HP97 Low Power warning



The low power lamp in my recently acquired HP97 remains on even after the battery is given several hours of a charge, and even when the AC adapter is plugged in.

The battery still takes and holds a charge and the calc runs fine (except for the card reader, of course, which needs the usual fix) on battery power alone.

This phenomenon was not present when I got the calculator a few weeks ago. Also, I thought at first the battery had been rebuilt but on closer inspection it may very well be an original one.

Is this an indication that the battery pack is finally dying, or is there something wrong with my calculator's circuitry or display?




Most likely one of the cells in the battery pack has failed (shorted) and the remaining cells generate enough potential when fully charged to keep the calculator running. The only way to know for sure is the measure the voltage on the battery pack. It should be over 5 volts when fully charged.

If the battery pack is fine, it's possible to adjust the set point for the low battery voltage detection circuit. You need to replace R1 on the display board. Look in the service manual for the location of this resistor. I've never done that so I can't recommend alternative values, but it can't hurt to experiment. I don't see comments about this in the service manual like they have for the print intensity adjustment, which is also done by resistor replacement.



There is a new wrinkle--I let the battery drain out over a few hours in a low drain state (decimal point only in the display).

Now when I plug in the adapter and flick on the calculator, I get no display at all, only the faint glimmer of the low battery lamp. Experience (albeit limited) tells me that as the battery picks up a little charge this condition quickly resolves and the calculator can be run off AC power while the battery pack charges in the background. Soon, the low battery lamp will go out.

As for servicing the electronics of this thing myself, you overestimate me greatly. I can't even get all the screws out to look at the card reader, which needs service (of course!)

I am expecting a 82143 printer anytime soon and understand it comes with working battery pack so I will be able to experiment in that regard since they use the same battery. I suspect the problem is the battery pack, though I have no idea what I did to induce the problem. When I got this thing I was amazed the battery pack seemed to hold a charge at all.



The thing has been plugged in for about an hour now. The battery pack is warm. I turn the unit on and the only thing in the display is the low battery lamp dimly lit. Hitting the paper advance button slowly advances the paper one line (with the adapter plugged in only), but that is the only response I can get out of the thing.

When I pop the battery out and "run" from AC alone, I get the usual response--flickering display, can't print, can't change digits displayed--BUT I can do a basic calculation.

So some time over the past several weeks this battery has gone wonky.

Is the behaviour I have described normal for a drained battery pack, or should I be able to at least see the display by now? How much of a charge must the battery have for the calc to be fully operational from AC current?

I hope this is just the battery pack, but as the thing goes to Randy in the new year for card reader fix and general refurbishment, I guess I while find out if the situation is more sinister.



Okay, about 4 hours of charging now and I got the display back and can run the thing with and without AC, but the low battery lamp remains lit.

I think this little "test" pinpoints things to the battery--evidently if the thing needs 4 hours of charging just to be able run the calculator with AC assist, it is time to replace it.

I will do the 24hour thing to see if I can shock some more life into it, but as it stands now I think it is time for a replacement.



Hi Les,

But one of these, and I think your troubles will be over. I got one for my 97, and it will go for at least a month (of intermittent use) before requiring charging.

Best regards, Hal


Glad that pack has a good review!

I also need a "classic" pack for my 45, and he has agreed to combine shipping for me.

I am also awaiting delivery on a 82143 printer, and I want to see what the battery is like in that before ordering.



Nearly one week later and things are worse than they were before.

The bottom of the calc gets very warm when I try charging, but it has gotten to the point where even several hours of charging will not render the calculator usable, even on AC power. Only the low battery light is faintly lit, but there is no other display and the calculator can't be used.

I guess this is the question that I have--every other device that I have every had has a rechargeable battery pack can still be run off of AC power long after the battery pack fails to accept a charge. My old TI57 is the best example I can think of. But it looks like that in the HP97, an adequate battery pack is a key feature in completely the circuit even if one wants to just run the calculator off of AC power.

Can someone clarify this for me? I can accept that the battery pack is dead, or shorted, or whatever, as I think it's pretty old. What I can't figure out is why I can even use the calc on AC power, even if the battery is dead or not. I am worried there is something going on beyond just the battery.

Many thanks and Happy Holidays--




If the battery pack is shorted the calculator will not work unless you replace it with a good one. Although you can power it directly from the AC adapter alone it won't work right. You'll probably get a flickering number display and some calculations might work, but the printer and card reader certainly won't work. However if you think that there is a good chance that something else is wrong, don't try that!

The 97 AC adapter puts out an AC voltage which is converted by a full-wave rectifier inside the calculator to a DC, this is used to charge the battery. There's also an over-voltage protection circuit inside the calculator to limit the voltage form the charging circuit/battery pack. If either the rectifier or the transistors in the protection circuit are bad however, the AC adapter can damage the rest of the calculator if it's run without a good battery pack installed.

If you don't have a volt meter or an external 6 volt DC power supply to use in testing, the only obvious option you have to figure out what's wrong is to get a good battery pack and test with that.


You'll probably get a flickering number display and some calculations might work, but the printer and card reader certainly won't work.

That is exactly what I get when I take the battery out, so the behaviour you describe is consistent with a dead or shorted battery. Too bad--when I first got the calculator the battery seemed to work beautifully, but it is feasible that reuse after years of apparent dormancy were probably more than it could take.

I am still expecting that HP41 printer with an apparently working battery that I can test. The calc itself going to FixThatCalc in the new year and I may ask Randy for a battery rebuild as well, or get one of Mark Hoskins' packs, or both.

Thanks for the quote from the service manual. I read it myself (I have the DVD), but I must confess it doesn't mean much to me!




I just got my 82143 and it shipped with the original pack which, though weak, doesn't seem shorted--there is enough juice to do calculations in the 97 but the printer tuckers out really quickly. I can't test the card reader--it still has to go to the doctor.

The supposedly shorted pack does seem adequate to do printing in the 82143, albeit with fairly light intensity. And the printer's BAT indicator is lit all the while, so this supports the shorted pack hypothesis--enough juice to operate the device, somewhat, but not enough to suppess the low bat light.

I have finally ordered one of Mark Hoskins' positively reviewed packs and when I get it in a week or so I can settle this once and for all.



Les, one theory is that nicads form "filaments" internally which short out a cell, and that these can be broken by rapping the battery hard against a solid surface.

I've tried this a few times with old batteries and have had success on at least one occasion. I slammed the battery against a brick wall, hard enough to sting my hand, and - mirabile diu - the battery came good.

Worth a shot in this case, perhaps?


--- Les



I will have to do this experiment away from the house. Wife has a skittish old shih tzu that goes ballistic at the sound of loud noise of any sort!



I don't remember the exact details of the failure mode, but I have seen a few machines where one transistor in the power supply failed and took a few of its neighbors with it. Check all the transistors on the keyboard where the power supply is (I think there are four of them in it). Also check that big power resistor. I have seen quite a few bad ones.

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