Time Machine is Leopard’s built-in backup solution. Backing up every hour, Time Machine keeps all of your files safe on either an external FireWire/USB drive, or a networked drive like Apple’s own Time Capsule. Time Machine is a pretty limited service at the moment – you can only have it back up every hour, and it will not prevent backups from being corrupted when your laptop runs out of battery.
According to AppleInsider, Mac OS 10.5.3 is going to bring us more options and features for Time Machine. The latest build of 10.5.3 (9D29) seeded to developers shows an updated version of Time Machine which lets laptop users stop background hourly backups when using battery power. No doubt this update also includes some various “bug fixes” for Time Machine, and hopefully makes it more speedy at preparing backups. It should eliminate a lot of battery loss from backing up over a Wi-Fi connection, and incomplete backups when your battery dies.
I think you've got it backwards. Currently Time Machine doesn't activate when you are on battery life. But now you will be able to turn it on even if you aren't plugged into a power source.