Monday, November 11, 2002

I... I got charts.

I've had my laptop (Sony Vaio FX-220) for about a year now, and have run it almost exclusively on AC power. This is mainly because Linux support for power management is sketchy, and so (until recently) I couldn't throttle the CPU back or dim the LCD (both vital for reducing power consumption). Windows does these automatically, of course, but I excised Windows from my life, remember?

Anyway, recent kernel upgrades mean I can now throttle performance to make the laptop's mediocre battery life slightly more useful. Faced with the prospect of actual mobile use, it occured to me to make some graphs of battery charge, so I can get a sense of how various features affect battery life.

One graph, which I thought would be very straightforward, turned out to be quite surprising. The graph on the right is one complete charge cycle: the battery starts out nearly empty, is charged to full, then is unplugged and discharges almost completely. (The vertical axis is fraction of full charge, and the horizontal axis is minutes. Yes, the battery life sucks.)

The first thing you might notice is the little upward and downward spikes. Those are measurement error, not sudden changes in charge. I don't know if the Linux driver has a bug, or if the Sony hardware that reports battery level is really that unstable. Either way, ignore those errors for now.

The real surprise, of course, is the kink in the graph that makes the graph look like Half Dome in Yosemite. Around 0.6, as it charges, the slope of the graph changes suddenly. As it discharges, there's a kink at 0.6 as well.

My first theory was that the battery might have multiple cells, and might switch from one to the other at some point.
Based on his experience with handheld devices, though, Noah thinks it's more likely that Sony is artificially fiddling the how-charged-is-the-battery function as reported in the BIOS. They might do this, he says, to make the function look more linear with respect to time. The irony is that the discharging function, which it seems would otherwise be perfectly linear, suffers significantly in the process.
I, for one, am much more interested in battery status when the laptop is discharging than when it's charging, so it seems strange that they'd screw with things in this way.

I'll run more tests if I get a chance, but does this behavior look familiar to anyone? Anybody know why you might want charge functions to be distorted like this?


  1. I... I got charts.I think they make an ointment for that.
    I would have expected the charging "kink" to be the opposite. My iPod will charge to ~80% capacity within the first hour of charging but will then take another two hours to charge completely. I suppose this is a feature of its "lithium polymer" battery and it's a damn good feature to have in a portable device. The discharge cycle seems normal to me (normal for a person who knows nothing about electrical engineering). What type of battery is in your Vaio?
    It is also my understanding that it is best to keep lithium batteries always topped off with juice. If you let them sit on a shelf (charged or uncharged) for an extended period of time, the "innards" can "dry out" and will refuse a charge later on. I learned this the $50 way by having to replace my camera battery. (My use of double-quotes in the previous sentences can be backed up by my mail order EE degree)

  2. I would have expected the charging "kink" to be the opposite. My iPod will charge to ~80% capacity within the first hour of charging
    I think that's what's going on here, too. If Noah is correct, the actual function looks something like the one on the right. I've just scaled the top 40% of the graph down (in gimp) so that the kinks go away. This graph looks like how I'd expect the battery to behave.
    So it's possible Sony thinks this graph would be too confusing for stupid users. ("It done charged most of the way, but now it's charging slowr'n mollasses. Gol-dang Sony!") Dunno.
    What type of battery is in your Vaio?
    Lithium Ion, I believe. And it's about a year old, and it's spent most of its life at full charge, either connected to AC power or shut down. That is, it's spent very little of its life actually providing power to the laptop.

  3. Palm, incidentally, solves this problem with their rechargeable handhelds by declaring that the battery is some percent full when discharging, but when it's charging up, the "percent full" is just "charging". The little battery shows up with a lightning bolt on it, at least in Palm's Launcher.
    Not that somebody couldn't repeatedly remove the handheld from the cradle to discover it was charging very slowly, but almost nobody bothers. Customer apathy saves the day!