Re-engineered AA Woodstocks


Hello all.

I just received another Woodstock for my collage. This time, it's the HP-21 (yes, the one I was considering previously).

When I opened up the battery compartment, I was surprised to discover disposable AAs inside the module.

Although these are 1.5v, how much danger is there in using these? Obviously, they'd need to be replaced every time. But, would it be better to replace these with a pair of rechargeable 1.2v AAs? Or would those not be compatible with the HP charger?

Please advise.

Thank you.


I have used AA 1.5Vdc alkaline battery cells in Woodstocks as have several other forum members w/o any problems. When you fully charge AA NiCd cells, they will have as much as 1.4Vdc open circuit (no load) voltage, so there really is not such a dramatic difference between the two types of cells.

Edited: 9 Apr 2012, 8:06 p.m.


So, please clarify.

Which rechargeable AAs batteries are 1.2v (NiCad, NiMH, etc.)?

Would a pair of 1.2v rechargeable AAs provide too low a voltage to power Woodstocks.

Which rechargeable batteries are 1.4v? But, wouldn't 2.8v for the pair provide too much voltage in the Woodstock?

Can either the pair of 1.2v or 1.4v batteries be charged safely and properly when the Woodstock is connected to the HP recharger?

Edited: 9 Apr 2012, 8:17 p.m.


NiCd batteries are nominally 1.2Vdc, but when they are fully charged the voltage is higher. When the open circuit voltage drops to 1.25Vdc or 2x1.25 = 2.5Vdc, the low voltage dots will light up on the display. The original batteries in all the HP calcs with rechargeable batteries are NiCd. Alkaline batteries are not rechargeable, so they start out when new at 1.5Vdc nominal, but even higher open circuit voltage. NiMh batteries are also rechargeable, and are rated at 1.2Vdc nominal. Regardless, you can use any of these types of battery cell safely in a Woodstock calc. You cannot recharge an alkaline cell. AFAIK, you can safely recharge a NiMh cell inside a Woodstock using the HP charger. When batteries are loaded in a calc that is powered on, the voltage across the battery will drop, which is why the battery must show more than the 2.5Vdc voltage for it to power the calculator properly.


"You cannot recharge an alkaline cell"

Actually, you can. I have not used them but I have 4AAA Rayovac rechargeable alkaline batteries that came with a used calculator I bought. They are rated 1.5V. Not sure if Rayovac still makes those batteries, I have not seen them.

For what I have read, alkaline rechargeable batteries can carry a charge for a long time, longer than NiMH and NiCd batteries can. Unfortunately, they have a limited life cycle (which is affected by deep discharge) and the relatively high internal resistance, makes them unsuitable for high discharge current.

Regards, HAL


It might have been Jake at an HHC in San Jose who made a presentation on recharging alkaline cells. Whoever it was; he said that the only thing those fancy Rayovacs have going for them is a good venting system, and that you can probably get about five charge cycles from a normal alkaline battery, if it doesn't pop on the first try. It was not recommended that we use these in irreplaceable RPN calculators, but flashlights or children's toys were good targets for them.


Which rechargeable AAs batteries are 1.2v (NiCad, NiMH, etc.)?

Yes, NiCd and NiMH. That's the nominal voltage, which is the voltage you'd measure when the are under load during most of their life.

Would a pair of 1.2v rechargeable AAs provide too low a voltage to power Woodstocks.


Which rechargeable batteries are 1.4v?

Freshly charged NiCd and NiMH with no load.

But, wouldn't 2.8v for the pair provide too much voltage in the Woodstock?


Can either the pair of 1.2v or 1.4v batteries be charged safely and properly when the Woodstock is connected to the HP recharger?

In theory, yes, it is perfectly safe as long as there are good cells in place and making a good connection. After all, that's how most people recharged their calculators. Nevertheless, it's not recommended. If there's a bad connection to the pack, the calculator may be damaged.

HP didn't care about this back in the day. While the calculator was in warranty, there was unlikely to be a problem with the battery contacts, and if there was, they'd fix it for the customer. Once it was out of warranty, the customer would have to pay to have it fixed. But after the end of the support life, the only repair is by substituting parts from another calculator.


Hello. Enlighten me, please. How is it possible that a combined voltage of 3.0v with the pair of 1.5v AA disposables does not provide an overload of current?


The rated voltage is the minimum at which the component can operate. Try using a battery that measures 2.5Vdc at no load in a Woodstock and see what happens. Another comparison can be made with a classic such as an HP-45, using the AC adapter only. Even though the calculator is rated to operate at 3.75 Vdc, using a 3 AA NiCd cell pack, the AC adapter will supply about 5 Vdc no load to the calculator w/o damaging it. Once the calc is turned on, the battery or dc source voltage will drop. Many calculators can operate on both NiCd or alkaline batteries w/o any problem and all electronic components have an over-voltage tolerance of at least 10%.


