Recently I was talking about portable audio and how it tears through batteries. I have two solutions for this, one is 2900 mAh AA rechargeables. When I feel that that just won’t be enough I go with my Bescor battery brick. I would not recommend this, but I have been known to plug in my lighter adapter and power multiple devices off of this thing. It will last longer than my SD card and any event I book, for that matter. It comes with a belt hook or a shoulder strap, although I usually either hang it on my tripod or throw it in my back pack.
As for charging, the one I bought came with a basic charger with no shut off, you had to calculate what you used and time it yourself. I dumped that and bought a tender. The Deltran Battery Tender Jr. has multiple cords so I can charge my scooter battery then swap the cord and charge my brick. The light shines green when it is fully charged and it is set up to not overcharge.
This battery has even supplied me with the little bit of power I needed to get through the Hurricane Ike power outages in Ohio a few years back. I ran my fish tank for a few minutes a day, charged my phone, and even ran a few lights. With conservative use it almost made it a full week, even if my fish didn’t.