[HD2 Android Q&A, Help & Troubleshooting and Genera] Slampisko:
Be warned that this thread is now outdated. There might be more current guides available…

THE ULTIMATE GUIDE TO MAKING YOUR BATTERY LAST LONGER ON YOUR HTC HD2 WITH ANDROID

OK I suppose this should be stickied as I've found numerous threads saying various stuff, but not a single thread, dedicated solely to this, providing useful and organized info.

Note: This guide is by no means definitive. Please feel free to correct me and add more tips, I'll be sure to add them to this post.

STEPS:

1. Try out a few ROMs and Radios to see which combination works best for keeping Android run as long as possible.

I have yet to find the best combo for my phone, but generally, the lightest ROMs with not much bloat in them (lots of items removed, fast startup) work best. With Radios it's all trial and error – a Radio that works miracles for one user is useless for another. You just have to try and see.

I know this is probably driving you crazy as you can read it everywhere around, but this can't be stressed enough: Don't flash your phone with a ROM/Radio you're not absolutely sure won't brick it. No one wants you to brick your expensive device. Read the flashing guides very carefully before you go on and do it.

Updating your build's Kernel is also a good idea.

2. Make sure you boot Android up as fast after running Windows as possible.

This one is crucial as it seems to affect your battery a lot.

To aid this, you can:

Download and install this (free) app (Exceller Multiple Build Loader is what I use, but you can choose something else) for the WM6. It enables you to autoboot into Android in the timespan of three seconds after running Windows, which is what you ideally want to do.

Disable HTC Sense in WM, it's unnecessary if you just want to boot into Android and takes a lot of time to load up. To do this, go to System settings, press Home, go to the Items tab and uncheck HTC Sense. You may want to check Windows Default to have at least the Zune-style interface to start from, but this isn't necessary if you're ok with an empty homescreen in WM.

3. Decrease the system load by automatically killing apps you no longer use.

This one gives a fair battery boost. By default, Android keeps all apps ready in RAM even if you press Home or Back to exit them to make them start quickly next time you need them and to keep their state. Most apps have a suspend mode which enables them to demand close to zero system resources as they run in the background, but some don't and close to zero isn't zero.

Download and install Automatic Task Killer (free) from the Market.

It gives you a list of the installed apps and enables you to uncheck those you don't want to autokill. It can also add your new apps to the list automatically.

Don't forget to uncheck applications you need to be running constantly, such as SetCPU (see below).

According to hastarin, this doesn't give you a battery boost. See the "What DOESN'T improve battery life" section. You can use AutoKiller Memory Optimizer, if you want to tweak your RAM – this app lets you set the limits for automatically killing apps only when you're low on RAM, which in turn may give you a battery boost.

4. Decrease the CPU performance when it doesn't need to perform at full speed.

I'm sure you're proud of your HD2's Snapdragon running at 1 GHz, I know I am, but it also drains a lot of battery when running on full speed.

NOTE: Using SetCPU with hastarin's R8 Kernel makes it redundant as the Kernel already has an interactive CPU governor in it. Thanks to socrated13 and atticus182 for this tip.

Download and install SetCPU ($1.99) from the Market.

It allows you to underclock your CPU to save battery. You can set profiles and play around with them. Generally, you don't want to set the minimum frequency too low as some tasks may then consume significantly more time to complete, wasting the energy you gained.

If you just want to try the app first to see if it helps, you can get it for free (search for SetCPU here at xda) and donate later by buying it on the Market (and please do, the developer put a lot of effort into it).

5. [INFORMATIVE] See how much energy your phone drains.

This one is not compulsory, but helps to get a general idea of how much energy your phone drains when you do such and such.

Download and install the Current Widget (free) from this forum. Just do a search on "current widget" and it should be the first post that comes up (don't worry, yes it's under Samsung Vibrant, but it works just as good for the HD2)

It allows you to put a widget on one of your home screens, where you can see the current your phone is currently draining. Can be set to update at various intervals. I prefer 30 s as it gives me enough time to shut down an app and look at it to see how much current the app itself is draining.

6. Decrease the brightness of your display.

This saves an incredible amount of battery life. I know you love your AutoBrightness, but it sets the brightness unnecessarily high, so until someone makes an AutoBrightness app with customizable overall brightness (once I get into programming apps for the Droid a bit, I might do it), you're left with doing this yourself.

Download and install the Brightness Rocker Lite (free) app from the market.

It displays a brightness bar everytime you bring up the volume control and thus lets you quickly change the brightness. Use the minimum brightness to see everything clearly and you should conserve a lot of juice.

7. Only use wireless functions when needed.

Having the mobile data connection, wifi, GPS or bluetooth enabled all the time causes severe battery drain. Sometimes even after you disconnect, the data connection connects again without you even knowing, downloading Over-the-Air updates or Weather, draining your battery unnecessarily.

Have a quick way to monitor which services are on and to turn them off (preferrably, have a widget/widgets on your home screen - I recommend the native Android widgets).

8. Recondition your phone for your battery after flashing each new Android build. (Thanks to t1h5ta3 for this tip)

Flashing a new Android build erases the battery stats and automatically rebuilds them based on voltages it sees, i.e. you need to recondition after each flash to get max battery life. The values are stored in the following file: data/system/batterystats.bin – if you delete this file, you set the new ground for making a new one from the value you want.

You can use the "battery life" widget by curvefish to show you temperature, voltage etc. What you're going for is voltage larger than 4.2 V on full charge.

To recondition:

Turn off the phone, plug it in, preferably overnight.

Boot the phone up while still pluged in.

Delete the batterystats.bin file ether through adb or terminal.

Drain the battery fully, until it powers down. Wait a minute or two and reboot it to drain it completely.

Congrats... your phone now knows corect voltage values for 100% and dead.

To delete the batterystats.bin file:

Find the means to be able to punch in lines of code. You can do this either by downloading and setting up the Android SDK or using a simplifying app like Droid Explorer. Do a google search for whichever you prefer.

Punch in the following code:

rm data/system/batterystats.bin

Or just find the file in Droid Explorer and delete it.

What DOESN'T improve battery life:

Getting a slower (and thus presumably less power-consuming) SD Card – because a slower SD card actually makes things worse as it needs more time to perform a task and thus keep the CPU busy a longer time.

Underclocking your CPU too much – similar reasons

Killing tasks automatically immediately after closing them – this apparently makes battery life worse, because when a program is in RAM, it doesn't consume system resources, but when you run it again, it does. Thanks to hastarin for the clarification.

Useful threads concerning battery life:

[REF] UPDATE: ALL GOVERNORS! :) Save battery while running ANY Android build (featured by the xda crew)

battery voltage / wipe batt stats

Battery Solution the **FINAL POST**

Time to Recharge to Full Battery

[TIPS] Thoughts on improving battery life for Android on the HD2

[UPD] 48hours + battery with mDeejay Froyo Z v1.7

Best SD Card for Overall Android Battery Life

Increase Battery Life [WinMo registry tweaks - Need confirmation] (do give this a try)

Hope this helps. If you have more tips or on-topic threads, I'll be delighted to add.