I'm wondering how many batteries I need to be carrying in my tr-21.  The old owner used only four, but I don't think it was ever used for a full 8 hours at a time.  I have a 36 volt motorguide trolling motor, a humminbird 797 in the console, a humminbird 585 on the bow, and a 225 merc that needs to be cranked.  Of course the boat has only a three bank charger.  Could I make do with only 4 batteries, or is 5 (better safe than sorry) the way to go?  

one dedicated crancking and 3 for the TM should be fine.  the fishfinders dont draw enough to hurt a battery in 8 hours.  As long as your not running the TM wide open for 3 or 4 hours you should be fine.

a single spare battery will do nothing for your 36V TM

one dedicated crancking and 3 for the TM should be fine. the fishfinders dont draw enough to hurt a battery in 8 hours. As long as your not running the TM wide open for 3 or 4 hours you should be fine.

a single spare battery will do nothing for your 36V TM

I guess my main question was whether or not I needed a single battery for the fishfinders.  Thanks surfer.

I run two fishfinders, my GPS and the livewells for a full tournament day on one cranking battery.  The electronics don't draw that much power.  

As someone else said for the trolling motor---1 battery p/12volts.

I wouldn't think you would have a problem at all provided the batteries are fresh. Do you know what size batteries your running?

A lot of guys with a 36v tmotor only use 3 batteries. You can do that if you have and EFI. If you have a DI then you need a cranking battery as the DIs need more juice to start them.

As long as you get a crank with 800+ CCA and tmotor batteries with 200 reserve amps then you would not need more than 4 batteries unless you plan to spend more than 24 hours on the water with no trips to the bank.