If I have WiFi enabled and 3G data disabled, does it save any battery relative to having both WiFi and 3G enabled? With WiFi enabled, I didn't know if 3G enabled would make any difference since it wouldn't be used.
Short answer: yes.
It depends on the currently enabled Wi-Fi sleep mode. By default, Android disables Wi-Fi when your device goes into a sleep. In this case, 3G is the remaining mode of network communication, and it gets used by Gmail/Gtalk push notifications, etc. Obviously, this network communication takes its toll on your battery time. If you disable 3G you'll save some battery, but your device will be completely offline while in sleep.
However, if your Wi-Fi is "always on", your 3G will never be used, so it doesn't really matter if it is on or off. However, "always on" Wi-Fi is a big battery drain by itself...
Update: Note that sleep modes and sub-system energy consumption differ from phone to phone. Moreover, 3G energy consumption varies greatly, depends on which background services are enabled, proximity of the cellular tower, etc. I haven't seen any "authoritative" documentation on this subject.
I suggest you check your battery stats (in
Settings -> Battery) and see how cellular communication affects your battery time overall. Usually, it is a very small percentage, so in general dialing your screen brightness a down a notch will have much greater effect on your battery life.
I have a Huawei U8510-1 (Android 2.3.4 Gingerbread) and when I got it my battery was having a low duration between recharges.
After some tests, by disabling and enabling stuff, the battery cycle started to last more, about 2x more, with the 3G disabled. For me was great since I always use WiFi.
But this was not enough, the battery cycle still given me a short period of time, forcing me to recharge the phone more or less every two days.
The best solution that saved my day and allowed me to really save the battery, thus increasing each battery charge cycle was this APP:
Easy Battery Saver free
It works and really makes a difference. Now I charge my phone one, max two times a week, depending on the usage. :)
So, by my personal experience, disabling 3G even if it is not being used, helps saving the battery life, but doesn't make that much of a difference compared to an APP solution.