Weird 42S battery problem

[jebem]

(03-17-2017 01:37 PM)pdicamillo Wrote:  When I took the batteries out of the calculator to measure them, I didn't keep track of their relative positions. But I'm trying again with 357 batteries, and I'll do that this time. I measure the batteries with a digital multimeter, and this time I measured the voltage across the calculator's battery terminals. When I first put the batteries in it was 4.6 volts. Thirteen hours later it's down to 4.2 volts. I used the calculator for just a few quick, simple calculations. It looks like the calculator will die again soon, and then I'll try the LR44 alkaline batteries.

Assuming that the calculator passes the full self-tests, this suggests one of two possible causes:

1) - The battery batch you are using presents very high internal resistance due to a manufacturing defect or maybe they are out of date.
This results in a very fast voltage drop at the battery terminals especially under load (with the machine in power on state).

2) - Your calculator current consumption is way above the average.
This one is easy to spot as long as your multi-meter is able to measure current in the micro Ampere range.

Here are the normal current consumption on a HP-42S, brand new, barely used:
(Test done using an external 4.5VDC power source. Any 4.5VDC 3R12 lantern battery will do, or else you may use 3 x 1.5VDC AA or AAA batteries in series installed in a battery holder)

Power off state: 9μA average (9 micro Ampere, to be clear)
Power On idle: 0.3mA average
Power On pressing keys: 2.5mA average

If you machine have much higher values, specially when in powered off state, then you need to check possible current drainage causes, starting from previous battery leakage (most common cause IMHO) to a bad bypass capacitor.




I was just checking the 357 / LR44 / SR 44 batteries and th manufacturers claim from 150mAh to 200mAh capacity, depending of the technology and vendor.

Taking the lower value of 150mAh for a LR44 alkaline battery, these are the running values that I found:

- In power off state: 150E-3 / 9E-6 = 16666 hours or 694 days or 1.9 years
- In power On state in idle all the time: 150E-3 / 0.3E-3 = 500 hours or 20.8 days
- In power On state doing calculations all the time: 150E-3 / 2.5E-3 = 60 hours or 2.5 days.