Master Tangler,

As an alternative to a heavy ATV battery, or fistfulls of AA or AAA batteries, you could try my solution for a power source.  I use 12V rechargeable lithium batteries from a Ryobee 12V drill set.  The drill, batteries (2) and charger kit cost anywhere from $39 on sale to $69 at regular price.  One of the batteries generally will last for a full weeks fishing.
I mounted a small waterproof "Pelican type" box under the seat of my Prism, which will securely hold either one or two of these Ryobee battery packs.
The initial cost may seem high, but I have been using the same set of batteries for three years now.  And have had the cordless drill to use at home...
Crimped on spade elctrical connectors available at any hardware store fit securely into the contacts on the batteries.  A very simple set up.  The batteries probably weigh less, and are easier to mount and pack than a AA or AAA's.
Setting the audible low depth alarm on my detector has helped prevent overnight depth checking in camp... One thing, as others have mentioned about Lithium though, they run until they quit, no getting weak or fading.  They just quit.