Disabling the APN settings which connect your phone to your providers network means that your phone loses all internet connectivity until it is enabled again.

Saves battery when you're sleeping ot have no intentions of using the net for a while.

Juicedefender includes some functionality such as turning the APN on for a few minutes in intervals to let services sync with android or turning it on when the screen is lit.

Try reading Latedroid: JuiceDefender

 

"spare parts" is a good app. will tell you what is using your battery when in standby mode. more detailed than the normal battery usage which is in settings on the phone already

 

On T-mobile it seems to eat MORE power turning on and off the APN.

Certainly my battery usage went UP from 3% overnight with everything on to 8% using juice defender!

Re-establishing connections, certainly on 3G seems to eat power.

 

It 100% uses more Battery than without the App... i was actually schocked how much it sucked up from my Battery at the End of the Day.

 

Perhaps it's just a T-mobile thing, or those that benefit have a heck of a lot of stuff using the net all the time.

I leave mobile data and syncs on constantly, and can easily see 36 hours. Wifi BT and GPS all off when not needed.

 

If you have your phone set up to sync every hour, then install JuiceDefender, and set it to do something like turn on APN for one minute every 15 minutes, then effectively your phone will be syncing 4 times an hour instead of 1, which obviously will use more battery.

If you set up JuiceDefender to match your existing syncing setup, then I'm sure it will use less battery.