NiMH bateries for HP-41 and 71

Hi, just a quick info for those using the actual machines at least aside the fantastic EMUxx et al versions:

I found a good source for n-cell NiMH batteries PLUS a charger. The nice benefit is that the charger is for AAA cells (the one the 71b likes) and with a little space charges the N-cell batteries. Only problem - it is in the US and I do not know if the company has any EU or UK thing to sell.

Here is the link to the charger and batteries

Scroll down about halve way and you will see it: the part number for the charger is P/N 850-0047 and the part number for the batteries is P/N 850-0049 (4-Pack).

Additionally, the company is super responsive and fantastic with Customer Care!




that's a great find. sooner or later the n cell is going the way of the dinosaur. i have nicads and a charger but your find will probably kept a lot of 41s (and those gameboy thingers) on the job.



I have been interested for a long time in rechargeables for the 41 calcs, and have never known the NiMHs even exist.

This seems like a great solution. I think one would need two sets, though, one set charged all the time. I believe the last bit of juice in an NiMH drops off fairly quick and you would want to pop in a fresh set ASAP to preserve constant memory while rejuicing the other set.



NiMH batteries are 1.2V each, the voltage at which my HP-41 says the batteries are low. New alkalines are about 1.6V, and a set lasts me a couple of years.


Thanks Garth,

I have found a Canadian distributor close to where I work, and was all set to spend CDN32 for the charger and CDN17 for 4 packs of the cells, plus taxes.

Alkalines last me about a year, but that is with pretty regular card reader use.

I have also found that when the low battery icon is in the display there is no way the card reader will function properly. Moreover, Randy Sloyer recommends changing the alkalines annually whether they are low yet or not.

The bottom line about the mimio N cells--I think I will hold off.

That said, I continue to be a strong advocate for using NiMHs in the 49G+ (and presumably the 50G when I get one). I have been thoroughly delighted with the performance of my formerly little used 49G+ since I got rechargeables for it. I was reluctant to use it much before since it consumed alkalines like mad. Now, the gold paint is beginning to wear off, I use it so much!



makes sense. Just FYI, I used those NiMH in my 41 with Cardreader with no problem. And it just 'feels' better for me to use rechargeable batteries.


Garth said:

NiMH batteries are 1.2V each, the voltage at which my HP-41 says the batteries are low

True, NiMH cells are nominally stated as 1.2 volts. But, that's somewhat misleading as most NiMH cells reach 1.4 volts when fully charged. Referring to the 41 documentation, HP has this to say about the supply voltage:

2-38: The power supply circuit monitors the input voltage level by comparing it to a reference voltage generated internally. A differential amplifier in the IC senses whether the voltage has fallen below 4.2V at the IC, and if so, it grounds the LLD line connected to the CPU.

4.2V distributed over the four cells would mean that the low battery indicator turns on at 1.05 volts per cell, well above the nominal 1.2 volt level of NiMH cells. But, it has been my experience that the 4.2V level in practice is a bit low as I see units turning on the BATT indicator at 4.4 and 4.5 volts which works out to ~1.12 volts per cell. All things considered, it's still well below the 1.2 volt level of a NiMH cell.

So, that works for me... and yes, I use both NiCD and NiMH N cells to test 41's. Even when buying case lots of alkaline N cells, those little monsters are still quite expensive.


Thanks Randy, I will reconsider then.

The only drawback is this could turn into a slightly more expensive proposition--the supplier in Mississauga is not going to wave his shipping charge, even though he is about 10 minutes from where I work, since he has to order the stuff from the US anyway!

Still, two sets of NiMH cells, the charger, and 15CDN shipping cost still works out to less that what some people are willing to pay to obtain and refurbish the original NiCad pacs for the 41 series.

I wonder if there is any way to fashion little conductive spring loaded sleeves for the little suckers so thay fit in NiMH chargers that accomodate AA and AAA cells? I really like the NiMH quick charger I use for the AAAs in my 49G+



I really like the NiMH quick charger I use for the AAAs in my 49G+

While it may be fine, the charge rate might be higher than what is required for the N cells. Most AAA's are around 850 mah, the N's are typically 400mah. If possible, compare the charger output with the cell charge rates - which are usually 1/10 the cell mah ratings. Quick charge rates can be as high as 1/rating.

I wonder if there is any way to fashion little conductive spring loaded sleeves for the little suckers so thay fit in NiMH chargers that accomodate AA and AAA cells?

The springs are usually present in the charger, if so, all that is required is a conductive spacer. I cut wooden dowels to the required length that I stuck copper foil across. Crude but it works.

Edited: 10 July 2007, 10:03 p.m.


"I cut wooden dowels to the required length that I stuck copper foil across. Crude but it works."

Slightly less crude perhaps: what about making your spacer out of conductive material itself - a piece of copper or aluminum rod?


I really wish that mimio sold directly outside of the US.

I am finding that the supposed mimio authorized resellers in Canada offer at best a partial list of mimio products and their pricing is all over the place.

I am really keen on the 12-pack, which seems unavailable here.

Peter, at the risk of opening the floodgates to similar requests, if you place an order to mimio on the future and wanted to add a 12-pack of cells to the order to forward to me for fair price, I wouldn't say no :)




I think it might be worth while to consider the following, as I have already all the cells and chargers I need (i ordered the 12-pack and a charger and they were so kind to send me two of the n-cell inlets to use their AAA charger for the N-cells, so I'm all set...)

Lets see if there is a greater interest outside of the US for those Nimh cells from Mimio. If so, I could make a bulk order and then ship the things to Canada. If there is interest, we can figure out the total shipping cost etc. However, I wonder if it would not be possible to send an email to them in the US and ask to buy those things in Canada. All people I contacted at Mimio have been outstandingly helpful (probably one my top 3 customer service experiences in my life). They might be willing to do something (I can provide my contact person if there is interest, just send me an email through here with MIMIO in the subject line)

Anyway, if there is some interest, I'd be certainly happy to be the conduit.



Edited: 11 July 2007, 6:49 p.m.

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