Found my old HP-34C from high school

[Graan]

Found my old HP-34C in the garage of my mother. Still looks fine and its working.
Fortunately the battery pack was taken out.
Manuals in Swedish.



/Andreas

[walter b]

Congratulations! Enjoy your find! And - as you presumably know - DON'T charge the batteries inside the calculator or your joy may well be a very short one.

d:-)

[Graan]

I charged the batteries for a while and it seems to work. I think I replaced them , but a long time ago (198X)
Seems not to be original batteries. Has a Mitshubishi logo (or equal) and "Made in Japan"
So how do I charge them? Outside the calculator in a charger?


/Andreas

[jebem]

(09-21-2014 07:43 PM)Graan Wrote:  I charged the batteries for a while and it seems to work. I think I replaced them , but a long time ago (198X)
Seems not to be original batteries. Has a Mitshubishi logo (or equal) and "Made in Japan"
So how do I charge them? Outside the calculator in a charger?

/Andreas

Hi, Andreas,

The HP charger was designed to work with the NiCd battery pack installed in the calculator.
(Edit: Charger technical details - The charger adapter is just a transformer supplying 10VAC, so the charger circuit is located inside the calculator and contains a rectifier diode and a current limiter resistor).


However if the battery contacts shows oxidation and/or the battery pack life has reached its end, there is a risk of damaging the calculator electronics if it is operated while plugged to the charger, due to a potential over voltage situation (the batteries works like a voltage regulator).

To minimize the risks, I would not power on the calculator while the charger is connected to the calculator.
Charge the batteries for 8 hours, then unplug the charger and use the calculator.

If you find that the battery autonomy is too short, replace them by the same NiCd types.
More info here.

[Steve Simpkin]

(09-21-2014 08:52 PM)jebem Wrote:  To minimize the risks, I would not power on the calculator while the charger is connected to the calculator.
More info here.

Unfortunately, since this is a 'C' model, charging it with the power switch off may not be safe as the CMOS RAM is always connected to power. The safest method is to take the batteries out and charge them in an external AA NiCad battery charger.

[Dieter]

(09-22-2014 11:13 AM)Steve Simpkin Wrote:  The safest method is to take the batteries out and charge them in an external AA NiCad battery charger.

For the record: here's how I use and charge batteries for my 34C, my first HP bought back in 1980.

First of all I replaced the original NiCd-pack of the day with current NiMH batteries. Two Sanyo/Panasonic AA Eneloops, to be specific, which IMHO may be the "best" general purpose NiMH batteries currently available. Besides other advantages, they have virtually no self-discharge. The Eneloops' capacity is 2000 mAh, i.e. you can expect about four times (!) the original operating time. I connected both batteries inside the battery chamber with a thin metal clip. I also tried a piece of aluminum foil (folded once or twice) and it worked fine for me either.

The batteries are charged in an external processor-controlled charger. This is both the most convenient way (insert batteries and let the charger do its job) as well as fast (typically 2...4 hours, depending on the charging current) and, most important, it is safe for the batteries themselves that are handled correctly without the danger of overcharge. A good charger does not have to be expensive. Over here in Europe, the good old BC700, for instance, can be had for less than 30 EUR.

Yes, I have to remove the batteries from the calculator every time I charge them. But due to the much higher NiMH capacity, charging is required four times less frequently than before. I got a pack of four Eneloops, so I can always have two spares in a drawer. Since there is virtually no self-discharge it does not matter if they received their last charge a week or several month ago. AFAIR the manufacturer claims something like 85% of the original capacity after a year (!).

Dieter

[Graan]

Hello Dieter
So if I understand you right you use 2 separate batteries and attach a metalstrip to connect them?


/Andreas

[Dieter]

(09-22-2014 01:24 PM)Graan Wrote:  So if I understand you right you use 2 separate batteries and attach a metalstrip to connect them?

Yes, I use two standard (button top) Sanyo Eneloop AA cells. Of course these have to be connected somehow. So I grabbed the 34C and looked into the battery compartment. The contacts of the two AA Eneloops are connected with a piece of alumin(i)um foil, folded several times. I also remember having used a thin metal strip, as described. The alumin(i)um foil method has the advantage of being flexible, so even standard button top batteries (with a protruding plus terminal) may be used.

Of course you can also use two (usually flat-top) AA NiMHs, professionally welded together. Which technically may be the better method, as long as it fits inside the battery compartment. But then - how do you charge such a pack externally?

Dieter

[Graan]

Actually I have a NiMH charger that charge from 2 to 10 cells. You dial in the number of cells in series. Used it for a battery pack for a bike light with 5 cells I built. Should work for a 2 cell pack for the calculator.

/Andreas

[Dieter]

(09-23-2014 08:49 AM)Graan Wrote:  Actually I have a NiMH charger that charge from 2 to 10 cells. You dial in the number of cells in series.

What kind of charge control does it use? How does it detect when the battery pack is fully charged?

The best and most reliable method handles each cell individually. Charging two cells in series might be okay as long as the charger can safely detect the point where the pack has reached 100% charge. On the other hand this method also requires a way to connect the charger with the battery pack. That's why I prefer charging the two cells individually.

Dieter

[Graan]

It uses deltaV detection and constant current.
See the link:

CHARGER FOR TOY BATTERY PACKS, NiMH AND NiCD