battery sizing question

Pls help with a battery sizing question. I'm powering simple stuff - nav lights, bilge, undergunnel lights, etc - no big stereo or TM. I'd like the smallest battery possible for weight considerations. My motor doesn't have the alternator capability so I need the juice to last for a night's worth of fishing. I was thinking the agm Odyssey batteries for durability/weight. Thought the pc680 was about the right size (and a nice alum hold down bracket is available). Thoughts? Thks.

Alex V

 

Thanks. I wound up getting on the blue sea circuit wizard page to try to sort this out myself (it's been a long time since my college electromagnetism classes). As I am really only powering a couple of one amp lights and two 3.7 amp bilge pumps, my whole circuit is pretty light (no tabs, stereo, etc.). I don't think I need the bigger amp hour batteries and can probably stay in the 15ah range. Should be able to give me steady nav lights for a few hours of nighttime fishing and still be able to crank a bilge pump if necessary.
Interestingly, the appropriate breaker for the system is much smaller than what seems to be seen in a lot of the other circuits. It was between the 15 and 20 amp breaker to protect the system. Winds up being one of the pushbutton type as the swing arm type doesn't go down that low.

Alex V

 

Ok so now I'm not quite so sure I should get the odyssey. It's seems its a starting battery with the ability to take some discharges down to 80% (per the website). Sort of dual purpose but not a 50% reduction-capable deep cycle battery. If I've got 2amps of lights, maybe a plug-in 1amp bait well pump, and the occasional bilge pump run, so I'll call it a 3.5amp draw. If I run say 4hr then I'll need the equivalent of 14ah. If I run a battery to 50% then I'll need a 28ah battery at the very least, right? An odyssey would have to be a 70ah battery.
So maybe just a mid-20's to 30ah deep cycle agm battery? Might wind up being one of those crazy scooter batteries. Thoughts?

Again, I'm running rope start, no tabs, no radio, etc.

Thks.