An XDA developer user has churned out a ROM that is deodexed and claims to improve battery life of the Nexus 5 smartphone. It uses stock kernel with init.d support and is pre rooted, which means you get utilities like super user and busy box pre installed. Here are the mod included in this ROM.
Tweaked system that makes it very fast
Raised contact sync resolution (requires re syncing of contacts if you don’t factory reset before installing)
Battery % Percent on status bar
Exchange security bypass
Relaxed MMS compression ratio (Hangouts & Messaging)
Enabled access to App Ops in Settings / Security
Enabled debug settings in Play Store
Blocked annoying ads in programs
Enabled NFC while on lockscreen
APN settings for a lot of carriers
Colored notification icons
So, how does it improves battery life? It comes with three CPU speed profiles that underclock the CPU. Here are the profiles included.
2.26GHz – 100-85 percent
1.95GHz – 84-36 percent
1.72GHz – 35-0 percent
Download the ROM from here and flash it. Before you do so, make sure to do a full wipe before install to avoid any problems.