WP 34S and 31S bugs and fixes

[Nick]

(02-08-2017 04:49 AM)efroymson Wrote:  ...about the BATT command not being accurate.

By itself, that's true as you observed (but not the whole story). The voltage was "enough" yet the calculator "shut down". Let me explain how both can be true at the same time:

Batteries are not only a voltage source, they have an internal resistance. If you were to measure the battery voltage outside of any load, you'll have the open circuit voltage (OCV). That's the highest voltage the battery can obtain under a 0 or near 0 load. As a calculator during idle is near-0, the voltage you read can appear high or sufficient, especially after the calculator has been off for a while.

Under load, the battery's voltage will drop. This is a combination of factors... internal resistance (which is really a net effect measure), temperature, discharge state, recent use, chemistry, abuse, defect, tolerance..... in short, a battery is not a perfect conductor and has limits to how much energy it can output under varying conditions. That's why a battery test routine gives you another measure of the battery's voltage that can be even more useful in determining the state of discharge. It tells you what is the voltage of this battery when the calculator is doing some work (consuming energy). That's a more critical measure of the battery as it indicates relatively how close the battery is to tripping SLOW/low-batt or cut-off due to voltage drop when the calculator is doing something other than idle or sleep.

(02-08-2017 04:49 AM)efroymson Wrote:  Also, I don't recall seeing a low battery indicator on my screen while this behavior was occurring.

Yep, that can also be true at the same time. If the battery had sufficient near-OCV to power on the device (basically just a residual / static charge) but the battery was still "bad", the voltage could drop so rapidly when you used the calculator under sufficient load that it apparently skipped past SLOW and low-batt and went directly to shut-down (it happened before the screen could refresh or just briefly flickered).

(02-08-2017 04:49 AM)efroymson Wrote:  Is this a unique problem to the WP-34S?

No, this behavior is not unique to the WP-34S. We observe it in laptops, cellular phones, and digital cameras. This is not the same thing as firmware / battery management behavior in these devices -- the "battery icon". If you recall early digital cameras that took 4-AA batteries, they would run perhaps 10 minutes on a set of alkaline batteries vs 1 hour on a set of NiMH batteries even though the alkaline batteries had a higher OCV and more energy within them. The critical difference was the lower internal resistance of the NiMH batteries that allowed the device to stay above the cut-off voltage for longer under load.

(02-08-2017 04:49 AM)efroymson Wrote:  I agree that having the BATT command work as I suggest would consume some power, but it would only do so on the request of the user ... I wouldn't think it a good idea to check automatically at power-on, as it might cause it to immediately switch off again (though that would presumably trigger the immediate thought "dead battery" in the user who could not get it to switch on).

Such a test could easily consume more than 1,000x the energy one would have consumed by performing a few interactive calculations. That would substantially shorten the battery life if automated. This is something you can easily explore yourself with a short program -- I've done it. You can store the program in your flash area so that it's fairly permanent. But it's not something I've ever repeated because the calculator is stable. It gives plenty of warning as-is when the batteries are becoming weak during long calculations and appears more frequently as they begin to fail.

That's why I say it's likely that your experience was just the batteries, that it won't repeat with good batteries, and that next time you'll probably observe the power-saving / low battery conditions the unit is already designed with. If the calculator sits unused until the batteries fail (like the new-in-box units or if left in a drawer) and then you turn it on, you might get the same behavior without prior warning. (a built-in or DIY load test couldn't even run to tell you that)

It's not anything I'd be concerned with. The battery management is stable and documented in the 3.1 PDF.

One good spare battery is sufficient to power it in a pinch (or flash it if it's a new, good battery -- I flash without a cable; the flashing procedure can accidentally discharge your batteries as well -- it happens).

Two like batteries in parallel operate with half the internal resistance and you'll be able to extract more energy from the batteries (more than twice what a single battery could provide) and be more stable under a wider range of conditions (ex. temperature). [Another benchmark -- test your batteries independently, one per slot; one could be "bad" or have a dirty connection (which increases resistance)]