[Sony Ericsson XPERIA X10 Mini] anancr:
Well I have tried roms since the times o Owain, and all the old devs. so I have tried eclair, custom eclair, froyo, GB with stock, gb with custom, cm7, cm9, cm10.

talking about battery life you have to take into account something first. the use that you give to the phone.

people tend to take a lot into account the % that the battery displays, and do lots of "clear battery stats" (and that doesn't work much) because if you have a max capacity of 970mAh, even if you clear the batstats 1000 times you will still have 970 mAh.

then with this, what you need to know is what kind of use you give to your phone, and when in that kind of use, how many hours of battery you can expect

like every electronic device, the mobile phone have a minimal battery drain.

because it keeps a signal between the cell tower and the phone, to this you have to add things like, the coverage, distance to the tower, the kind of signal (3G - 2G )

that's the minimal drainage of the battery. to this you need to add the usage of bluetooth, and the data.

until here we are talking about the expected drainage of the battery. and here you can see that the same phone can have different levels of drainage, that depends of factors like the location of the phone.

so up to here we can say that a phone has a minimal battery drainage that is affected for external factors.

[up to here with eclair, the drainage was terrible, even with bluetooth off, wifi off, data off, the battery life was between 10 to 13 hours to me, froyo wasn't better, GB with stock bosteed to 15-17 hours, and GB, JB with custom, can get between 19-27 hours, having tweaked the settings. (using the phone just for calling having the most battery duration the days with less calls), with wifi enabled, I can get between 6 hours to 15 hours, it depends of what I'm doing with the phone. (this means, whatsapp, line or whatever app you use for chat and social). and with data, this can be tricky. you can get between 3 to 7 hours data is tricky or special because it gets the drain for being on, the kind of network and the signal strength.

so with all this I want to say, that using the %isn't really accurate to read the battery duration. because it will use more or less battery depending of what you do with the mobile phone.

and there are other things that makes the phone use more or less battery, things like the kind and the number of apps that you install in the phone.

so going back to your question. regarding battery life, the best roms in my opinion will be the GB+ roms, and better if you can use a custom kernel. but again you can't expect a long battery life, if you use your phone intensively

In this moment I'm using kernel 2.6.29.6 nAa-jb-03 rom Jelly Beam mini project release 2 my phone is an U20 mimmi. and my battery times are.

standby mode: 24 - 27 hours

calls: when I have the phone in mode just for calls I can get 16 - 20 hours (and no using the phone for anything else, just calls, no wifi, no data, no bluetooth )

listen to music: I normally use the phone with bluetooth headset. and I combine it with calls so I get 7 - 12 hours.

3G - WIFI in standby: between 10 - 19 hours. (stanby just using to get notifications, mails)

3G on the move; 5 - 6 hours

WIFI: 6 hours if I'm using it. it goes up if mix with pauses etc.

so you can see that the battery life depends a lot of how, and where you use your mobile phone. hope this help a bit. and JFYI, these are my times. it's my personal experience. after using the phone in the conditions described. other users may say other things, but I feel confident, that in any time of the day, I can say, how much time I have left of battery, and how much time will the phone be on, if I do X, or Y thing with it.

tried to write this, with my best english, but again, you can find some grammar off, because Is not my mother language, and my level is just B2.