I use this 12v battery as a dedicated battery for my fish finder. It lasts for a long time. Certainly longer than a day but I still give it a charge before every trip to be sure.
It's only $26, compared to the same thing from Hobie for $69
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
I hang it from the lid rim of my back hatch with one of these:
https://www.amazon.com/Hobie-12V-Bat...battery+holder
I have a similar second 6v battery for my bait tank pump. It also lasts more than a day.