I have 12/24 set up. Trolling motor 19 yrs old. I have three bank onboard charger that is plugged in unless fishing. Batteries are two years old this Tuesday. Go to lake run on calm day, all day rigging, lite use bumping to move slowly, all well. Windy day, running on high 12 or lower 24 after a few hours on the lake less and less power. Tie up or drag chain let it set awhile and more power back, not seemingly as much as at start. Come home check with volt meter, 12.11 and 12.8 in the other. Put meter on battery and run trolling motor voltage drops to 7-8 volts. Let off and voltage back up. Plug charger in, leave overnight, shows full charge for all. Voltage is 12.87 run trolling motor, voltage drops to 12.5 or so. About the same on 24v side drops to 23.9 while running. Took one of the batteries out and checked fluid all full, checked with Hydrometer and all cells are in the fair range, out of bad, fair, good. I do not know how accurate they are, never used one before. Is there a true way to load test a deep cycle battery?

While having the problem my thought was the trolling motor was drawing to much current, getting hot, let it cool and works again.

Now thinking the batteries are just getting weak. I just thought that the batteries would show a lower voltage on a standby reading. Did not get to check voltage while having the problem, probably 3 hours after having problem voltage reading was taken. Would the voltage come back up just sitting if they were down 3 hours before? But lack the amps to run the TM! I think I am going to be buying battery's. I have Interstate now, any thoughts on better ones that might last longer, or is this all you can expect?
Thanks for reading, and any thoughts are appreciated! Mark