Why newer android versions drain the battery faster? I've got an Android phone whose stock Android version is 2.1. I've tried several times to install some custom ROMs (mostly Cyanogenmod based) with Android 2.2 and 2.3.

The full charged battery lasts:


>3 days with the 2.1 stock version
1.5~2 days with android 2.2
~1 day with android 2.3


...And they say that android 4 drains the battery even faster!

I'm wondering why this happens:


Is the newer software using the hardware more intensively? What pieces of hardware? What pieces of software? And why?
In general, should I expect a custom ROM to drain battery faster than a stock one (same Android version)? The ROMs I installed were supposed to be more optimized than the stock one, and have less crappy services running, but meh!
If the phone was designed for a newer Android version (say 2.3) would I get a big battery boost by switching to a former version (2.1)?
I doubt this is a software issue but moreso a hardware issue. Consider the following, Newer phones typically have:


Bigger Screens
More Antennas (Wifi N, 4G, NFC, etc.)
More sensors (This includes cameras)


Either more or more advanced of what's mentioned above. This comes at a cost, as each draws its own amperage. While it may be minuscule in an isolated system, when combined together these features present a non-negligible drain on the battery.

And, While newer batteries will perform great, over time the battery will not hold the same charge as it once did, this is not a limitation of the phone but battery technology in general. I'm specifically talking about Lithium Ion batteries right now, b/c that's the most common battery in phones today.

While there are things you can do to mitigate the problem, such as staying in airplane mode throughout the day or buying power management apps for your phone, downgrading your OS likely won't help much. If anything, it will provide old power management code, as well as no support for newer hardware.

Custom ROMs are a different story, and aren't created equal. Some may have modified code to provide better battery life, some may be performance focused and not taken battery life into account. 

Some phones also have extended batteries available for them. I can't speak firsthand to their effectiveness, but I obtained one for my Dell laptop and it made a huge difference. Some people are reluctant to this approach because it changes the weight and/or profile of the phone, however one must weigh that against having it tethered to a charger all day as well.