Lots of us run our ice fishing electronics on tool batteries. I had a stash of ryobi batteries so I went that route last year. Just need an adapter with pigtails (mine was $8), an adjustable DC voltage regulator ($9), and some kind of project box to keep water out. I've ran them down to 0F and had no issues. With garmin units, you dont have to use the DC regulator as they will run at 20-22V without issue. My humminbirds will not run over 18V even though they are supposed to be rated to 20V (all 18-20V tool batteries should be 20.5V charged and 17V dead). I knocked mine down to 13V with the regulator so it just outputs that until the batteries are dead. I will be using garmin this coming season, and will use a normal 12V lithium 12ah battery.

Math is pretty easy, you just take your draw and divide your battery capacity by that draw. So a 6ah battery will power a echomap 7sv (1a draw) for roughly 6 hours. That's max draw from garmin, and as you increase voltage the power draw is reduced slightly so keep that in mind.

The new 12V lithium batteries out now are pretty impressive, you can get a 12V 18AH lifepo4 battery for around $100. That'll run all your electronics for the day without issue if you aren't running massive stuff. The lithium is 5 pounds and takes up hardly any room.