About 15 months ago I bought an new battery for my Apple PowerBook, and after 158 cycles, it only holds 36% of a full charge. I work at home, so a vast majority of the time, I’ll unplug my laptop from the outlet upstairs and plug it back in downstairs a minute later. Occasionally, I’ll run it off the battery for 15 or 20 minutes, and on very rare occasions, I’ll use it until the battery is completely drained.
It seems to me that keeping my computer plugged in most of the time actually shortens the life span of the battery. Before I replace the battery again, what usage pattern would give it the longest life?
Depends in what battery type you have: NiMH (Nickel metal hydride) or LiIon (Lithium ion). Each has different characteristics and likes to be treated differently. (a recent battery primer: http://www.straightdope.com/columns/061103.html)
Also-- not familiar with Apples, so I cannot tell you-- many mfrs give you a utility that tells you what it thinks the charge is (rather than how much juice the battery actually has left). If the utility has disconnected from reality. If so-- for example, it tells you that you have 36% but you get 3 hours-- there should be a utility provided to get its attention.
This may not be low but not completely out of line: LiIons are supposedly good for about 300 cycles.
Unfortunately LiIon batteries have a limited lifespan, no matter how you treat them. 3 years is about it. 15 months seems early, but being at 50% capacity after 2 years is not unreasonable. I leave my MacBook plugged in all the time when I’m at work, and asleep over the weekends.
For Li-ion the standard used to be the longest life will be at 40% capacity, will have a shorter life at 100% and much shorter at 0%. I’m not sure if the newer ones still hold.
Also they tend to be effected by time and cycle life independandly.
For the longest life take your new battery discharge or charge to 40% capacity, take it out of the laptop and put it in the refrigerator, use your old battery and a/c power normally and the battery in the fridge for times you need the extra run time (though it will be at 40% capacity when you first use it)
I use the a/c plug ( connected to my desKtop UPC ) and keep the battery unplugged until I go traveling. Charge it up once a month after running it pretty far down. Lasts a long time that way.