[Android Software/Hacking General [Developers Only]] DarkDvr:
So after noticing how much of a difference people get in their battery lives, I've decided to do some research and make a guide-line that will give us all we need to know about properly using our batteries. First part is a general information and usage techniques for LIBs, second part is taken from Google materials on Android-powered devices (G1, Magic, Droid, Nexus One, etc).

Sources:

Wikipedia - http://en.wikipedia.org/wiki/Lithium-ion_battery

BatteryUniversity - http://www.batteryuniversity.com/parttwo-34.htm

Google IO Conference 2009 - http://www.youtube.com/watch?v=OUemfrKe65c

Electropaedia - http://www.mpoweruk.com/life.htm

General Lithium-Ion Battery (LIB) Usage:

• Discharging your LIB fully (or less than 2.4 Volt per cell) is bad for the battery. Every time you do that, it can be said that small part of your battery (some cells) dies (they forever lose their charge). Do not store your batteries depleted, there's a high chance they will die completely or will become very "weak".

• You cannot restore bad LIBs by overloading/heating/praying. You gotta go buy a new one. They DO degrade overtime, some cells naturally lose the ability to gain/give electricity.

• Although it is said that LIBs do not have memory, it's not entirely true. LIBs have gauges that monitor performance of cells, and if you do a lot of small charges, it won't let those gauges to monitor a full battery potential, causing an invalid indication of charge level. A complete charge/discharge should be made when battery capacity seems reduced, that will calibrate gauges and they will provide your phone with correct charge level status. A full charge/discharge cycle should be done every 30 (or so) partial charges.

• LIBs have a shelf-life. Do not buy them to store them. Use them early, use them often, they will die whether you use them or not. Do not buy LIBs to use them in 6 months/year/etc, buy them right before actually using them.

• LIBs have short lives (in comparison to NiCa batteries, etc). You should expect to buy a new battery in 2-3 years after being manufactured. It is caused by internal oxidation and there's nothing you can do to stop or prevent that.

• Worst LIB treatment is to keep it at 100% charge level at high temperature (think laptop/phone under direct sunlight, like car dashboard).

• Best LIB treatment, or LIBs "favorite" charge level - 40%. That's also the usual charge level you buy them with.

• LIBs don't like heat. For example, while always at 100%, typical LIB in a laptop, at temperatures of 25C (77F) will lose 20% (twenty percent!) of full capacity per year. That capacity loss is reduced to 6% (six percent) at 0C (32F), and increased to 35% loss at 40C (104F). So, keep them cool (LIBs like fridges), don't let your devices sit in the sun or overheat at charge. Also, keep in mind that while in use, battery will be significantly hotter than phone/outside environment

• LIBs like frequent partial charges/discharges more than they like full charges/discharges.

General Android power usage advice from google:

• Although this part is somewhat controversial, they do recommend having a complete, full FIRST charge to be made. If time allows, a preferred time for the first charge is 12 hours. This may have more to do with the OS than the battery.

• Battery on a Android device, in average, will last about a full day with normal use (some videos, mail, calls). That's what you should kind-of expect.

• Speaking in averages, "idling" 3G/EDGE connection (when phone is sleeping and no data is transferred through 3G), drains almost no energy. Just a little more than having 3G/EDGE radio off completely. So when no apps are using 3G, you don't need to keep it off.

• Same goes to WiFi connection - although it's on, if there is no data flowing through it, it uses almost no energy.

• At full throughput (100% data flow), EDGE is using more energy than 3G. In average, 3G is more energy-efficient than EDGE.

• WiFi is using more energy than 3G (when both are at 100% use), but since it transfers files much faster and then goes to "sleep", it's actually recommended to use WiFi whenever possible. Since it'll "sleep" more often than 3G, overall it will use much less battery than using 3G.

• Some bad apps or widgets can use android's "WakeLock", keeping CPU at 100%, screen always-on, or both. I myself have encountered such widget (I won't mention the name, it's in the market) that used a WakeLock to keep CPU spun-up at 100% all the time. That makes a huge impact on battery life. My advice - use a CPU profiling app to monitor the CPU - make sure that CPU slows down by itself when it's not used. So, beware of such widgets/apps. To check for CPU cycles, many recommend OSMonitor (free from market, install it, go to options, sort by "Load" in descending order. It'll give you "busiest" processes at the top). At rest you should be getting about 10-20% for OSMonitor itself, and 1-10% Android System. At rest, everything else should be 0-1%.

• Android slows down CPU when not in use by itself, as a built-in feature. Apps that throttle/change CPU frequency, are not necessarily needed.

All this info comes from those reputable resources I mentioned above, I didn't make any of it up.