Laptop battery care
Making the most of your battery
Apple's advice is here: http://www.apple.com/batteries/notebooks.html
Keeping a battery under charge all the time is not a good idea. Best battery life is achieved with complete charge and discharge/use cycles. If you do not regularly use it on battery power, then set yourself a monthly reminder to discharge and recharge the battery.
Three to five years is a normal battery life for a Mac. The worst that you can do is to leave a laptop plugged in 24/7. Batteries have to have occasional use, not plugged in, or they will die prematurely.
Utilities
Coconut Battery (MacUpdate page) is a long-standing popular free utility which gives additional info on your battery.
iStat Pro (see MacUpdate page) is a dashboard widget which displays data on battery usage, as well as system info such as processes that are running. It is not fully compatible with OS X 10.8 "Mountain Lion", but see the end of this thread on MacRumors.com for a fix.
"Service Battery" warning
When your Mac laptop displays "Service Battery" in the Battery menu, it will not hold a charge for as long as it used to. If you need to use the computer where you do not have access to an electrical outlet, then see about changing your battery soon.
However, if you do not need to use it on battery power, then you can safely carry on using it plugged into the power socket. Modern batteries are better than the old alkali ones, and there should be no risks to the other components such as leakage.
The time on battery will get shorter and shorter, but its final death is hard to predict. It may fall to under an hour within a few weeks, or even if you spin it out for some time, it may collapse quickly over days. If you are trying to get the last ounce of life from your battery, keep following the above advice about sometimes using the computer on battery only to discharge it in cycles.
Replacement batteries
For replacement batteries that are reliable but less expensive than Apple's, CATUG members have recommended LaptopBatteryOne.com and macsales.com.
If you live near an Apple Store, check whether they give you a lower price if you hand in your old battery.
iFixit.com gives instructions on changing the battery yourself.
See also
Credits
Compiled from a conversation on the Mac-Ministry List by Neville Reid in January 2013.