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This may seem a simple question but I can't seem to locate a solid answer.

Anyone here know the charging current for the subject line devices, powered with the standard charger?

I just built myself a Topcat pack with fairly high capacity cells (3300 mAh), and I am trying to guestimate how many days a full charge a depleted pack will require.



Hi, Les;

I was wondering about to measure it too, because I rebuilt two packs with four 1800mAh NiMH batteries and let them charging for 24 hours. And I happen to have at least one of each of these three models (two working HP82143A).

I'll try finding some time this weekend and post about it as soon as I get it. Meanwhile, have someone else done it though, I'll compare results. There is a third 1800mAh pack and I have no battery shell to hold it, so it will be easier to use clamps in their bare terminals.


Luiz (Brazil).

It will take its time...

The charging current is adapted to the old standard cells from the seventies with a capacity of about 1Ah.

Once I tried to build myself a pack with the fanciest cells I could get on the market. The refurbished pack was dead within a few months (inner resistance increased - no charging possible anymore).

The newest cells are sensitive to low charging current. To perform well and to reach their estimated duty cycles they need to be quick charged with a current higher than a few milliamps.

I recommend that you charge your pack externally because within the devices there is not enough current to prevent the cells from failing because of low charging current.

And as a reminder: The rechargeable battery in this type of calculator is an important part of the whole power management system.
The plug-in charger only contains a rectifier and a small capacitor. Your battery pack does all the voltage regulation and there is no 7805 or zener diode!
A dead pack can kill the whole calculator when the voltage runs high.

Hallo, Stefan.

The plug-in charger only contains a rectifier and a small capacitor.
I know some AC adapters are rectified, but the ones used in these models - HP82143A, HP82162, HP97 - deliver AC only, not DC. I measured the output of the one I use the most - 82066B EUROCHARGER, we have 220Vac outlet here - and found approximately 10.7Vac with no DC component. Seems to be a single line transformer.

About the packs and cell's amps capacity: I am using these two 1800mAh packs for no more than a month, but I have been using two 1200mAh packs for more than a year - one with the HP97 and the other with an HP82143A, not so often though - and they are doing fine. This agrees with what you say, I mean, the 1200mAh cells are closer to the original 1000mAh ones and their charging current, although lower than what they were designed to, has not caused any damage... at least so far.


Luiz (Brazil)

For the 97:

First off, it is not a constant current charger. It is similar in function to the woodstock charging circuit, charging current decreases as battery voltage rises. However, in the case of the 97, if the battery is bad or missing, there is a voltage clamp to prevent circuit damage. Also, the 97 varies the current depending upon the position of the on/off switch.

The circuit consists of a 10 vac transformer rectified by a full wave bridge. The dc output is connected to the battery in series with two resistors (in series). When the calculator is switched off, the effective resistance is 12.9 ohms, when on, 4.7 ohms. When the calculator is on and the display current exceeds a preset limit, the 4.7 ohm resistor is shunted by a pnp transistor to increase the available current.

Given those variables, it is not going to be an easy estimate.

I do not have schematics for the printers so I cannot comment on their charging circuits.

Thanks, Randy!

Evidently I couldn't find a simple answer because there isn't one.

I am getting more interested of late in the finer points of batteries and charging, so probably the best thing for me to do is get a decent multimeter and measure currents and voltages directly and attend to good battery hygiene--i.e., don't overheat them, etc. I pretty well cooked the pack you rebuilt for me a few years ago due to inattention. I was able to zap some life back into it, but its self-discharge rate is still higher than when new and of course isn't the same. I am striving to be more attentive with my battery charging now.