, intelligent chargers do not specify charging times for NiMH, only passive chargers do. And in this case, NiMH batteries demand almost twice the time to charge.

Does anybody have better technical information than my suppositions? Maybe I'm wrong and there is no trouble, but I do not want to risk the power circuit of an HP25, an HP55 and an HP31E...



I have rebuilt the battery pack for the HP 34C by using 2 Radio Shack NiCd, 600mAh cells in series. These fit nicely in the battery area and the solder tabs make good contact with the unit's tabs. I recently charged these in the calculator with the stock charger for about 1 1/2 days and it seems to be working well. Of course, there may be problems later but so far the batteries have held their charge in the calculator for about 1 week to 10 days now.



please, forgive-me: in the first post, where you read:

I remember reading that the original NiCad cells were 250 mAh, 1mA max. recommend load current.

please, read:

I remember reading that the original NiCad cells were 250 mAh, 100mA max. recommend load current.

Thank you and sorry.


Higher capacity rechargable batteries shouldn't hurt electronic equipment. The equipment will draw the current it always did, the charge would just last longer. If a piece of equipment would operate for 5 hours on 250 mAh, then on 500 mAh, they would operate about twice as long, assuming that the equipment draws a steady current.

Charging of higher capacity batteries is a totally different issue. The charging current to the batteries determines how fast they will charge. A rough rule of thumb to calculate charging time is to divide the battery capacity by the charging current and multiple by 50 percent. For example, a 250 mAh battery, charged at 100 mA would take about 3.75 hours to fully charge. At a 50 mA current, it would take about 7.5 hours to fully charge. Using higher charge current on batteries not designed for it will result in the battery overheating during the charge cycle and shortened battery life. New high capacity batteries are designed to take the higher current of the fast chargers.

As to using higher capacity batteries in the calculator. It should be fine. The calculator will operate for a longer period of time on each charge. If the original HP charger is used to charge them, again no damage "should" occur - they will just take longer to achieve a full charge.

I would not recommend using higher capacity chargers to directly charge the batteries thru the calculator. I do not know if HP put a current limiting circuit in their equipment or not. If they did, then no harm would happen, since the circuit would limit the adapter to provide a set limited amout of current to operate the calculator and to charge the battery. Of course, in this situation, since the current is being limited, there would be no improvement in charging times.

Since HP makes a point of saying to limit the current to 100 mAh, I would think they did NOT provide a current limiting circuit and that a higher current adapter could harm the calculator.

To be safe with high capacity batteries, I would suggest either increase the charging times using the original HP adapter or remove the battery pack and charge it separately using a fast charger.

Hope above helps.

Maybe someone else here knows more about HP charging circuits used.


One other quick note. In calculating charge times, the current listed on the adapter may not be the current value to use. If the adapter is directly connected to the battery, then the current listed would be currect to use.

But if the adapter is connected to the equipment which houses the battery pack, then the equipment may very well limit the currect to a set value. In that case you'd need to know that value to calculated charging time.

One last note - some battery packs have built in limiting circuits. In these cases, even hooking a higher current adapter directly to the battery pack will not shorten the charge time.


I use NiMH cells in all my rechargeable calculators (classic, woodstock, spice, etc). The standard HP chargers works fine. Basically if you have a slow charge NiCad system like the HP calculators, NiMH cells will work just fine.

It takes a longer time to recharge the cells in direct proportion to their incrased capacity. Original AA cells were 600 maH. My 1800 maH cells take 3 times as long to fully charge. You do not have to worry about fully discharging/charging NiMH cells like you do with NiCads.


I've discussed this in previous threads, but it seems a good time to bring it up again:

I've been rebuilding almost all of my calculator collection with AA lithium batteries. This allows me to pluck any calculator off the shelf and have it run well. (I still have some original (rebuilt) NiCd packs of course). Even my daily use HP 67 is still on its first set. (I build them to fit in the calculators without the original battery frame, so I can keep the original rechargeable for resale, etc.)

As I've been warned: you don't ever want to accidentally charge these batteries. (Not a problem... I don't even have the chargers near the calcs, and they always work right off the shelf.) There's also the concern about their leakage.

I found some halloween toys that used these batteries that were left on for at least two years. No leakage. In fact, I've still never seen this type of battery leak. Nor has anyone ever mentioned on the forum that they've seen this type of battery leak. With up to 5 times the capacity of alkaline batteries and at least a 10 year shelf life, they're still my preference in the calculators. Still... it's an ongoing experiment. At least the track record for leakage has been perfect, certainly better than NiCds or alkaline AA cells.

