Found this thread after doing a google search for captivate rom with the best battery life.
I also recommend Dlev 4.2, even though dlev has stopped development since he broke his cappy. And even after dlev 5.0 has been released. Dlev 4.2 JVQ has the best battery life of any rom I have found (and I have tried dozens in the last 15 months), is screaming fast, is highly customizable, and stable. Before dlev, I used Firefly for the longest time of any rom, but upgrading to gb was worth it. Plus, unlike some gb roms, dlev is easy to flash.
Since going to dlev 4.2, I have flashed a few other roms including Apex 9.2, apex 10, and dlev 5.0, but none of them provide the battery life on my phone as 4.2, likely because it is jvq which my phone seems to like best. So I always come back.
Downsides to dlev 4.2 (and some of the other dlev versions):
I do not like the included swype skin (alt characters too hard to read), but had saved the one with good visibility from dlev 4.1 so this is not a problem.
Like all the other i9000 based GB roms, it has the charging bug when using most usb chargers (try some of the other ones you have lying around the house until you find one that does not cause it), but I will put up with this since an i9000 rom is required for hsupa/hsdpa speeds.
Once it is flashed you must immediately go to the market and update titanium backup since if you use the version included in the rom it seems to corrupt everything it restores and lots of stuff force closes. You are immediately going to the market to restore Tibu Pro, anyway.
I think you have to go into the status bar settings after flashing and turn off and on the battery display to get the graphic (I use the textual battery display myself).
You should flash h3llstorm's gps fix afterward. This fix was incorporated into 5.0 so it did not have to be flashed separately, but oddly the GPS in 5.0 appeared to have lots of problems, unlike in 4.2 with the fix. Note that GPS works much better on my phone than most cappys (it works perfectly in other words) since I have a plastic battery cover.
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I am finally to the point where I think, "Why would I ever flash a new rom?" But I am sure I will since I have never gone as long as a month without trying a new one. And I am almost as sure that I will flash back to dlev 4.2 soon afterward.
With the standard battery, I can easily go a whole day with normal use (text, phone, surf, some videos, some games, and a lot of bluetooth streaming). With an extended battery I can add a couple hours of GPS (nav and mytracks), 8 hours of tethering (I leave my tablet tethered most of the day), and a couple more hours of gaming. It is almost impossible to completely drain the entire extended battery in one day to recalibrate it.
