NiMH batteries in the HP48 and 49 series


As I get exasperated with how my 49G+ gobbles batteries, and I fret about the the cost and the waste, I am contemplating investing in a few of the highest rated AAA NiMH batteries I can find and seeing how I make out. I understand that the main beef with rechargeables and the 48 and 49 series is that they can conk out without adequate "low bat" warning and you lose the memory of the calculator. As the life of newer rechargeables increases, I am wondering if anyone has had better luck with the NiMH solution as opposed to plowing thru piles of alkalines. I find that just sitting around doing nothing is enough for my 49G+ to kill a set of batteries in a few short weeks. The 48G is much kinder to batteries, but still once under regular use it gobbles the alkalines too.



I posted some info about this on comp.sys.hp48 last year:

I used a variable bench power supply to run 3 calculators, and reduced the voltage until the low battery indicator came on in the display. These are the results.

Calculator       Total voltage when             Per cell voltage when
low batt came on low batt came on

HP-48G 3.20 volts 1.067 volts
HP-49G+ 3.45 volts 1.15 volts
HP-50G 4.85 volts 1.2125 volts

If you replace your (alkaline) batteries when the low battery indicator first comes on, the HP-50G will have significantly shorter run time.

See the discharge characteristic at:
and you will see that if you only discharge to 1.2125 volts per cell, you will have used less than half the energy in the cell. Also, note that the HP-48G in allowing the cell voltage to drop to 1.067 volts has used about 85% of the energy in the cell. (These are eyeball estimates)

The high turn-on voltage for the low battery indicator on the HP50 may be a good thing when using NiMH or NiCAD batteries.

If the low battery indicator turned on at slightly more than 1 volt per cell (which is good when using alkalines), the NiMH cells would be on a very steep part of the discharge curve, only minutes away from exhaustion.

But the HP50's low battery indicator comes on at 1.2125 volts per cell (my HP50 does so; I don't know about any others). Eyeballing the NiMH discharge curve, this looks to me like the cells would be about 70% discharged, so there is ample time to change them.

The slightly lower per cell voltage when the low battery indicator comes on will mean that NiMH cells will be more discharged in an HP49G+ than in an HP50, but I think you will still have a bit of time before they completely die.

Note that the HP49G+ has a lithium memory backup battery, so that even if the main batteries die, you won't lose your memory contents, although some of the settings may be lost if you leave the main batteries out for a long time.

I've been using NiMH's in my HP50 with great success for about 9 months now.



I've been using 750mAh to 1100mAh NiMH cells in about all of my 48/49/50 series. Only my HP48SX, the oldest of them, is still powered by regular alkalines.

I cannot complaint. Although the NiMH cells need constant recharging, about one time a month if I use the calculator in a daily basis, I can tell I have not been buying batteries for some time. As a consequence, no disposal chemicals...

My 2¢.

Luiz (Brazil)


Interesting; the other measurements that would be useful would be the current draw under various conditions.

Although the calculators still run after the low battery indicator comes on, if I recall correctly, they'll refuse to do any external I/O.

Unfortunately, my first impression is that my 50g is as much of a battery hog as my 49g+.

I seem to have good luck using NiMH cells in the 49g+ and 50g, but still use alkaline cells in the 48 series and 49G, which aren't so hard on batteries. But if you're going to be doing a lot of external I/O, particularly via IR, it may be better to use NiMH cells in the 48 series as well.

Of course, if you use rechargeable cells, you should have a spare set easily available.

My general impression is that the power requirements (from highest to lowest) are something like this: writing to flash, via IR I/O, via wire I/O (except via USB with the 50g), running a program, on (idle), off, and for the 48 series, coma mode.

Of course the 50g can be powered externally via the USB port, in which case the battery isn't even required.



Also from the newsgroup:

Here's what I posted back on March 24, 2006:

" I made some measurements of the current consumption of the HP49G+ with
standard alkalines installed:

  Condition of              Current
measurement drain

Calc on, doing nothing 18 mA

Calc on, holding ON down 75 mA

Calc running a program 65 mA

Calc on, pressing RED key 30 mA

Calc on, pressing BLUE key 30 mA"


" I have two HP49G+'s. The older one draws 29 uA when off and the newer draws 14 uA when off."

and finally:

"  Here are the measurements from my new HP50:

Condition of Current
measurement drain

Calc off 28.4 uA

Calc on, doing nothing 14.6 mA

Calc on, holding ON down 82.5 mA

Calc running a program 73.1 mA

Calc on, pressing ORANGE key 25.8 mA

Calc on, pressing WHITE key 25.8 mA

So, you can expect about the same battery life with the HP50 as with the HP49G+, although the HP50 will wear out 4 rather than 3."


