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Battery Issues
07-29-2019, 06:03 PM
Post: #1
Battery Issues
I’d like some help concerning a current issue with batteries for my HP-35 and HP-67 calculators. I’ve been using battery packs marketed online by Mark Hoskins and been very pleased with their performance, but he has recommended me to this forum for tech help.

Some history, in case it’s important:

Both calculators use the same battery pack, so I bought one for each of them in August 2015 (usually good for at least 3 yrs, which has been found to be true). In case it matters, the 35 was bought new with its charger in 1974. The 67 was bought new with its charger in 1979 and was refurbished by HP when its card reader wore out. It’s been my experience that the 35 will function without the battery installed when used on the charger, but the 67 MUST have the battery installed even when using the charger.

The current issues:

HP-35

When I checked the 35’s battery voltage with my volt meter it read 4.26 volts, even though the battery pack was labeled 3.6 volts. Strange! I plugged in its charger without the battery installed, turned it on, and the display showed faint flashing numbers (similar to the 67 when performing its calculations). I put the battery back in the 35 and plugged in the charger but got the same display only brighter. The battery compartment is pristine, with no corrosion.

HP-67

The 67’s battery obviously died and was discarded. I installed the remaining battery without the charger, turned it on, and the display showed it was ready to use as usual (no “red dot” indicating a low battery). But when I put it on its charger with the battery installed and turned it on, the display showed a bright flickering “0.00” and a bright red dot. The battery compartment is pristine, with no corrosion.

Any comments are appreciated.
Jerry B.
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07-30-2019, 12:12 AM (This post was last modified: 07-30-2019 12:14 AM by teenix.)
Post: #2
RE: Battery Issues
Maybe both battery packs, being of the same age, have partially failed and are causing the problems. Both calculators appear to have a CPU board issue which may be caused by the batteries. The bright red dot indicates very low battery voltage which may be too low to operate the calculator properly. Have you tried a new battery? Are the CPU boards clean and free of corrosion?

As I understand NiCads, (if that is what they are) they should have fairly constant voltage until discharged, (3 x 1.2V = 3.6V). 4.26V seems very odd then.

If the battery is damaged, the charger/battery combination, could also cause a problem, which may risk damage to the calculator. You could load the battery out of the calculator with a 27 ohm 1 watt resistor (about 130mA) and measure the voltage. If it is charged, it should be close to nominal 3.6V if that is what they are specified to be. The resistor will heat up a bit.

I think the 35 can handle 3xAA cells (4.5V), but not sure with the 67. This voltage feeds the card reader chip directly but I don't know if the chip is spec'd to handle the higher voltage directly. The HP-97 uses the same chip with a higher 5V battery voltage but has extra circuitry which may limit current/voltage for the motor.

cheers

Tony
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07-30-2019, 12:50 AM
Post: #3
RE: Battery Issues
Tony, I'm mainly hoping to learn if simply trying a new battery pack will solve all issues; but if the calculators are already damaged (not likely), that'd be money down the drain. That may be the best path though. I can't identify the battery type without opening up the pack which would render it useless for sure, and I'm not keen on attempting a 4.5 volts supply anyway. I may just order one new pack to see if both calculators are still OK; I can always order a second pack if need be.
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07-30-2019, 01:09 AM
Post: #4
RE: Battery Issues
I run my 67 on 3 AAA alkaline batteries, and the card reader works fine that way.

I recently bought a pack of AAA lithium batteries, which have a slightly higher voltage, but I haven't tried running the calculator and using the card reader with those yet...
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07-30-2019, 07:59 AM (This post was last modified: 07-30-2019 08:00 AM by Hans Brueggemann.)
Post: #5
RE: Battery Issues
(07-29-2019 06:03 PM)jbodinephotog@gmail.com Wrote:  HP-67

The 67’s battery obviously died and was discarded. I installed the remaining battery without the charger, turned it on, and the display showed it was ready to use as usual (no “red dot” indicating a low battery). But when I put it on its charger with the battery installed and turned it on, the display showed a bright flickering “0.00” and a bright red dot. The battery compartment is pristine, with no corrosion.

depending on the cell technology used in the battery pack, voltages may indeed vary quite a bit. thus, the NICAD cell is characterized (give and take) for
0.9 VDC: fully discharged (for a 3 x AA batt pack: 2.7 VDC)
1.25 VDC: "nominal" (somewhere in the range of 75% charge and leaving it on the shelf for some time)
1.40 VDC: fully charged, measured immediately after removing the cell from the charger.
thus, 4.25 VDC sound reasonable for a fully charged pack and indicates that the charger doesn't have an integrated charging voltage limiter. this is actually true for the HP-67 charger.

as for the flickering with battery pack installed and operating off the charger:
in this operating mode, the HP-67 is actually not using the battery at all, it is taking its power solely from the charger, while the charger is simultaneously charging the battery.
the flickering indicates a faulty component inside the charger.
do not continue to power the HP-67 with it.
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