Thanks, V1. I was hoping that you would jump in. Does this mean that, if the battery runs down, there are no real consequences because I have the computer set (via System Preferences > Date & Time to set the date and time automatically?
EDIT: I don't know how long my iMac was sitting in Apple's warehouse before I bought it, especially because it's a refurb unit. I have noticed that, when waking from sleep, the time is wrong but it quickly corrects itself. It may be reading the time that I put it to sleep but I'm not sure. Is it possible that the battery ran down while it was in storage?
The battery maintains some settings and runs the SMC, as well as powering the clock. When plugged into AC power, the battery is not used. Computers that are unplugged will slowly discharge their SMC battery. I've seen examples of computers that were at people's "summer cabins" where they unplugged all electronics between seasons go through a number of PRAM or SMC batteries as a result.
If the battery is totally drained, the clock isn't updating and the SMC is in a Reset state. When you plug it in, the SMC "boots up", resets the clock the default, and starts it running again. You may have to press the power button twice instead of once to turn the computer on. The clock will eventually update when it connects to the internet, but that may take awhile, and other services may try to start up and use the clock before it gets updated. For example, you may find if Mail auto-launches, it could fail its SSL certificate checks on IMAP mailboxes because "the certificate is
not YET valid". Mail often hast to be relaunched after the clock updates to fix that error.
The biggest problem is if the battery is
down but not
dead. In that case, it may cause the SMC to crash/hang due to low power. It's what watches the power button and turns on the computer when you press the button. So if IT is crashed, you can't turn on the computer. Unplugging the computer from the wall won't reset it, because it'll fall back on the failing battery and continue to be in its crashed state.
So if your SMC battery is low, you will need to familiarize yourself with the SMC rest procedure for your model of mac, as you may need to use it from time to time until the battery no longer will keep the SMC running in any state when AC power is lost. In most cases, the reset procedure for a desktop is to remove AC power, and press/hold the power button for 10 seconds and then release, wait a few sec, and reconnect AC power. (again you may need to press the power button more than once to power it on initially, as the SMC boots up on the first, and turns the computer on at the second)