HP-19c battery pack replacement (again...)

[Xavier A. (Brazil)]

Hello there,

I acquired a few weeks ago an old full-working HP-19c, delivered with 4 'new' - in box - Clartech Ni-Cad 800 mAh cells ; it seems to be a French brand, since user's instructions are in french only.

The first instruction recommends to "charge before use", which is obvious, but I am wondering if it is possible to charge them within the calculator, with the official HP adapter. I already charged the cells out of the calc and everything works perfectly.

Of course I read a lot of threads on the subject in this forum, Katie's recommendations included; it seems that some cheap Ni-Cad cells are not recommended, but is it a risk to dammage the calc charging it (with my cells) directly from HP adapter?

Any feedback will be appreciated, thank you.

(edited to correct my english!)

Edited: 22 Sept 2011, 9:46 a.m. after one or more responses were posted

[Katie Wasserman]

It's fine to charge the cells inside the calculator while the calculator is off. The risk comes into play when running the calculator with the AC adulator and the batteries not making contact with the prongs from on the PCB inside the batter compartment.

So, charge indies the calculator when it's off and unplug from the AC to run it and you'll always be fine.

HP never made a charging cradle for this battery pock so the only easy option is to charge the battery pack inside the calculator.

[Xavier A. (Brazil)]

Thank you so much, Katie.

[Tony Duell]

Are you sure that's correct?

The 19C has continuous memory, which means the RAM chip is powered even when the machine is turned off. The battery pack acts as a shunt regulator and limits the voltage applied to the RAM when the charger is connected.

If the battery is missing or open-circut, the voltage applied across the RAM chip can be high enough to do damage.

However, when the machine is turned on, the current drawn by the rest of the logic is normally enough to pull the voltage from the charger down (the official HP transformer has a deliberately high internal resistance) and normally no damage is done.

So I believe that -C models are more at risk from charger damage when turned off than when turned on. Non-C models are totally safe when turned off because none of the electronics is powered in that state.

AFAIK you're right, there never was a 'reserve power pack' for the Sting models. There is for all the others, including the Spice series (that one is probably the hardest to find).

[Xavier A. (Brazil)]

Thank you Tony for this new advice.

Well, the best is perhaps to charge the cells outside the calc, which unfortunately can not be applied for long computing - my goal actually.

[Katie Wasserman]

I agree with Tony, partially. You're still at risk powering the any "C" Woodstock off an AC supply even when it's off. But I'm not sure that the risk is greater than when it's on. Even though the voltage is higher when off it's power is only going to one chip not all of them.

At any rate, charging outside the calculator -- however difficult -- is the safest way to go. I power almost all my calculators with primary cells of one type or another so never deal with charging. The Woodstock battery packs (2x series, 19C, 10A) lend themselves to this pretty well because of the built in springs. I use Energizer lithium cells for this because they have near zero self discharge and as far as I can tell never leak.

[Xavier A. (Brazil)]

I do agree with you, Katie: charging outside is safer. Any idea about average computing time (before batteries fail) with my 800 mAh cells? How can this time be calculated?

[Katie Wasserman]

It all depends on how much you use the printer. That requires a huge amount of current. I haven't measured the current draw, but without using the printer I'd guess around 6 hours of use on a charge.

[Xavier A. (Brazil)]

Thank you Katie.