I'll let you know if anything bad ever happens, but so far I'm so happy with the results, that nearly all of my calculators now contain them. They're expensive, but I've found them at the office store for $8.99 a four pack, then up to half off with coupons. A little grinding and they pick up solder instantly, so I don't think heat has ever harmed them either.



Lithium cells are GREAT. And they are more durable. But that is because they are also more hazardous. A leak from a Li-ion cell is a nasty leak for health reasons as well. It is true they are packaged better, but that is because of this very reason. Would I use Li-ion? Of course, but bear in mind the extra precautions. Actually, a Li-ion cell can ignite in open air if cut open. Just some extra info on the subject.


The lithium cells that Dr. Mikey uses are not rechargeables, but a lithium-manganese primary cell that produces 1.5V output... compatible with standard AA cells.


... and we used NiCd for years...


The cadmium in NiCad batteries tends to stay put even when the batteries leak. The problem arrises when 6 billion people toss their old batteries in the land fill/incinerator and spew it into the environment.

An intresting aside... my old company was supplying some electronics to IBM. Our contracts specifically stated that we would not use cadmium anywhere in the product (batteries, metal plating, etc). This was apparently their requirement for at least 30 years. Now the EU is trying to ban lead based solders. And don't forget that 6+ pounds of barium in every color CRT.


I have used lithium batteries for several years in all sorts of equipment. Never thought about the HP Calculators. What would be great would be to design a replaceable battery holder that would fit into the old classic series. Load it with lithium's and go forever on a set. We use lithium's in all our digital cameras at work and I'm really amazed how long they last.

I also have never seen one leak. Almost all the new rechargable battery packs are lithium now days. I remember when I first saw a rechargeable lithium pack. It was for an Amateur Radio Icon Handheld, the IC-02/03/04 series. The Canadian firm would only sell it as a kit, with the individual cells not installed. This was due to the potential explosion possibilty. I had to assemble the pac including the charging circuits. There was a disconnect piece to it that you were to remove if you flew on a plane since it was illega to fly with more than two cells connected together. Removing the piece opended the circuit between the cells. There was a fear that if the pack shorted out on the plane, it could explode. I never tested the theory out!!


Glad to hear support for these nice disposables.

It sounds like the "bottom line" is to treat them with respect and never attempt to charge or allow them to short. (I don't think this is a particular disadvantage, as NiCd's and NiMH batteries are also quite capable of doing nasty things when shorted!)

I'm waiting to hear if anyone's ever seen one leak. It makes sense that they're built better to prevent leaks, as a leak would be nasty....but maybe not TOO bad. I've seen devices unharmed with slightly leaky button-cell lithium batteries left in place. And again, no battery is without that risk.....

It is amazing to not have to charge my calculators and have them run for a LONG time (never had to even replace one yet). For this advantage of having them ALWAYS ready to run, I'm willing to take on some risks.... hopefully, with all the batteries sold for the digital cameras, they don't spontaneously combust. Either it's never happened, or Eveready does a nice job with covering up any incidents...... !


Just to corroborate some of the info in this thread:

The cells Dr. Meyer is referring to are Energizer Lithium AA 1.5v cells. They are a specially constructed "consumer-driven" battery style-- the chemistry of prior cells using Li-Mn technology were 3v nominal primary cells. Energizer is the one manufacturer I know of currently making these 1.5v jobs: I believe they developed it or bought the exclusive rights to it. I'm told Panasonic do have a version, but I haven't seen it on shelves yet.

But Li-Mn 3.0v (and Lithium-chloride 3.6v) technology has been used by our calc community for a long time... if you have any devices using CR2032's or 2016's or many of the other "coin-cell" lithium batteries, that's what they are.

Other than an extremely fast discharging (a short), I can think of no condition that could arise in which these cells would ever cause damage to a calculator.

Note also that the "cmos" backup battery in your PC, laptop or PDA computer will almost always be a Lithium primary cell; a 3v coin cell, or, as in most Macs, a 3.6v "1/2 AA" short cylinder cell. These have proven VERY reliable for these low-drain applications.

Why a primary Lithium instead of a rechargeable NiCd or NiMH? Because rechargeables, even whether used-down regularly or hardly ever drawn upon, can only stand the pressures of constant trickle-charging for a limited time. The NiCd might last a few years; the NiMH is more charge-sensitive and might last MAYBE two years as a constantly-charged backup; but the ideal in a PC is a backup that doesn't even NEED a trickle, and lasts more like six to ten years. That's the Lithium primary.

