Mysterious HP-19C power-on behavior-- solved with interesting cause.

Well, I got back to my two 19C's with problems powering on after sitting for a few days. They would both power on with a regulated supply, but not with battery packs. I soldered a "c" shaped wire over the battery contacts with no luck. Couldn't figure it out-- very frustrating.

Both battery packs-- rebuilt with brand new Energizer Ultimate Lithium batteries... show full voltage.

When I switched to NiMH batteries, both calculators work fine. I haven't had time to investigate further, but I'm guessing that they may have changed the formulation of the batteries or that I got a bad batch... they seemed to work fine in all of my other calculators and have historically had enough amperage!!

Any thoughts? When Energizer came out with "Advanced Lithium" (Cheaper), did they change the "Ultimate Lithium" formulation?


Yes, cheap batteries can be a problem. Not just lithium, but also NiCad and even alkaline. Case in point, I have a digital camera that works off both rechargeable or disposable AA batteries. I have been using name brand alkaline AA's, but replaced them with some cheap store brand batteries that would not work at all, even though they all checked out fine with a voltmeter. I tested different cells from the lot, but none of them would work. As soon as I returned to some name brand cells, the camera worked fine. Not being any sort of battery expert, I'm not sure what the problem was, but in any case your situation is very credible.


I run one of my 19C calculators on Energizer lithium cells too with no problems. However I have seen problems with the cells themselves at times. The "button" on the end of the cell is often a bit shorter than the ones on alkaline cells and sometimes the rim of the cell makes contact with the spring (or whatever) in the battery case instead of the button. The effect of this is to have an open in the batter pack, a bypassed cell or, worst case, a shorted cell that can get dangerously hot.



You bring up another very good point in that the choice of battery form can also contribute to this problem. I was having mysterious intermittent contact problems with some of my Woodstocks when using high top (button) AA cells, and the problem was completely solved by replacing them with flat top AAs. Also, the damage to the battery contacts in Spices is IMO often due to the use of regular high top AAs.


Lithium batteries aren't supposed to be "cheap"... they're the most expensive and I buy them because, so far, there have been no reports here of them ever leaking. Alas, companies always seem to find a way to make things cheaper.....

Katie-- one pack was altered with extra springs so that all four batteries can be inserted as bare cells and "float" between springs. The other pack was soldered together and only had two springs to push the end cells against the calculator contacts. THAT'S what was so baffling. I "knew" that the batteries were making contact, and I'd switch the packs around, and eventually BOTH of the calculators would turn on and work fine, only to fail to turn on and to act squirrely a few days later!

I never suspected the cells themselves due to my long history of success with them....


I'd guess it's either a contact problem, or high resistance in the cells. NiCd and NiMh cells have very low internal resistance and consequently can deliver high surge currents (like when a motor starts or a circuit powers up) without a voltage sag. In cells with greater internal resistance, a large current demand may cause a brief voltage sag and subsequent malfunction of digital electronics.

I took a quick look at the data sheets for Energizer lithium and compared it to the data for their NiMh AA batteries. I noticed that the internal resistance of the lithium batteries is about 3 - 5 times higher than that of the NiMh.

Without some detailed measurements it's tough to say that's the cause, though. Another possibility would be one out-of-spec cell (low power, high IR, etc) that's causing the problem.

Interesting that it's an 19C being fussy. I wonder if there's something that draws a lot of current on power-up (like some big capacitors in the power supply or motor drive circuitry)?



Thanks, Bob.

Again, strangely, once the calculators would power on corrrectly, they'd work fine, including the printers. And they'd work just fine and turn on and off until put away for a few days. Then, it would take a long time and lots of strange behaviors before they'd power up again! It was really baffling me!

Edited: 5 Feb 2010, 3:32 p.m.


Probably a capacitor to recharge and causing a voltage drop in the batteries?


Marcus-- that was my very first thought when I posted this puzzle last month. It makes sense that the batteries ALMOST provide enough power and the calculators eventually power up and "stay up". I'll test the batteries for their shorted current capacity against some older lithium cells also. I'm still a bit baffled. They could power the LED's and printer just fine once the calculator(s) would "start"... you'd think they'd turn them on o.k.!


The other possibility is that the voltage is too high. Lithium cells with no load are 1.7V so the calculator when shut off -- but the 19C is never really "off" -- sees 6.8 volts. This might be causing the CMOS to "latch up" preventing the calculator from turning on.

Here's an easy test for that...

Remove one of the cells from the pack and replace it with a metal screw or something that's just the right size. The calculator will see a more reasonable 5.1 volts with no load and probably around 4.8 volts when it's running.



Katie... you could be right, and I had thought about that. But I'm afraid it might take some more time to solve this puzzle because they are both starting up easily while I try to figure this out.

I tested each of the lithium cells... they put out between 6 and 7 amps each, and over two amps in the packs. So, providing enough power wouldn't seem to be the problem. And, of course, both calculators ran perfectly last night. I ran them both for quite a while, printing listings, etc.

This morning, one would not turn on. When I hooked up the pack externally with leads, the calculator turned on. Some kind of stubborn contact problem? Or no over-voltage problem because they haven't sat long enough.

I'll have to wait a few days and then see if they'll start if hooked up externally and if not, if they'll start with 3 cells fed in instead of 4.

If it's still a problem with the battery contacts, why would they work so reliably once turned on, only to quit and be so stubborn after a few days? And why would NiMH cells work when the lithium cells are causing problems? Your answer could be correct. I'll give your over-voltage theory a try when they stop working again....

Such a strange, annoying, and stubborn mystery that I should be smart enough to figure out!


Too high a voltage is an interesting hypothesis. It would be certainly be worse with a four cell pack (I've had no problems using Li primaries in Woodstock 2 cell packs).

Let us know what you find out!



Well, it's Sunday. I completely rebuilt one 19C battery pack; cutting away the inside front frame (as I've done with the other to allow standard cells to be removed and inserted), installing new springs and even a curved m-shaped gold-plated contact which connects the middle two cells but doesn't block the stock circuit board "prongs" in the 19C.

Results: The calculator turns on with NiMH cells, but not Lithium cells.

Complete disclosure: One of the two calculators seems to be fine, now, with lithium cells. I'll let it sit a month or so and re-check it. Also, this stubborn calculator would turn on with lithium cells when connected externally. Still, I can't believe poor contact alone would explain this phenomenon, especially with a completely rebuilt pack.

Well, all of this is interesting.... but most important... I have red LLLEEEDDD's. I think I'll just have to use this one to do my taxes instead of the 97.....




Quick follow-up: Both of my 19C's turn on perfectly, even after a week of sitting idle, with NiMH cells rather than lithium cells. Thanks for all the input, and I'd love to hear if anyone else has trouble with this model using the lithium batteries.


Thanks for the followup! I'm always concerned about over voltage issues with several lithium cells in series, that extra 0.2 volts can add up fast.



Indeed - great remote diagnosis! Thanks for the follow-up.

I use my 19C a lot so I'm using NiMH in it now. If I ever change it to a shelf queen, I'll be certain to take Katie's caution into account.

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