Does Leopard reduce a portable's battery-powered working time?

Some users of Mac portables are discovering problems when running the computer on battery power.

This note from a MacFixIt reader is fairly typical of one strand of complaint:

On my Powerbook G4 (15 inch), I installed Leopard and the update. I find that when the battery runs down now, I hardly get any warning, and the computer shuts down and does not go to sleep. Also, I noticed that the date and time get lost, and I need to reset the time and date.

A similar issue arises also, for example, in this Apple discussions thread. There, one reader suggests, besides resetting the PMU and the PRAM, that one might try moving /Library/Preferences/SystemConfiguration/com.apple.PowerManagement.plist to the desktop and restart. (If you elect to do this, though, I would recommend doing it while booted in single-user mode, or off another drive entirely, so that the same plist values do not get written back to disk as you restart.) However, this does not seem to work for everyone.

It is not entirely clear whether running Leopard is implicated in this matter (whether, for example, it aggravates the problem even if it doesn't cause it). There are certainly threads, though, accusing Leopard of diminishing the amount of time one can run the computer from the battery, such as this one. In this thread, some users lament having half the battery life they had before installing Leopard, while others say that battery life is actually better with Leopard. Another thread on the topic is this one.

Note that the System Profiler (under Hardware > Power) will provide information about the health of your battery. The "cycle count" and "battery health" settings are particularly useful. If you feel that your battery is problematic, it can be worth while contacting Apple; they will ask for details about these readings, and may elect to replace your battery for free if its health has dropped prematurely (i.e. with a low number of cycle counts).

Let's separate, though, the issue where running on battery does not last as long as it used to (or even as long as it does under Tiger) and the issue where the computer shuts off under battery power without sufficient warning. The latter is just wrong behavior, and the business where the time and date are reset is particularly disturbing; this is not the only message we've received where the date and time mysteriously reset themselves to the wrong value after installing Leopard. Short of suspecting a permissions problem, or recommending a cleaner form of reinstall (our usual fallback suggestions), it's hard to know what to make of this, since it's acting like a hardware issue but seems to have been triggered by software (the installation of Leopard).

On the other hand, a reduction in how long you can run off a single charge may be disappointing, but might not be the sign of any serious problem. Leopard does a lot of new graphics calculations, and uses a new threading model, so perhaps it exercises your power in a different way. Users can employ a variety of tactics to eke longer life out of a single battery charge: turn off AirPort and Bluetooth, disconnect all peripherals, turn down the screen brightness, make sure you check the first two boxes in the Energy Save Options pane of System Preferences, avoid excess reading to and writing from the hard disk, and avoid doing anything that causes the fans to spin up. The third-party application CoolBook claims to extend battery life on certain machines by lowering the CPU voltage and speed.

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