Saturday, November 24, 2007

What "reserve power" means on a 15" powerbook

Short answer: It means it'll run a 12V/200mA fan for about six hours before quitting.

So my 15" powerbook's battery has been misbehaving -- after charging, it didn't last as long as it used to. I heard from somebody the idea of hooking a fan up to it to give it a deep discharge, so I did that today.

The elder teen asked me, "Why does that work?" Well, I don't know, but I think what it does is convince the computer's charging circuit that when the battery is fully discharged, its output is like this rather than like that. Of course if it never gets fully discharged, then the computer might "think" it's completely outta gas when it actually has a lot more mAH left in it. And what would the harm be in that?
Well, if your battery has a lot more electrons that it can pump through your computer, but the computer says, "Goodness gracious me, the battery is out of electrons!" and refuses to work any more, you might feel a bit peeved. Like if your car refused to start once you got down to ¼ tank of gasoline. So this condition would be Highly Annoying, and if the technique works, it would be a Good Thing.
So here's how I did it: First, I removed the battery from the computer (turn the lock clockwise with a coin) and set it on the table. Here's a picture, proving that I cannot read:

The fine print says there's 10.8 volts DC. Unheeding, I took out a Sharpie® marker and wrote "12V" near the terminals so I'll know next time where to connect the wires:

The wires (red for +, black for –) are connected to a computer cooling fan. You might notice that the wires look a little hinky. I had to try to jam them into the slots that you see there. Eventually they stayed, and the fan ran. For about six hours. Here's a close-up of it. The fan was running when I shot this photo; the camera's flash stopped motion:

I replaced the battery in the computer and plugged the AC adapter in. It's charging now.

Theoretically, the computer will now run well over 3 hours (if I'm not watching a movie or something) on a charge. If not, I'll whine about it here.

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