Thank you for that very detailed explanation. This tells me the why, how and that is very much what helps me learn about my HP calculators.


Matt; If you have any lingering doubt about overpowering your calculators with alkaline cells you can do what someone here said several years ago. Use slightly used cells. I've done that, but i usually just burn MINH cells that i charge outside the calculator in a "smart" charger. I use de-tabbed, formerly soldered cells in spice calcs so that they are not so long they will stress the tabs.


You can also get low self-discharge NiMH AA cells - I've used those with no problem in my Woodstocks. I use those in my daily driver, the new high capacity NiCd's or NiMH usually go flat from self discharge long before I'd run them down from actual use.

Eneloop is one brand but you'll find them sold by most major battery retailers. The tip off to the fact that they're low self discharge is package markings like "Pre-charged - Ready to Use" or something similar.

I haven't found them in flat-top or solder tab packages, unfortunately, but a local battery shop has welded button top cells into packs for me.

I wouldn't charge NiMH in a Woodstock (or any LED HP) since their charging circuits were designed for NiCd and may damage either the cells or the calculator. Use an external charger designed for that chemistry. Another advantage of that approach is if there's a leak during charging it won't happen in the calculator, and it gives you the opportunity of regularly inspecting the battery pack.



I do like the low-discharge cells. Costco usually has a nice price on multi-packs of the eneloop. I've seen the similar duracell NiMH there as well, but not recently.

I wouldn't charge NiMH in a Woodstock (or any LED HP) since their charging circuits were designed for NiCd and may damage either the cells or the calculator.

That is the safer approach, if you don't break the calculator constantly taking the battery in and out.

However the chemistry in NiMH and NiCd cells is so similar, that the simple low-rate charger that HP provides is perfectly fine for NiMH, and that is a lower risk than constantly messing about in the battery compartment. (IMHO)

Since NiMH has larger capacity than the old 300mAh cells provided by HP, the charge rate will be lower when charging NiMH. Most likely it will result in either a very slow or even a trickle charge rate. Luckily NiMH is pretty efficient so will usually charge even at the very low rates provided by the HP.


The slow charge for both chemistries is specified as C/10 (C is the amp-hour capacity of the cell) and usually needs 140-160% or 14-16hours to full charge. (Even tho C/10 is typically considered the "forever" charge rate I wouldn't leave it on charge for more than a total of 2x the capacity or 20 hours, especially with low self-discharge cells.)

In general faster charge rates are safe for both chemistries as long as the cells don't get hot. Obviously the faster the rate, the more risk of overcharging / overheating the cell if you don't stop charging when full. Even C/5 is usually OK if you don't forget them for too long. If you are babysitting the cells then up to C or 2C is OK but will likely reduce the lifespan of the cells. Any faster than that you'd better have a smart charger and a shield over the cells just in case a cell violently vents.

And the key difference between NiMH and NiCd? When doing a fast charge (C/3 or more), the "full now" voltage does not dip as much in NiMH as it does in NiCd, so fast chargers don't know when to stop unless designed for NiMH. You won't have that problem with the HP charger. It is nowhere near fast.

PowerStream has some good info:


It's the I-left-it-connected-to-the-charger-and-forgot-about-it scenario that primarily makes me recommend against charging inside the calculator. The site you mentioned said:

In a standby mode you might want to keep a nickel metal hydride battery topped up without damaging the battery. This can be done safely at a current of between 0.03 C and .05 C. The voltage required for this is dependent on temperature, so be sure to regulate the current in the charger.

I believe the calculator provides a charging current much higher than this. That aside, I'm generally not nuts about *any* charging circuit that doesn't provide for charge termination (which I don't think any of the LED HP's do?), hence my recommendation to use an external charger appropriate to the battery chemistry.

Excellent point about wear and tear on the battery compartment. I usually get months of use out of a single charge, so that's not an issue for me. I'm not sure which makes me more nervous - the battery contacts on the Woodstocks or the door latches on the classics, but I like to avoid stressing them, too.


Edited: 15 Apr 2012, 4:12 p.m.


The HP charging circuitry doesn't do anything that will damage NiMH cells, nor do the NiMH cells do anything different than NiCd cells that might damage the calculator. The only drawback of using NiMH cells rather than NiCd is that it will take much longer to charge them, because they have so much higher capacity. (That's also true for using modern NiCd that have higher capacity than the original NiCd.)

However, charging them in an external charger is safer for the calculator due to the damage that can occur if for any reason the battery pack isn't making good contact with the terminals of the calculator. That problem exists regardless of the battery chemistry.

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