HP 41 batteries


I've got my HP 41 (CV) and the rechargeable batteries do not charge very well any more. There are some battery holders online (ebay), but read a thread where some handy users replaced the N-batteries with what I think are AAA batteries (I think 4 ea) and where they described having done some sanding and grinding work to make it fit. I am not adventurous to do this and am afraid to kill my battery pack altogether. (mine is a rechargeable module, so I'm guessing this person was talking to start from the non-rechargeable pack). Any suggestions, or since I am not too handy in these kind of things, is there anyone who has done this in the past (and available to do this again?)
Many thanks,


Hi Peter,

If I were you, I would buy an N size battery holder and be done with it. For the more adventurous, you can rebuild the rechargeable packs. I did two a few months ago, but had a little trouble because the springy metal contacts in one had corroded and needed to be replaced. I have not tried, nor see, photos of a holder modified to take AAAs.

You can also sell your rechargeable pack to help pay the cost of the regular holder and let someone else work on it.



Both you and Etienne (see message below) suggest the same. I was thinking in the rebuilt type because of "short" battery life. I've been working with the rechargeable ones, and these didn't last very long (any more, and I can't remember how long they lasted in the beginning). But the non-rechargeable batteries should last well enough. So I'll just buy the battery holder (I only seem to find this on eBay), and fit it with the correct size battery.

Many thanks, I do agree this is probably the easiest solution.


Hi Peter,

- AAA rechargeable batteries will not fit: too long.

- 1/3 AAA will fit

- 1/3 AA will fit with great effort and pain but hold more power. I've done it once and the fit is extremely tight.

Therefore, if you're not absolutely comfortable doing it yourself, I suggest you get a regular battery holder and feed it with either regular N's or N-sized Nicads or NiMHs. You would have to recharge the latter externally though.



Edited: 12 June 2010, 2:11 p.m.


Etienne, thank you to you as well. I will as explained in my response to Dave buy a battery pack and fit this with the N- Nicad batteries.
Many thanks to you to Etienne.
Thank you both to your support to the HP Museum forum !!!



NiCads are 1.2V, meaning my 41's low-battery indicator would never go off. With alkalines, I normally got two years to a set, but it dropped to about six months when I got the double extended memory module. That's still way, way longer than it takes NiCads to self-discharge, even if they did have a high enough voltage.

Edited: 13 June 2010, 5:45 p.m.


Anyone know where to buy a holder for N batteries? I'm looking for a generic holder to use for a charger, not one specific for the calculator. I can put together the charging circuit, but I would rather not try to cobble a battery holder.

-J. P.


Keystone makes them and you can order them in single quantity from Mouser, but apparently they don't normally keep these in stock.

Edited: 13 June 2010, 10:31 p.m.


This place has an incredible selection of battery holders, including several holders for 1, 2 or 4 N cells. They have a really lame order form page and prices too in a weird way. Batteryholders.com, of course!

Edited: 13 June 2010, 10:54 p.m.



about four (maybe five) years ago I started using a pack of four Sanyo Cadnica 150mAh N-cells, and they are still working. I have at least five working HP41´s (halfnut and coconut units), and two CX kept without batteries (the internal clock is not a good friend of rechargeable batteries...), so using disposable batteries is not an option. I also have three 82242 IR modules, three card readers and one 82153A (optical wand), and all of these guys use battery power. Using rechargeables makes perfect sense, right?

Now for the facts. The current capacity in batteries is measured in Ampere X hours, and the nominal value (e.g. 150mAh in this case) means the maximum current in one hour till the nominal voltage in the battery terminal drops to 90% (I also found 85% in some literature). So, if a NiCad, 1.2Vcc, 150mAh battery keeps a 150mA load, after one hour its terminals might be measuring about 1,08 (or 1,02 if 85% is the factor). The 150mAh pack I have still keeps an HP41CV memory contents (no modules at all) for about three to four months before the battery indicator lits.

I ended up buying a brandless Chinese pack of four NiMH 400mAh, which lasts less than double the time of the four Sanyo. I was also given twelve 190mAh Sanyo Cadnica (three packs of four) that work almost the same time as the 400mAh Chinese units.

But the secret lies in the way you charge them, not the way you use them.

The average 1,000 charging cycles applies only if you respect the '10% nominal current' charge limit while charging (that's why the charging time is always 10 hours). Any fast charger will charge the batteries according to a non-linear charging curve, but chances are that fast charging will also reduce battery life or discharging characteristic. Also, many of the standard charges apply high current to the batteries, and most of them heat the batteries up somehow. I measured a regular charger right after getting my first 150mAh NiCads and it gave me almost 100mA charging current. It would kill the batteries very soon, I guess.

While measuring the charge current I noticed that the charger had about 3.8Vcc in its terminals with no batteries, so I measured its current and voltage in many circumstances and I realized it was just a passive current source. So I used a pair of NiCads in series (about 2.4Vcc) and added a small series resistor. Please, look at the pictures: (link to photobucket)

Assembly with two batteries

I used two-side coated PCB´s to accommodate the resistors

Assembly with batteries and resistors

As I did not open the charger to see the circuit, I simply tested with two different resistor values, measured their voltage drop with the two NiCads in series then I computed the correct one that would give me something closer to 15mA needed to charge the two NiCads (47 ohms) and another one closer to the 40mA for the two NiMH (27 ohms - should be smaller, about 23 ohms, but I decided to use a higher, commercially available value, hence a smaller current).

The batteries never heat up after their ten hours charging time (they do not even get warm to the touch) and their discharging periods did not significantly reduce so far.

Hope this helps a bit.

Luiz (Brazil)

Edited: 14 June 2010, 1:59 a.m.



Nifty idea, and nicely done! Yes, high charge rates result in high heat and that's what kills batteries. Some nicads tolerate heat better than other and these are rated for faster charging, but the best rule of thumb is to not charge at a rate more than C/10 where C is their capacity in mAh. Nimh cells are similar but to maximize their life you need to not fully charge nor fully discharge them. In fact, hybrid cars like the Prius carefully control their battery charge/discharge to vary +/- 15% around a set point of about 60% of a full charge.



Hi, Katie;

the Prius carefully control their battery charge/discharge to vary +/- 15% around a set point of about 60% of a full charge
Thanks for the valuable information! From now on I´ll charge the NiMH´s for no more than 7 hours. I measured their charging current and found about 36mA with the 27 ohms resistor. Considering that the batteries voltage increase as they charge and the series resistor does not significantly changes its value, the charging current tends to reduce as the batteries charge.

Thanks again!

Luiz (Brazil)

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