Rechargeable 2/3 AAAs in 41C-series

Inspired by this recent thread, I acquired eight 350 mAh 2/3 AAA batteries quite cheaply on TAS. Actually, they started out life as 9.6V battery pack for a radio controlled plane, but it was a simple enough matter to harvest the eight flat-top cells from the pack.

As reported by Luiz in that thread, they are about the same length as N cells, but a bit narrower. They work fine in the calcs, but the only glitch I have encountered is that occasionally the positive-end button doesn't make adequate contact with the spring tip in the holder, and I have to line it up just so before reinserting the lot into the calculator. I have been testing things out with some pretty heavy use of my card reader, which draws batteries more so than any other peripheral I have, and things are going strong--not even a flicker of the BAT indicator or objections from the card reader.

I am waiting on a bunch of AAA-to-AA converter sleeves (see here) to aid in charging. I intend to make up the centimetre or so space in the holders with springs or little balls of foil. In the meantime, I charged the cells successfully in my very good LaCrosse charger. The cells fit width-wise in the AAA grooves (of course), and I made up the space with little foil cylinders. This is NOT the best way to charge these cells--the lowest current the LaCrosse offers is 200 mA, which is fine for AAs of over 2000 mAh capacity, but is a little steep for cells in the 200-400 mAh range. That said, I initially discharged and charged the cells fully before use, and even at that high current they didn't get warm. Of course, I paid close attention--I have cooked a few NiMH AAs in my day in so-called rapid chargers, so I have learned my lesson. That said, fashioning some PCB resistors like Luiz suggests to cut the current and lengthen the charging time seems wise.

Just thought I would share this info with anyone interested. I am looking at the CL upgrade for my CV. The CL puts a higher draw on batteries. I hate chewing up alkalines, so it will be interesting to see if this is a viable way of keeping powered up.

Edited: 14 June 2012, 4:01 p.m. after one or more responses were posted


Hi there,

Although not directly related with the battery/charger type selection for the HP-41, it may well worth to mention this note regarding power from USB port.

It is certainly useful for desktop operation (I use it all the time) and *extremely* cheaper than any battery type... :-)

Note that although USB-41 module also provides HP-41 power thru the USB interface, it CANNOT be used to run the Card Reader

All the best.


(edited: typos)

Edited: 14 June 2012, 7:07 p.m. after one or more responses were posted


Ok, so now my post for some competition here :-)

If you don't want a PC all the time running in parallel while you use your HP-41 or if you don't have a power adapter with an USB outlet so comes my solution here.

Basically, all you need is a spare HP-41 module, some tinkering and soldering. Any power adapter with 6V DC does the job. Instead of using plugs for the connection to the "power module" you may solder the wires of the power adapter directly to the connectors in the module.




I admit I didn't know anything about a manufacturer named "LaCrosse", but obviously you are using a charger that resembles quite closely a device that is as well quite popular in Europe, or at least in Germany: the BC900 or one of its companions. This is a quite sophisticated processor controlled charger that uses the well-known -dU charge termination control. This means that the charging current has to exceed a certain minimum in order to make this technique work reliably. You said:

the lowest current the LaCrosse offers is 200 mA, which is fine for AAs of over 2000 mAh capacity, but is a little steep for cells in the 200-400 mAh range

I would say that charging a 2000+ mAh cell with just 200 mAh, i.e. a relative charging current below 0.1 C, will not result in a reliable voltage drop that could be detected by the -dU control. Here currents > 0.3C are recommended, maybe a bit less for small-sized cells since they warm up faster (which essentially causes the voltage drop, indicating the right moment for charge termination).

I think 200 mA is "just right" for your charger and a 400 mAh battery - unless the manufacturer says that 0.5C exceeds the range of recommended charging rates. For instance, let's take a look at the GP40AAAM data sheet. It says this battery can be charged with 200 to 400 mAh in a charger with automatic charge termination control.



I use a Maha MH-C9000 charger. They specifically recommend against using charge current less than 0.33C for exactly that reason: "Charging too slow may prevent the charger from terminating correctly.". For a 2000 mAh cell, that would be a minimum of 660 mA. Since the MH-C9000 is configurable in 100 mA increments, the minimum I use when charging 2000 mAh Sanyo eneloop AA cells (2000 mAh NiMH low self-discharge) is 700 mA.

I use the 200 mA setting when charging 750-800 mAh AAA cells.

If I had to charge 200 mAh cells, I'd try to find a charger that had a 50-100 mA setting, or build a custom charger.


Diego, I must confess that USB41 is very tempting, especially since you added TRACE mode, which I use all the time.

I will give it some thought. There has been some personal upheaval for me in the past 24 hours in my family, so I may put further HP41 expansion on the back burner for a bit.


Hi Les,

Hope everything end up well for you and your family.

As previously noted on another post USB-41 first 20 units were sold out, so there's no rush whatsoever. It'll be by end summer or so when I'll start preparing another production run.

Will place an ad at the corresponding section as usual.




Hi Frido,

Good to read from you, and certainly no "competition" here... ;-)

Both ideas go to the same point: an external reliable power source and a module housing to feed that power into the HP-41.

Althogh I built and sell about 24 such USB-power cables by mid 2011, I no longer produce them. Mostly because I think that HP-41 modules and connectors are too scarcy and hence too valuable to use them with that single purpose.

The idea is nonetheless perfectly valid regardless the power source chosen for the project and it's also very easy to build so almost any user with minimal soldering skills can face the task and save a few bucks in batteries... :-)

Enjoy your 41's!


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