My two trolling motor batteries finally have up the ghost. It's a 24v Minn Min Kota. My starting battery for my 115 horse Mercury is at least 4 years old, maybe up to 6 but still appears to be fine.  Do most of you change all of your batteries at the same time or just like all of your trolling motor batteries at the same time?

 

Should I change all of the batteries or just the trolling motor ones that are dead?

If your starting battery tests good I would probably continue to run it if it were mine. Others can give a lot better advice on battery sizing and type so hopefully they'll chime in directly. 

I change my 3 T/M batteries at the same time. Cranking battery as needed. I get between 4 to 5 years out of them.I keep them watered & the on board charger on 24/7.

It's weakest link issue, the bad battery weakens the good battery when in series or paralell.

Cranking battery shouldn't' be part of the trolling motor power system, separate independent battery.

Tom

The engine battery is separate and so I would run it till it drops.  The trolling motor batteries work together, so replace them together with the biggest size you can fit in the boat. Also recharge them immediately upon returning from the water.

I'd run it until it's dead unless your series is connected for some reason. With that said I'd test it every time before I went out and expect the one time I said "I'm running late, it should be fine without me checking this time" to be the time it's dead for life.

I also wouldn't worry about the Cranking battery, if it's still working well.  It's not connected to your TM batteries, so its age/condition wont affect them in any way.