HP-21 battery drain


Hi, and thanks in advance for helping me!

My HP-21 have a problem, when i put fresh NI-MH batteries in it, and don't use it, 24 hours after batteries are dead. So the calculator is eatin very fast my batteries when off...

But when on, i estimate batteries last the same time as a "normal HP-21"

I don'T know the story of the calculator, so i don'T know if there was any modifications on it.

The calculator itself works perfectly and is in mint (...) condition...

Michel Beaulieu

Edited: 25 Jan 2005, 4:51 p.m.


I also realize that hte batteries in it are new NiMh batteries that i charge with the normal HP-25c charger

Coud't it be possible that NI-Mh are not well charged in the HP-21?


Michel Beaulieu


FWIW: I charge my nimh cells outside the calc and they have a long life. I think it's safer that way. You might try that as an experiment once.


Ok, i'll try your advice. The only drawback i can see is that everytime i remove my battery from the holder i pray for not breaking the central plastic that retain batteries.


do you use the HP-21 *that* often?

I seldom use mine and I find that alkaline (non rechargable)
cells are much better (they last forever).



Hi, Michel;

I have the HP21 schematics in hands right now (no, it is not an HP-original, it is a "volunteer" achievement, and there are no credits mentioning the author) and there is nothing but a series resistor connected to the batteries when the calculator is switched to OFF. The circuit ia almost like this:

                      /- - - - -| ON/OFF SW
_____/ ____ |
| | |-/
AC from___|\|__|_/\ /\ __|_____/ ___
charger |/| \/ \/ |
D1 R1 _|_
_ +
___ BATT
_ -

The series resistor is the only 1W (big guy), 8.2 ohms resistor that is connected to the charger when charging the calculator while it is switched to OFF. When ON, the resistor is bypassed, while the charger is used both for charging batteries and feeding the circuit. So, the only possible problem (if everything else is correct) resides inside the batteries themselves. OR (big OR) a problem with the ON/OFF switch (tabs printed in the keyboard PCB, must remove the plastic rivets).

Yes, I can send you a copy of the complete diagram (I cannot remember where did I downloaded it from), just drop me a line (I lost almost all of my friend's e-mail addresses... Sorry). I also have some news about your 41 MBoard: the equipments I borrowed (a two-trace oscilloscope and an eight-channel capture board with a digital-analyser Software) will be with me at the end of this very week. The "lender" needed them in the first minute after he offered them to me.

Hope this helps for now.


luiz (Brazil)

Edited: 26 Jan 2005, 12:35 a.m.


Hi Michel,

VP's suggestion for regular alkaline (non-rechargable) is a good one. Even better is to use the Energizer Photo Lithium AA (non-rechargable) batteries. They last several times as long as alkaline. Just be sure you don't accidently try to recharge them.

I very seldom purchase alkaline AA or AAA batteries anymore - I just pick up several packs of the Lithium when they are on sale and they last forever. Granted, they are a lot more expensive but every now and then Target has them on sale.



Charge the battery pack (in the calculator, if that's what you normally do), then take it out and leave it for a day or so. If the batteries go flat when you do that, then either they're defective or you're not charging them properly.

Assuming the batteries hold their charge when you do that, then there is some leakage in the calculator. I don't think there's any capacitor across the battery terminals before the on/off switch that could be leaking.

I would open the machine up (2 screws under the feet) and see if anything has been added. It's possible somebody has stuck a capacitor or zener across the battery contacts (in a misguided attempt to prevent damage if the battery goes open-circuit, although only the -C models suffer from this!). You could also connect the battery pack to the machine with a milliammeter in series to see if any current is drawn when the machine is turned off (it shouldn't be!). Maybe there's leakage round the on/off switch (liquid spilt into the keyboard?)

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