|Capacity of the CR2032 Batteries as used in the 15c LE|
Message #1 Posted by Jeff O. on 29 Jan 2012, 2:08 p.m.
There has been a lot of discussion regarding just how much capacity the pair of CR2032 cells will provide as utilized by the 15c LE. I decided to go ahead and run a fresh set of batteries all the way down to see for myself. I did not want to just enter a simple continuous addition loop program and set it running, for a couple of reasons. First, continuous running of a program is not a very real-world test, i.e., I don’t think anyone would run a program continuously for a real purpose. Second, and more importantly, I did not want to have to watch it continuously for an unknown amount of time to see when it quit. So I used the following process:
1. Entered Katie Wasserman’s “306 digits of pi on a 15C" program. This program takes approximately 16.5 minutes to run on my 15c LE.
2. Ran the program a few times over the course of a morning or a day
3. checked battery voltage
4. repeated steps 2 and 3 until the unit finally quit during a run of the program, keeping track of the number of runs and battery voltage at the checkpoints.
I am happy to report that the unit ran 60 times without issue, each time in the expected execution time of approximately 16.5 minutes. On the 61st run, it finally quit during execution. I’m not sure how long it ran, the display was blank when I checked at about 16 minutes run time. (Each time I ran the program I set an alarm on my 17bII for 16 minutes later. Then when the alarm went off, I watched the “running” display” until it stopped, then verified that it performed correctly by checking the first 20 or so digits and the last 6.) I turned it on and got the “Pr Error” message. It seemed to just clear memory, no weird data corruption or other effects. I reentered the program (it is only 79 steps) and ran it again, this time watching to see when it would quit. It ran for 13 minutes, then the display went blank, and I got the “Pr Error” message when I turned it on. So then I entered a simple addition loop and watched, it ran for 6 minutes. Did that again, and it ran for 5 minutes. That was Thursday afternoon, I believe. Friday morning I entered another simple loop and set it running; it ran for 20 minutes, so the overnight rest allowed the batteries to bounce back a bit. But they are obviously essentially depleted, although I may continue to use the calculator for normal day-to-day calculations to see just how long they last.
At 60 reliable runs of a 16.5 minute program, that represents 16.5 hours of program run time. Based on Katie’s reported draw of 10.5 mA per cell during program execution, I managed to extract over 170 mA-Hours from each cell. The cells are Panasonic brand, rated 225 mA-Hours when running at 0.2 mA. So I managed to get over 75% of rated capacity running at 50 times rated current.
As I said, I measured the voltage along the way:
After 4 runs: 2.8 Volts
After 8 runs: 2.7 Volts
After 12 runs: 2.7 Volts
After 16 runs: 2.7 Volts
After 20 runs: 2.8 Volts (yes it seemed to go up.)
After 28 runs: 2.7 Volts
After 36 runs: 2.7 Volts
After 44 runs: 2.6 Volts
After 52 runs: 2.7 Volts (yes, increased again)
After quitting on the 61st run: 2.6 Volts
After quitting 13 minutes into the next run: 2.4 Volts
After quitting 6 minutes into the next run: 2.4 Volts
After quitting 5 minutes into the next run: did not measure
After quitting 20 minutes into the next run the following day: 2.5 Volts
Unfortunately, I forgot to check the initial battery voltage. I used my 15c LE that I received in September and did not use at all beyond doing the keyboard test to insure I had a good unit, so I assume the batteries were in as-new condition. I measured the battery voltage by swapping the cells into a 30b running wp34s and using the BATT function to check voltage. It seems like fresh batteries show 3.1 Volts in wp34s, so I’ll assume that’s where mine started. If they were actually lower, that means battery life should be even better with new batteries.
16.5 hours of run time on a 15c LE is equivalent to around 2000 hours, or 86 days of run time on a 15C Classic. Would a 15C Classic be able to run that long? Based on these results, I for one feel that battery life with the 15c LE is acceptable. I don’t think that HP needs to slow it down to extend battery life, although a user-settable speed setting would certainly be a welcome addition. The high current draw on key press should also be fixed to eliminate any adverse impact on battery life during normal use.
Although battery life may not be as bad as feared, I did confirm that no low-battery indication is given, and that when battery voltage drops too low during program execution to feed the processor, it shuts down and flushes memory. I also discovered another troubling behavior. As I said, I measured the battery voltage by swapping the batteries into a wp34s. I changed the batteries one at a time as directed, putting a new set in the 15c while measuring the test set, but about half the time I got a Pr Error and had to re-enter the program. (That’s why I starting measuring every eight runs instead of every four.) If you cannot swap batteries while preserving memory, it does not really matter if there is no low-battery warning or it shuts down and flushes memory during program execution.
Edited: 29 Jan 2012, 2:17 p.m.