Hp-67 Power life

[Fouad M. Kaadou]

Hello Hp Museum,

I am new here and this is my first post. I read that their is nothing you guys can't answer.
My question is how long is the ON life of an Hp-67 with a fully charged battery pack (around 4.2V).

Thanks in advance,
Fouad

[M. Joury]

These days that would depend very heavily on the capacity of the cells you are using. I would hazard to say that everyone running an HP67 is using a rebuilt pack and it probably has much higher capacity than the original.

[Jim Horn]

Indeed - it also depends on what you're doing with it. Sitting and siplaying "0" takes far less power than continuous program operation with lots of card use.

Back when I used mine heavily, it would typically give me about 2 - 2.5 hours of use on a charge (1976 - 1979). Today's cells have a much larger capacity (over 3X) so it should be good for a lot more than that if using a modern battery pack.

[Fouad M. Kaadou]

The cells are 1.2V NiCd Tenergy 1000mAh. I have charged it for about 3 hours to 4.2V and I left it on with the "0.00" display and it lasted less than 1 hour. You should know that when I first received it the charger was set to 110V and I plugged the calculator into 220V, but the charger still works find. Any thoughts?

[M. Joury]

The charger is a trickle charger and I suspect that you are not getting a full charge from 3 hours. Are you testing the cell voltage under load? I would suggest charging overnight and trying again.

[Fouad M. Kaadou]

Thanks. I will try it tonight and get back to you tomorrow.

[Jim Horn]

I know that when I used my '67 for a final exam in an Air Force orbital mechanics and perturbations class around 1978, I expected it to last for the two hour exam. Just for humor I took along my 15cm Post slide rule as well. To my dismay, I had forgotten to recharge the '67 that night so it died halfway through the exam and I had to resort to using the slide rule after all. Once I finished the exam, another student loaned me his TI calculator and I re-ran my numbers, getting more than 2 or 3 digits of accuracy for the answers.

That's my personal reference for '67 operating life...

[Fouad M. Kaadou]

Hello again.
I took your suggestion and charged it over night - around 10 hours - I did not use the calculator till the afternoon but before I did I charged it for another 2 hours. I left it on idle at "0.00" display and it lasted around 2 hours and 15 minutes. I measured the voltage when it turned off and it was at 3.7V, that does not look good . I noticed the longer I charge it the longer it stays on but when I measured both voltages after 3 and 10 hours of charging they were the same - 4.2V.

HELP!

FOUAD

[M. Joury]

You have to measure the voltage under load otherwise the reading is probably off.

Also, note that a NiCd or NiMH battery has a VERY flat discharge curve. That means that it will display high voltage until just before it dies. An example of such is the old HP-41 battery pack. With regular Alkaline batteries the calculator could run for hours, days, even weeks, with a low battery indication on. But with a NiCd pack you were lucky if you got 30 minutes, or even less, after the low batt lit up.

The point here is that the voltage difference will be very small between a fully charged battery and one the is at 50% charge. This is particularly true of the battery is tested with no load on it.

Cheers,

-Marwan

[M. Joury]

One more thing: If this is a new battery pack it can take as many as 10 charge/discharge cycles for it to reach full capacity.

[Dave Shaffer (Arizona)]

Similar for a battery that has just been sitting around for a while - you may need several charge/discharge cycles to get it back to "normal". Check the various battery manufacturer pages for more info.

[M. Joury]

Good point. I forgot to mention that.

[Fouad M. Kaadou]

I have measured the voltage at the end of the calculators battery life - a little after the low power LED turned on. The voltage started at 3.8V and was quickly decreasing and in around 25 seconds it stopped at 2.29V. I am now working on charging the calculator for 3 hours and then discharging it. Is 3 hours of charging enough?

Thanks for all the help,
Fouad

[M. Joury]

Quote:

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I am now working on charging the calculator for 3 hours and then discharging it. Is 3 hours of charging enough?

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If you mean to cycle the batteries through 10 charges? Probably not.
This takes several *full* charge discharge cycles. What I would do is simply to charge it overnight, use it until it turns off and then repeat. After 10 cycles or so I would run your test again.

Also, I was more interested in how you measured the voltage after a charge than at the end of the charge. What you are seeing at the end of a charge is typical NiCd/NiMH behavior but at the beginning of a charge testing under load gives you more information than without.

Cheers,

-Marwan

[Fouad M. Kaadou]

Quote:

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Also, I was more interested in how you measured the voltage after a charge than at the end of the charge. What you are seeing at the end of a charge is typical NiCd/NiMH behavior but at the beginning of a charge testing under load gives you more information than without.

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I don't recall checking the voltage after charging the battery pack. I will measure it under and without load after fully charging it and come back to you.

Regards,

Fouad

[sylvandb]

Quote:

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What I would do is simply to charge it overnight

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The 67's charger was a slow charger for the original NiCds. A full charge of those took overnight. I believe the original cells were about 400mAh. 350mAh were common then, and the best was about 600mAh.

If the new cells have 4x the capacity, it will take at least 4x the charge time.

The NiMH in my 67 take about 30 hours to full charge (from flat) using the 67's slow charger. I think I used 1600mAh but they might be only 1400's.