Battery Times under Linux

Out of curiosity more than anything else, I decided to start checking how long my battery would last under different conditions, depending on what i was doing at the time.

Rik Faith has been doing some more detailed and precise work on battery times for his computer, which he's sent to me. I've got it all laid out below, but keep in mind that he's working with a different computer (and a different battery).

For all of my examples, the battery was fully charged (charge light glowing green), and a simple timing program that I wrote in perl was started as soon as I unplugged the power cord. I've rounded to the nearest minute, since seconds in this context are fairly ridiculous. The stated time is the difference between power disconnect and the low-battery alarm, which probably leaves you with a few minutes of power left. Note that these times are for the T-3400 monochrome; active matrix computers will have different (usually lower) times.

1st try: 4 hours 26 minutes

For this run, I was doing a wide range of things, probably a reasonable sample of everyday use: running X with three open windows; text editing using vi; PCMCIA modem usage for about 30 minutes; mounting and I/O to a floppy (only long enough to get one 200k file and unmount); 85% of a kernel compile (battery alarm went off mid-compile); perhaps 40 minutes of 'idle' time in which the machine wasn't doing anything actively; no 'suspend' at all, but the screen backlighting was off about half the time.

For this run, I was using the 1.2.8 kernel with PCMCIA but no APM (the kernel compile that happened was to install APM, actually). This and all subsequent runs will be using the 540 MB drive (not the original 120).

2nd try: 3 hours, 52 minutes

This was a killer session in terms of battery life: I decided to grab TeX off the net, and the only download option I had was floppy disk. So, I spent a good hour accessing the floppy and doing a lot of HD work. Once I had it downloaded, I started playing with TeX and xdvi, which ate up even more HD access. (You can imagine that using TeX, under X, on 8 MB of RAM would swap quite a bit.) I'm very impressed the battery lasted as long as it did. There was also about an hour of text editing, so that probably helped preserve the battery life a bit. Due to the active nature of what I was doing, the backlight was on almost the entire time the computer was powered up.

This session was run on a 1.2.13 kernel with the PCMCIA patches installed, and no APM. APM and PCMCIA aren't co-operating, so I've chosen PCMCIA as the far more important module to have... If anyone can tell me how to get these two packages to work together, I'd be quite happy (and yes, I can guess that the immediate response will be 'upgrade to the latest kernel!' but that's a bit more than I'm up for at the moment...)

Rik's results

As I said above, Rik Faith contributed this information, so you should speak to him about it if you have any questions. The text below is verbatim from a message he sent me via email -- I've added some HTML to make it look better...

In January, I posted a note about a daemon I wrote for Linux that monitors battery status information (apmd, available from ftp.cs.unc.edu: /pub/users/faith/linux; sunsite; and tsx-11). I've collected data during February, and I thought I'd share them. Implications are for 95% instead of 100%, since the battery jumps from 5% to 0% when the machine starts beeping. So you really only have 95% of the battery to play with. I have a standard 3300mAh battery in my T3600CT.

Discharge rates (124 data points):

Minimum: 0.120219 %/min => 13.2 hours
Maximum: 0.508475 %/min => 3.1 hours
Average: 0.361665 %/min => 4.4 hours

[This is based on leaving the laptop in resume mode for between 6 and 35 hours. I've never left it off for longer than that. I can't explain the variation -- it is unrelated to the amount of time being off. I can't imagine how it might be related to memory contents...]