Hi, Les;

I find that just sitting around doing nothing is enough for my 49G+ to kill a set of batteries in a few short weeks. The 48G is much kinder to batteries, but still once under regular use it gobbles the alkalines too.
I'd guess it is due to the base clock, somehow higher in the 48GII, HP49G+ and HP50G.


Luiz (Brazil)


In powering our data collection system ( we've learned lots about batteries.

Most interesting is that certain units respond differently to the same sets of batteries. Some calculators appear to respond with a low voltage message much earlier than others.

One in particular, will give 8 full hours of use after the first low voltage warning. That is powering two long range (up to 1700' working range!) Bluetooth adapters and an external communication circuit board. The same set of batteries, charged in the same manner, will only last 3 hours past the warning on other units.

Some units will run for 3 days consistently moving in and out of the "low voltage" warning level, and then moving above the threshold.

In short, I've learned never to replace the batteries until the calculator refuses to turn on. I often get several more days out of my rechargeables on my "non" data collector units.



I am thinking of getting the 50G and I don't know how much of a power hog it is. With my 48SX and 48GX the alkalines last a long time so I don't think it's good idea to use NiMH with them as the NiMH has short shelf life.


NiMH cells do self discharge faster than alkaline cells. I think you could do an exchange on a regular basis so you are always on full. I have 2 smart chargers from La Crosse US or foreign voltage through Thomas distributing. I use them in toothbrushes, flashlights, TV remotes, RF headphones, pocket rasios, CD players and even to run a self opening trashbasket!


"even to run a self opening trashbasket!"

Wow - does it open as you (or what it senses to be trash) approach, or does it open periodically and demand to be fed?!?!

PS don't let your calculators get too close!


My niece who cares for me said she wanted a trashbasket with a lid, and found a battery operated one. It has an optical proximity sensor and is supposed to run from 4 D cells. The charger kits I bought for AA and AAA had bodies that used AA to substitute for C or D cells and that runs it..for a long time. we had it near the fridge but it kept opening, so it is nearer the wall. Priced at 100 she bought it at a discount warehouse for 50. Nice seal too.
Sorry Dave,


Panasonic has a new series of NiMHs, the Infinium series (AA and AAA sizes are available). They claim that after one year of storage, this new battery still has 80% of its initially stored capacity. So this could be a good alternative to alkalines and standard NiMHs. I use them in my digital SLR.

Nevertheless I continue using alkalines in my HP48 and HP49.


I have bought a set of 1000mAh NiMH Duracells, about which I am optimistic given the great experience I have with 2650 mAh AA Duracells in my 82240 printer.

After a full charge, Edwin Cordoba's BatStatus reports the cells as having 85% capacity, which makes sense to me since I believe NiMHs have slightly lower voltage than alkalines.

It will be interesting to see how much use I get out of this single charge.



I am finding that with pretty regular use a full charge gives me a few days of use at least. BatStatus seems to suggest that the battery drains quickly to about 50% capacity, and after that the drain seems to slow.

Thanks to the backup battery, I don't worry about memory loss. Even if I didn't have the backup cell, I regularly back up my HOME directory to SD card using ARCHIVE and my flags to a list variable using RCLF. That way, if the calculator resets or loses memory, I can recover most if not all of its last saved state.



NiCad cells peak at about 1.4V and NiMH about 1.33 V after charging. This voltage rapidly drops to a fairly steady 1.2V for about 90% of the charge remaining. It then falls very rapdily. Consider it fully discharged at 1V. The 1.2V plateau is probably where your calc says 50% remaining... I suspect it will stay around 50% for quite a long time. Also the battery will self discharge at about 1% a day.


This is proving to be my experience exactly.

I recently went out of town on a business trip and since I couldn't take my entire collection I chose one toy and made it the 49G+, which has seen more use in the past two weeks than in the last year. With quite a bit of use this week BatStatus reports a capacity of about 50% and it has hovered around there for a couple of days.

With the decent quality of the NiMH batteries, the backup wafer cell, the availability of pretty rapid chargers (my Sony camera charger will give the cells a full charge in a couple of hours, albeit only two at at time), and the ability to back up regularly and quickly to an SD card (and keep and mind that the smaller capacity cards which are more than adequate for calculator use are now dirt cheap), I can think of no good reason at all to go back to using alkalines, unless I am in a real pinch.

This was a really good discovery to make. Until now, I used the 49G+ only lightly, since the high battery drain seemed so wasteful. Now, I don't worry about this. Of course, now that the calculator is seeing more use, it may be a matter of time before the infamous keypad failure sets in. I have put off getting a 50G--frankly, it seems to me that it is basically the same calculator, albeit with an improved keyboard and some extra goodies and USB power if desired and a broader repertoire of interface options.


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