Right now I'm using a 3S 2250ma soft pack lipo for the lights and horn. This battery is quite small but that only gets me close to an hour on a charge with the head light on. I'd like that to be a bit longer if possible. I'm not sure how long for just the taillight and/or brake light on a charge. That would probably last many hours.
Past experience with lipo packs in the rc hobby has me a bit cautious about leaving this battery on the bike unattended. When a lipo fails...and it's not unheard of...they make a big fire ball if shorted.
I figure a few of the 18650 LiIon cells in series/parallel are much safer as far as leaving it unattended and not in a fireproof box. They're fairly reasonably priced to make up a power source.
I have 18v LiIon power tool packs but those cost $100 each new. Not a reasonable option for a power supply.
There are adapters for the power tool batteries that provide 2 wire leads. Add in a voltage reducer from 18v to 12v and use the power tool battery as needed. We have one that uses my Dewalt batteries to supply power to our 12v camping accessories.
This is what I am using for all my lights.. with 5 amps it lasts about 3-4 hours. A $25 charger and the right connectors make charging and powering your bike lights easy. A bit bulky, but very light weight. You can get smaller ones with less amps and less power time. I find my horn uses the most current in my setup as it is a typical motorcycle horn with the "vibrating" sounding surface that has been around for ages. I am going to replace it with an electronic version if I find one that is loud enough.