Like someone else said, it's like the epa sticker on a new car. I drive way faster than that.
Back to batterys. If you dim your screen, spin down the hard drive, reduce processer performance (energy saver options), and type slow, You might make it. If you turn the screen up bright, and watch a DVD while compressing a movie, you drive like I do.
I own a Wallstreet 266, a 15" G4 667, and a 15" 1G . I travel quite a bit, and carry 3 batteries.
Since I don't know what you are using your laptop for it's hard to compare my experience to yours, but one thing most people have in common is watching movies. On either of the G4 Powerbooks, I can get through a DVD without a battery swap (with the screen dimmed) on a good day, but I don't count on it. If I copy the DVD to my hard drive, and play it from there, I've got battery left over. That works even if I'm on battery durring the copy session. The hard drive uses way less power than the DVD drive.
I've never lost a battery to the cold, but that might just be luck. Any one from Canada, or better yet, Alaska here? What happens in winter if you order a laptop or battery by regular ground shipping?
A properly working powerbook will not over charge the battery, and in fact may leave the battery alone even though it is not full. Here's what Apple has to say about it on their web site.
Some PowerBook G4 and iBook computers may not show the battery as 100 percent charged in Mac OS X, even when the power adapter is plugged in. The battery appears to stop charging between 95 percent and 99 percent.
This is normal. The batteries used in these computers are designed to avoid short discharge/charge cycles in order to prolong the overall life of the battery. Because of this, when setting the Mac OS X battery status menu bar icon to display charge state by percentage, you may notice that the reported charge stays between 95 percent and 99 percent. When the battery level eventually drops below 95 percent, it will charge all the way to 100 percent.
Here's a link to the Powerbook G4 support page at Apple, It has a section on batteries that should answer you pretty well.
As for software updates, it's a personal thing, but I always wait a week after they come out, and then download them. I got caught once by downloading an update within hours of Apple posting it. Apple pulled it back when they found it had serious problems, about 3 hours after I installed it. It was an OS 9 thing, years ago, but I learned my lesson.
As for upgrading to the next number, and paying for it? Your mileage may vary...