I am sure there is a way. There is connecting batteries in series and parallel. I can't remember which does what, but one way adds the voltage of all batteries connected and the other keeps the same voltage. You want the one that adds all the power of the batteries with the same voltage.

Actually I think its parallel. Someone can tell me if I'm wrong. If you hook a 12volt batt to motor you have 12volts of power to it. If you take another batt and connect the positives together and the negatives together, the motor still has 12 volts, but should last longer because you have 2 batteries.

I guess that's not a for sure answer.

Rich, thanks for the info.
Larry G.

Larry,
Think of it like your flash light... you put the batteries in positive to negitive to increase voltage this is called series.
Now to keep the same voltage but increase amps hook the batteries positive to positive and negetive to negitive this is perallel.

But since you want to add batteries to your 24 volt system, you need to add two 12 volt batteries. like this:

your current batteries are in series to make the 24 volts, so you do the same thing with the other two and treat each pair like two individual 24 volt batteries.
Now hook your two 24 volt batteries in parellel.

If all of this makes sense it should go like this:

12volt A to 12volt B == positive A to Negitive B == 24volt AA

12volt C to 12volt D == Positive C to Negitive D == 24volt BB

24volt AA to 24volt BB ==
Empty Negitive AA to Negitve BB and to negitive side of trolling motor
Empty Positive AA to Positive BB and to Positive side of trolling motor

And to charge you would use a 4 bank 12 volt charger
or a 2 bank 24volt charger

Note:
You would NOT want to add one battery to this because:

If you hooked it up in parellel it would try and charge the 3rd battery to 24volts.  Bad idea for the 12 volt Battery

If you hooked the 3rd battery in series it would then be  a 36 volt system. Again Bad Idea for a 24 volt motor  



I hope this helps
Jim

Thanks everyone for your posts.
After reading all the comments, I have decided my attempt to add one more battery to my 24 volt system has come to an end.
It seems you can add 2 more or no more.
It was a just a one day need, for sockeye fishing on Sat.
Larry G.

Jim's a perfeshunal mekanick.  He smart.