It's that time of year when we need to plan ahead and set our clocks ahead one hour before retiring for the day. When you think of time, have you ever thought about the battery in your computer? Does it need maintenance? How long will it last? We use Mac computers in our office for web design, so we'll look at battery life from that perspective. The same thought process will be relevant no matter the brand of computer you use.
The Mac maintains its settings while turned off by allowing a tiny trickle of current around the power supply. This powers any components that must retain information as long as the computer get power from the wall socket. The battery only starts to work if the computer is unplugged from the wall, and is intended for times when the computer is being moved or when maintenance is required. Therefore during the life of a computer the internal backup battery may need to run only hours or days, thus its long life in "normal" use.
If you unplug the computer or turn it off using the switch on a power strip, backup power supply, or "power center" located between the computer and the wall socket, the battery will work overtime and can fail in months, not years.
Turning off the computer through the Mac's "Shut Down..." command in the Apple menu does not stress the backup battery.
Some well-meaning pundits of "green living" who have little knowledge of computers have recommended we unplug our computers anytime we are not using them. By doing so you may save $7 worth of electricity in a year but incur a computer repair of several hundred dollars.
The backup battery is buried deep in modern iMacs and, for most of us, is not a user-serviceable part. Therefore the cost of replacement is a few dollars for the battery but one to two hours of technicians time for accessing it.
Source: Apple Community