hp 67, battery



When I connect the calculator to the charger(with the battery),
it is working correctly, but when not connected to the charger,
I can't turn it on, even if the battery is charged.

Anyone know what might be the problem?

Thank you :)


The battery pack needs rebuilding with new cells.


If you do, you will fry the calculator. The battery is part of the voltage regulator in all HP LED calculators apart from the original 35/45/55/65/70/80 models. 19/25/29/67/31/32/33/34/37/38 are all easily killed when connected to power supply with a dead or dying battery or without battery, since the charger is not voltage regulated and relies on the battery to limit the maximum voltage supplied to the calculator. The calculator is exposed to overvoltage and circuits are damaged if the battery is not good.



The 3 pin charger used with the classic series has 2 outputs, a constant current output for charging the NiCd battery and a constant voltage output to run the calculator. When you plug this charger into a 35,45,55,70 or 80, the battery is electrically disconnected from the rest of the calculator. The calculator logic is run from the constant voltage output of the charger and this does not depend on having a good battery fitted. It is safe to run those models from the charger without having a good (or any) battery installed.

The 65 and 67 as similar except for one important difference. The card reader sense amplifier/motor driver IC is connected directly to the battery back. So in those models, the votlage across this IC when the charger is connected is determined by the battery pack acting as a shunt regualtor. If there is no battery, or a defective battery, fitted, then that voltage will be much higher than normal. The HP65 manual acutally warns that this can do damage. The HP67 card reader IC was redesigned, hopefully to prevent such damage, but I certainly don't risk it!

The later models with 2-pin charagers all use the battery as a shunt regulator to limit the voltage. In the case of the non-contiuous-memory models, there is no danger in trying to charge the battery in the calculator with the calculator turned off (the logic is then entirely disconnected from the battery by the on/off switch). Turning such a non-continuous-memory model on while connected to the charger without a good battery is unlikely to do any damage, the calcualtor draws enough current to reduce the voltage at the output of the charger circuit to a safe level. However the calculator will not work correctly.

The continuous memory models (19C, 25C, 29C, 33C, 34C, 38C at least) use the battery to power the RAM chip even when the machine is turned off. This is how the memory is maintained. If you connect such a machine to the charger without a good battery fitted, a higher-than-normal votlage is applied across the RAM chip -- the RAM chip on its own does not draw enough current to pull the voltage down. This is very likely to do damage. With the machine turned on, it shouldn't do any damage to connect the charger without a good battery, but the machine can't work correctly. I don't risk it.

The Topcat (91, 92, 97) machines have a circuit in them designed to apply a load to the charger to keep the voltage down if it rises too high (that is, if the battery is defective or missing). As with the non-continuous-memory models above, it should be safe to connect the charger without a good battery fitted, but the machine won't work correctly. Now, this circuit only works with the machine turned on, and the (very rare) 95C uses the battery to maintain the continuous memory as you might expect. In the case of the 95C, therefore, connecting the charger without a good battery installed may well do damage (and on a machine where spares are next-to-impossible to find).

So to sum up, connecting a charger to a machine without a good battery :

35, 45, 55, 70, 80 -- Works fine

65, 67 -- May damage the card reader sense amplifier IC

Non-continous-memory Woodstocks, Spices, etc -- Won't work, may do damage, but it's unlikely

Continuous memory Woodstocks, Spices, etc -- Is very likely to damage ICs

Non-continuous-memory Topcats -- Won't work, but there is a protection circuit that should prevent damage

95C -- Is very likely to damage ICs.


Note to the original poster: Adding anything to instructions by Tony is like correcting Newton's math, but there is one more thing. When you get that new battery, remember that a bit of dust or corrosion on the contacts can completely take the battery out of the system. Make sure you have a good connection when charging.
I charge my woodstock and spice batteries in a cradle and the bats for the 65/67 in a 35 for that reason.

