My tracker has one cranking battery and one trolling motor battery. I want to add another battery for backup on the trolling motor. Do I string the batteries together? If I do will that make it a 24 volt system? All I am wanting is some extra juice if the first battery runs out. What is the easiest and cheepest way to do it? I would also like to use an onboard charger. At present I have a single bank on board and two or three regular chargers. Thanks for any help.

Wiring 2 trolling motor batteries in parallel will give you 12 volts and more current. Positive to Positive and Negative to Negative.

As far as charging, get yourself a 3 bank on board charger. Check it's dimensions and make sure you have room to mount it. You'll be glad you did.

2 Batteries in series will give you 24 voltS

Don't forget circuit protection if it's not already there.

Wiring two batteries (of any type) in parallel will insure that as one discharges, it will also discharge the other. Then you have TWO unusable batteries. Does that sound like a good plan?
If one battery begins to weaken, it will pull the voltage down on the other, discharging it.
If one battery begins to fail electrically (not charging fully, or a cell shorts out, resulting in less than design voltage) it will summarily discharge the second battery.

You want a back-up? Carry two batteries. add a short 6 gauge jumper between the two batteries on the "NEG" terminal along with the BLACK cable from the trolling motor. This will always stay in place. Connect the RED cable for your trolling motor to the battery you want to use. This cable should go through a breaker of some sort before it goes to the trolling motor. When that battery gets low, move ONLY the RED cable to the other battery. It will be fresh and ready to go if you had it fully charged.

If you want to do it without making a battery cable move, you need two breakers that can be manually switched off. Each battery POS goes to a breaker, and the outputs of those two breakers are tied together to go to the trolling motor. ONLY ONE of these breakers should be turned ON at any given moment. This gives you a more convenient means of isolating the two batteries vs. moving a cable from one to the other. Both breakers ON together results in dead battery #1 making battery #2 dead also, in short order.

Three batteries is a lot of weight for a small alum. bass boat. You will have less weight in total if you get one new LARGE battery for the TM. The best pricepoint for a high reserve deep cycle battery, in my opinion, is: Deep cycle battery, Group 31, $100

If you want to go with AGM (sealed, never add water, will also start your outboard if the other battery dies), you get almost as many amp-hrs from this one: AGM Battery, Group 31, $180

Don't add an extra 50+ pounds of weight to your BassTracker if you don't really need it.

I think I got him to see the approach of a single Group 31 as the best way to move up in trolling motor runtime and keep weight in consideration.

if you go with another battery get a perko switch, simple install and then you have a fresh battery when the first goes down, can also use trolling battery for a jump start just by turning the switch.

best solution, can also use a 3 bank without having to unhook a battery, parallel wired you would have to take off the cross connections to charge.

https://www.westmarine.com/buy/perko--medium-duty-battery-selector-switch--281469?cm_mmc=PS-_-Google-_-GSC%3ENonB%3EVendors-_-281469&product_id=281469&creative=108421560124&device=c&matchtype=&network=g&gclid=Cj0KCQjwoPL2BRDxARIsAEMm9y8DztrB5T29NDxmscH5k3DqjS4h2XJBHW29pnZy3cDa9qAF3_k83dcaAmw9EALw_wcB