Also, the 1/2-AA and AA-sized 3.6v (and sometimes the camera-sized cr-2 or cr-123) are used quite often in remote alarm systems for the glass-breakage detectors mounted on ceilings or high-walls. You only have to replace them after 5 years of use or so-- unlike akalines which would make you climb a ladder every 18 months.

I have never seen a corroded, leaky or bulging Li-Mn cylinder or coin-- never. Saft, Maxell, Tadiran, Sanyo, Panasonic, Sony, Renata, Ultralife or Energizer. I handled lots of these, sold trays full of these-- they just don't have this failure mode, UNLIKE alkalines or silver-oxide cells, which do rupture/leak on occasion. Dr. Mikey is, in my opinion, on totally safe ground with these. I think, with his experience of their utility in calcs, he has convinced me to try some in my 33c, in fact.

True, a "short" CAN cause the LiMn batteries to occasionally explode in a most spectacular manner-- like a "black cat" firework-- and I have actually seen this happen once; I tossed a 2032 into a box of dead coin cells and it hit them just right. The result was a loud percussion and a black powder-mark on the wall just above the box. The battery was separated neatly in two like an oreo cookie-- but I'm glad I wasn't hovering my fingers over that box.

But a short is really a rare (maybe non-existent) thing if installed in equipment. I've never seen a Casio databank, for example, damaged by batteries (it uses 3 coin lithiums: 2 for display, 1 for data-backup). Chances are, if you have a car made in the last 5 years, you are carrying a remote keyless entry fob in your pocket. Most of them contain one or two coin-style lithium cells. They undergo real abuse, as you can probably imagine. They don't leak though, and I've never heard of one exploding.

While I was working at a Batteries Plus, I would occasionally tack-weld solder-tabs onto coin cells, mainly for laptop computer CMOS backup-battery replacements. The metal can on these is extremely thin, and so the pulse of the welder is set to a much lower level when working on these than, say, to weld tabs on a NiCd. I believe that the REASON the metal is so thin is for safety: if it DID become shrapnel, it would tear and deform and lose energy there rather than propelling sharp hard pieces... you gun-owners probably know that ammunition cabinets are made of soft containment metals for a similar reason.

Anyway, in tack-welding to them, a couple of times I melted through the thin metal can of a lithium cell (this'll wake you up during a long shift). It spits out a watery clear-grey substance which does not seem to be very corrosive, nor does it much irritate the skin-- BUT it SMELLS really nasty. So bad, in fact, that it'll take several minutes to get the icky sweet, metallic smell out of your nostrils. I have no doubt it is poisonous. Um, don't try it at home.

I have heard of R/C modellers and scale-racing car enthusiasts building pulsed rechargers from schematics on the internet so that they can recharge LiMn cells (CR-123 camera cells usually). This is dangerous-- though the camera-typs cr-2 and cr-123 are designed as wide-bodied and capable of handling large drains, they are NOT intended for recharge. If you are not extremely cautious, you will cause them to develop a hot-spot inside, and then they MAY explode. Radio-control guys live on the bleeding-edge of battery performance, so they do risky things and experiment with all technologies-- which is fine, but is *no* guide to those of us who want reliable and SAFE power sources.


Now, what about Lithium-ION? Well, that is a slightly different set of technologies from the LiMn/LiCl primary cells we have been discussing. Unlike the primaries, Li-Ion CAN be charged and recharged for hundreds of cycles (typical lifetime: two years). It has the highest storage density of any rechargeable; it has a VERY small self-discharge rate; it can deliver to moderate loads without sagging; UNFORTUNATELY, it is still really expensive and requires a fairly sophisticated circuit around it, especially in packs, or it becomes unstable and can explode or catch fire. (Which is, I believe, the reason for the story related by Bill Hemphill earlier-- an early ICOM Li-Ion setup).

Currently, most laptops use them, and most handheld Videocameras, and some PDAs and digital cameras. They will ALL have an ABS fire-resistant plastic case, and ALL have a failsafe coulometry circuit inside them, which will DISABLE THE PACK if any anomaly develops in any ONE cell in the pack. So you may have a LiIon battery that works fine for months until suddenly, it is dead to the world-- this is the fail-safe circuit having done its job of protecting the user and his equipment.

You will see Li-Ion EVENTUALLY in AA and similar consumer sizes, I think, IF nothing better comes along in the meantime, and IF the cost of the cell INCLUDING the charge-limiting, delivery-limiting and failure-protection circuits can be made reasonably cheap. Right now a single AA made this way would cost about $10-15, with a $50-75 charger for it to go into... this hasn't really got the marketing guys all excited just yet.


Environmental note: all RECHARGEABLES (NiCd, NiMH, LiIon, lead-acid, etc) pose a potential problem when considered "en masse".

They all have Heavy Metals, that is, elements that exist in nature in trace quantities everywhere, but when you concentrate them for a purpose like making a battery, you are changing the way Nature decays and disperses their effects.

Lead was a terrible problem for years-- so useful in so many ways to us, but until we got wiser, it was building up in biopotentially harmful ways. Now it's out of paints, fuels, eating utensils and landfills-- and so lead-poisoning is fairly rare now. The Battery Council International (a manufacturers coalition) is proud of statistics which show lead to be the most recycled material on earth-- some 97% of lead used in batteries is eventually returned for re-smelting.

After a heavy awareness set in on mercury's effects in fish and waterfowl, it was taken out of formulations of alkaline and other batteries. Now, mercury poisoning is rare-- except a few places in the world where agricultural chemicals are still allowed to contain the stuff.

Nickel, Cadmium, and Lithium are disposal problems in the making because they often DO end up in the landfill, and thus their concentrations in groundwater become a concern. While these elements play important roles in life-processes, too much of any of them has been shown to increase birth defects and stillbirths. Sterility is suspected to be one effect of excessive cadmium exposure. So communities would do well to support recycling efforts for these materials. You may have to call around in your area to find the proper place in your municipality for disposal of rechargeable batteries-- in some places, they classify them as hazardous waste and seal them in drums to be buried on the high side of the city dump.

(Batteries Plus, by the way, accepts all rechargeable batteries for recycling, and they ship them to a company that does metal-reclamation in a large-scale operation, which makes it economically feasible. This is something I was proud my employer supported so fully.)

Lithium PRIMARY coin-cells and camera batteries and the consumer-sized Li-Mn and Li-Cl have YET to be considered by the U.S. government for either hazardous status or for serious recycling. Some states say they are mere "household waste". A couple of states are of the opinion that they are hazardous, yet have no disposal guidelines for consumers, only industrial users. My personal advice is to put them in plastic bags for the time your locality invites hazardous waste disposal, and give it to them then. To drop it into the trashcan only is to put it eventually in your community's water-table; you don't really want that.



THANK YOU for the wonderful, detailed information about these cells. Months ago I raised this topic and several respondents were uneasy about their use.

Even I didn't think about the huge user base of digital cameras and their track record of safety.

I feel even better about using these, thanks to your response. (And glad that I've collected every dead NiCd cell I've removed for recycling. Great reminder, though.

By the way: the positive terminal of these cells is a raised nipple (as in many user-device cells) and is thus pretty safe to solder, from my experience. The negative end is the bottom of the "can", so I'm hoping it dissipates heat well. When sanded, the solder sticks and flows instantaneously, and I've had no problem here, either.

One last thing I forgot to mention: I usually solder these cells together with a single strand of wire. Though probably "overkill", I figure the wires themselves would make a nice fuse should the circuit ever somehow short. They're short pieces and since calcs don't use that much current, I'm hoping they don't cause any resistance problems.

THANK YOU AGAIN! I can continue to feel confident as I pluck my "babies" (thanks to Luiz!) off the shelf and admire and play with all the wonderful machines without worrying about recharging, dead cells for many many years, and hopefully, leakage. The cells I've been installing lately say, "Use before 2012..."!!



That nasty metalic smell may be quite dangerous.

I read a story (only a story mind) in a camera mag about 10 years ago...

Some guy put some batteries in his shirt pocket along with some other metal stuff (keys, coins or something). He smelt something funny so put his nose to the pocket and breathed in. Some time later he suffered from all sorts of health problems (joints I think) and couldn't work etc.

Now this may have been a scare story, or today's batteries are much safer than they used to be. I can't immagine companies being allowed to sell batteries that (if shorted) would produce highly toxic gas. But on all batteries they do say not to tamper with them.

So in conclusion I don't think there is a problem using Lithium cells in a calculator but I would not risk messing about with them unless I knew the contents were mostly harmless. The cases are leak proof for a reason - this is likely to be costly for the manufacturer so they would not do it lightly.


One of my engineers came from an undersea project at TI that had long strings (like 300 volts worth) of lithium "D" cells loaded into steel tubes. One of these strings shorted out (oops, somebody forgot the liner). The building was uninhabitable for days. The lab was crispy critters.



I can install a "self-destruct" on my calculators. If they should happen to fall into enemy hands, they could be programmed to self-destruct in 30 seconds!


Watching too much Star Trek.


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