Tony; While you are on the subject of battery charging:
Is the 75c protected or should i charge it's battery in the 35 too?
Would it be safest to charge the batteries for the Topcat series in a 41 printer or IL device, or do those have potential weaknesses too? I sometimes really use my 41's peripherals and would hate to fry one just to charge the 92's battery



First, thank you all for these instructions, they're worth a printing. That leads me to a question for which I didn't get an answer by now :

Has anyone already designed an electronic shunt regulator that can be connected instead of the battery ? It could be a convenient way to power a (non continuous memory) calculator with its charger, when you haven't any good battery at hand -and when you're too lazy to build a custom power supply :-)

Ideally, that device would be small enough to fit in the battery compartment ; the challenge is, not to dissipate too much heat due to the voltage drop.

Any ideas ? Thanks in advance !

Best regards from France,


In theory, a 2.5V or 2.7V Zener diode rated for at least 500mW connected across the battery terminals should suffice. I would also recommend adding a 1000 uF capacitor across the battery terminal as well to filter the half-wave rectified power from the charging power supply. Assuming the maximum charging current is 125mA, the Zener will dissipate less than 400mW. In fact, HP could have just added a 3V, 500mW Zener across the battery side to eliminate the possibility of damaging these calculators when the battery was missing or had gone open. Oh well.....


Hello Steve,

Thank you for your answer ; to say the least it's quite simple a solution... I'll soon give it a try.

That simplicity makes me think that HP really fought to save even the smallest fraction of a cent in their BOM.... years before the Chinese era !

Troll mode ON : was it already a strategy to easily get more calculators serviced ? Argh :-)



Please be very careful....

I would not want my "theory" to be proven wrong and result in a damaged calculator.

I was very lucky. When the batteries started to go bad in my HP-25 around 1985, I replaced them with alkaline batteries and inserted a diode in series with the battery holder springs. If the original rechargeable batteries had gone open, I might have lost my first HP.


Hi Steve,

Please do not worry, I don't want to "brick" my 21 and my 25.
I got some schematics from Jacques Laporte's web pages ; so when I have enough time, I'll give this trick a try by building some kind of a test environment : charger + load + Zener, and so on...

Thanks again,



Here is some additional information about running/protecting HP calculators while using just the changer and no batteries.






Thank you very much Steve,

Now all I need is some spare time...

Regards, Marc


Yes, indeed. A battery that's not electrically connected to the machine (e.g. because of corroded contacts) is the same as no battery at all .

AFAIK the HP75 is protected. I think the service manual says that the machine will even operate correctly from the AC adapter with no battery fitted.

With non-continuous-memory machines (like the 92), it is quite safe to charge the battery with the machine turned off. With the switch off, there is no power to any of the logic. In fact with the topcats, there's the protection circuit I mentioned, so turning them on shouldn't be a problem either. But this doesn't apply to the HP95C.

I suppose the safest way is to use the 'reserve power packs' -- the external chargers. They exist for the classic series, woodstocks, topcats and spices. But they are not easy to find.

Possibly Related Threads…
Thread Author Replies Views Last Post
  HP 67 Calculator Battery Terminal plus/minus Steve E. 6 2,206 04-30-2012, 03:29 PM
Last Post: Nick R
  aOT--Battery/battery contact care advice needed Matt Agajanian 0 1,142 04-24-2012, 07:35 PM
Last Post: Matt Agajanian
  HP-67 Battery door latch Matt Agajanian 23 5,542 04-13-2012, 01:57 PM
Last Post: Matt Agajanian
  HP-67 battery question Csaba Tizedes (Hungary) 3 1,302 05-05-2011, 06:33 PM
Last Post: sylvandb
  HP-67 battery questions Thomas Okken 2 1,207 01-08-2011, 02:44 PM
Last Post: Thomas Okken
  HP-67 Battery Pack Robert Hollyer 1 974 04-09-2010, 10:41 PM
Last Post: John Robinson
  HP-67 Battery Pack AlbertTocarLoseggs 5 1,804 10-15-2009, 01:26 AM
Last Post: sylvandb
  TDS 500K dead battery, pictures of the inside of the card and the battery. Geoff Quickfall 4 2,003 04-02-2009, 11:23 PM
Last Post: Geoff Quickfall
  HP-67: The Red Dot Low Battery LED almost always lighting AlbertTocarLoseggs 2 1,103 03-15-2009, 02:41 PM
Last Post: Charles
  HP-67 Battery Pack Trent Moseley 11 2,561 08-17-2006, 02:26 AM
Last Post: Klaus

Forum Jump: