|Re: HP20B low battery warning doesn't work|
Message #7 Posted by BobW on 12 Oct 2008, 6:06 p.m.,
in response to message #6 by BobW
One question, did you deplete the battery by using the calculator or outside?
The low battery indicator comes up if the battery are detected low for 100 keystrokes (this is on order to avoid the indicator turning on and off all the time when it's 'on the limit' between too low and good enough).... Are you just putting the low battery in and then trying to use the calculator for a bit, or are you leaving them in for a while... Obviously, if all you are doing is placing already bad battery in the calc and doing a quick test, the low battery warning will not come up because you are not 'pressing enough keys'...
basically, what it boils down to is: if the battery deplete in the calculator from use, you should see the low battery sign coming up (and if it does not, then I am verry interested in trying to figure out why), if you are just placing already bad battery in the calc, and it does not come up after a short test, then it's 'normal'...
Your response is truly appreciated. It has inspired me to do some testing. Here are my results.
I hooked up an external power supply to my original 20B ( firmware version 6 24 2008 2).
When the voltage was set to 2.35V, no matter how many keystrokes I typed, the low battery icon never lit.
When the voltage was set to 2.16V, after about 100 keystrokes, the low battery icon did turn on. When I hit OFF and then ON/CE, the icon remained on.
When the voltage was set to 2.00V, the 20B would not turn on.
I also measured the current drain:
20B off - about 100uA.
20B on but no keystrokes - about 244uA.
20B on with rapid keystrokes - about 4600uA.
Per two separate CR2032 battery datasheets, the expected life (down to 2.0V), with 100uA load, would be about 2000 hours. Since there are two CR2032 batteries in parallel, on the 20B, the expected shelf life would be about 4000 hours (not quite 6 months).
So, yes, the 20B low battery icon will come on under the right conditions (in the lab). However, I truly believe that in the real world, the low battery icon will never get a chance to work. This is because of two factors:
1) the increasing output resistance of the batteries as they deplete.
2) the large difference in idle vs operating current (during keystrokes) on the 20B.
The 20B's detection circuitry/algorithm will never get a chance to activate the low battery icon because when the batteries are getting low, the large delta V caused by the large delta I, due to high battery source impedance, will cause the 20B to completely reset.
It seems that, with a little work, the algorithm can be changed to allow the 20B low battery icon to work properly.
After thinking about this issue some more, I believe that there are couple of factors that will limit a 20B user from knowing that either his/her work in progress will be lost, or having a calculator that goes dead without proper warning, with the current battery-low-detection algorithm.
One factor is the relatively high "off" current drawn by the 20B that depletes these poor little CR2032 quickly. Another factor is the rapid decrease in battery voltage and its associated rapid increase in source resistance as the batteries near the end of their life.
As I recall, the 20B is shipped with the batteries installed. So, the user will most likely receive their 20B with the batteries somewhat, if not very, depleted. Even after installing fresh batteries, however, there will be a very limited time window when the batteries are high enough for the 20B to collect its 100 "below threshold" keystrokes and yet have a low enough source resistance so as not to dip below the processor reset voltage (2.1V?) during high current events (i.e. during keystrokes).
In my case (to answer Cyrille's original question), I bought the 20B new in the middle of July. I've used it sparingly since then. My first indication that something was wrong was that it had lost my RPN mode setting. So, between the time that the batteries were okay (i.e. no low battery warning) and the time it got reset, the HP-installed batteries had gone low enough to allow the high-current keystroke events to reset the 20B.
It's going to take some work to come up with an algorithm that takes into consideration the decreasing battery voltage and its associated increase of source resistance. I think that it's necessary to do because, I suspect, that you will get a lot of complaints as more 20B's are sold and used.
One good thing is that I see that the user manual has been updated so as to inform users to replace the batteries one-at-a-time (so internal memory is not lost).
If you (HP) would like me to do some more testing, I would be glad to help out.