[Touch Diamond, MDA Compact IV ROM Development] number16:
Ok guys, I've made a few discoveries that I figure I could share with everybody. Maybe you'll find it useful.

Last week I went on a trip and turned off the data connection while I was out of the country. Instead of letting the roaming charges kill me, I was relying on wifi to check emails and browse the web.

I ended up checking emails just as often, because where I worked had wifi AP.

Now, I could usually get 8, maybe 9 hrs out of my battery with moderate use before it's completely empty previously. So I was very surprised to find that with a similar usage pattern, but using wifi instead of EDGE/GPRS I still had 30-50% battery left at the end of 8-9 hours day everyday during the trip. I know data uses a lot of battery, but I always thought wifi drains even more, so that's quite a stunning discovery.

Now, I don't think it was due to wifi draining less than GPRS/EDGE (can't be true), so it must be something else. In trying to figure out what made the difference, I did a bunch of tests after the trip, and this is what I find-

1. Data channel dis-connection / re-connection is BAD

I used to always set my phone to auto-disconnect data channel (EDGE/GPRS) after 5 min of inactivity, in an attempt to save battery. What I found, was keeping the data channel open does NOT actually drain more battery than leaving it off at all. Transmitting data drains battery, but not leaving the channel open. However, disconnecting it, and re-connecting it all the times actually drains quite a bit of battery. I set my phone to check email every 30 minutes, and then there's also the odd weather forecast that needs data channel. In a 9 hrs day, that means channel disconnection + reconnection of about 40 times.

The last couple days I have left my data connection ON all the times, and I actually get more hours out of my battery. My battery used to drop ~ 10-15% per hour with moderate use. By keeping the channel on all the times it's been kept to under 10% per hour!!! I've only tested it for a couple days. I'll report more on it once I get to test it for longer, but the idea that 'keeping data connection off when you're not using it to save battery" seems to be a complete myth. The opposite actually saves battery!!!! And as a bonus, I don't even have to wait for the data channel to connect when I need it!!

2. Recycling the radio is VERY BAD

Everybody knows 3G is a real battery killer. However, similar to EDGE/GPRS, keeping the 3G channel open does NOT drain any more battery than turning it off, or turning on EDGE/GPRS channel. When the data channel is idling, it doesn't matter whether it's on EDGE, GPRS, 3G, or even completely turned off, the battery drain is close to zero in all cases.

Now, you do see a 1.5 - 2 times battery drain with 3G compared to EDGE/GPRS, so I've always turned 3G on only for web browsing or watching YouTube, and use GPRS / EDGE for regular emails update. The thing is though, if you're not transmitting much data (which you won't for regular email update), the difference in battery drain is minimal. SWITCHING between 2G and 3G though, requires a radio power cycle (turn off then back on to switch frequency) and THAT drains a lot of battery!!!

So if you're often switching between 3G and 2G, and you only transmit little data in 2G mode, you might actually be better off keeping it in 3G all the times instead of forcing the radio to power-cycle all the times.

I've tried keeping it in 3G all day long and I noticed minimal increase in battery drain. However, there might be another reason you want to consider - RADIATION. 3G not only drains more battery than 2G, it also transmits at a stronger power than 2G and as a result create more radiation. For that reason, I'm still keeping my phone to 2G for email updates and what not, and switch to 3G only for web browsing. For radiation you may try this thread if you want to read more about it.

3. VGA screen is a REAL battery killer

I do quite a bit of reading on my phone (ebook, on-line magazines etc) and reading ebook was never a battery concern in my days with the Touch (QVGA screen).

That's why I was quite surprised on the Diamond, reading the ebook for 1 hour, with EVERYTHING else turned off (GSM, EDGE, GPRS, 3G, BT, wifi), my battery level went down by 12% in ~1 hour.

The VGA screen drains a lot more battery than the QVGA screen. Now, if you need to use the phone you need to use the screen, there isn't much of a choice. It does make sense, however, that if you're using the screen for a while (like reading ebook) switching from a high brightness level to a lower brightness level.

Oh, and the auto-adjust brightness thing? That doesn't help you save battery at all. This is because it polls the light sensor every 2 sec (default value, but you can change it) and adjust screen brightness accordingly. This mechanism drains battery in itself, and in most cases end up using more battery than keeping the brightness constant at a low to medium level.

The auto-adjust thing is cool, and in theory it sounds like it can save you battery, but unless you constantly set the brightness to max even when you're in a dark environment, disable the auto-adjust and just set it to a constant 50-60% instead.

These are the few things I've noticed and I'm still trying things out, but over the last 2 days I've seen a significant drop in battery drain. I would be lucky to go through a 8-9 hrs day with moderate to heavy use before, the first 2 days I tried this I still had 60% battery left after 5 hrs of moderate use. The Diamond is very weak on battery life so every bit helps! I hope these tips are useful to you!