battery charger question

[wiperhunter2]

That is an unusual battery configuration IMHO.  I have 2 engines (drive motor & kicker motor) and both have their own battery. 
It is always left in the 1+2 position so that both batteries are being charged if either motor is running.  My sonar runs off this power source, not its own battery.  While fishing, my kicker motor is always running and driving the boat forward in a straight line.  There is no need to charge my motor batteries when the boat is out of the water.

I have a 24 volt electric motor up front that is used every fishing trip to steer the boat.  I have a 2-bank charger that is plugged in and charging after every day of fishing.  My 24-volt system will run for anywhere from 6 to 8 hours depending on the winds on any given day.

I guess I'm questioning why you have a separate battery for just your sonar instead of hooking the sonar to the boats power buss fed by your drive motors battery like every other boat accessory like boat lights, bilge pump, live well pump, downriggers, etc.  A typical sonar will draw about 1 amp at 13 vdc and a Gp 24 battery (75Ah) will keep it going for at least 75 hours.  Even if you don't run the gas motor for fishing, you aren't going to run the sonar 75 hours and make your gas motor battery dead for cranking the gas motor.  I have never had a gas motor battery go dead from running accessories while fishing.

I would recommend you run the sonar off the gas motor accessory buss in the boat and buy a 1-bank charger for the electric trolling motor.  Whatever current rating you buy will fully charge the battery overnight.  If you really want to leave the sonar on its own battery, then buy a 2-bank.  I can't imagine you would be running a gas motor that doesn't re-charge the crank battery while the gas motor is running.  There is no reason I know of that would dictate charging the crank battery at home.

Even if you're running a mudd motor type boat and don't run the mudd motor to fish, you would still be okay to run the sonar off the crank battery.  You never completely drain the crank battery.  If your really feel uncomfortable doing that, then buy a 2-bank charger and charge the electric motor battery & crank battery after each fishing trip.

Bob, I guess you have not got into fligging or fishing without running your two gas motors. I do that in the Spring and Fall most every year now, so I can understand what Richard is doing plus I sometime run up to three finders off one battery, one for a stand alone GPS and one for fish finder from the transom and the third from the bow for side scan, on my electric trolling motor. Two units running at the same time is bad enough but adding a third unit really draws the juice, so for that reason alone, having one battery for just my finders makes sense. That way I'll never get stranded. This isn't a problem if I run my kicker but it seems more and more I'm fligging, so this setup makes sense for my needs.
I have a two bank charger for my two electric trolling motor batteries, can't remember the brand name for sure but it could be from Cabela's. I run a second, single bank charger that I bought from Harbor Freight that is just for the fish finder battery. This setup works for me but my forth battery is just for my main motor and like you said, it charges that battery, so I don't usually have to charge it.