Trolling battery bank, starting battery and a 35 amp alternator.

[Trimitup]

Hey y'all,

I'm just putting together a trolling motor battery bank for the new (to me) boat. The plan is to have 2 batteries in parallel for the trolling motor (a 12 volt - 50 lb thrust Minn Kota) and a separate starting battery.
I bought a good onboard charger (ProMar 13 amp 5/5/3) for dry land charging of all 3 batteries. So I'm covered there.
The question I have is...
Is it worth buying a 3 battery isolator and hooking all 3 batteries to the outboard alternator or just to the starting battery?
When the engine is running I know it will charge the trolling batteries as well as the starting battery because of the isolator but would it be worth it?

I know my alternator (2004 Evinrude 115 2-stroke DI) supplies 35 amps peak according to the manual. That would be at full rpm of course and I don't expect the alternator to keep all 3 topped up but I would feel pretty silly if dry land 120AC was my only source of charging my trolling batteries. I would also feel pretty silly if I tax the crap out of the alternator/motor.

I don't post often as you can see but I'm always looking for info here. Great resource!!

 

[alldodge]

Hey y'all,

I'm just putting together a trolling motor battery bank for the new (to me) boat. The plan is to have 2 batteries in parallel for the trolling motor (a 12 volt - 50 lb thrust Minn Kota) and a separate starting battery.
I bought a good onboard charger (ProMar 13 amp 5/5/3) for dry land charging of all 3 batteries. So I'm covered there.
The question I have is...
Is it worth buying a 3 battery isolator and hooking all 3 batteries to the outboard alternator or just to the starting battery?
When the engine is running I know it will charge the trolling batteries as well as the starting battery because of the isolator but would it be worth it?

I know my alternator (2004 Evinrude 115 2-stroke DI) supplies 35 amps peak according to the manual. That would be at full rpm of course and I don't expect the alternator to keep all 3 topped up but I would feel pretty silly if dry land 120AC was my only source of charging my trolling batteries. I would also feel pretty silly if I tax the crap out of the alternator/motor.

I don't post often as you can see but I'm always looking for info here. Great resource!!

You could but I don't see it being worth it. Your probably not going to be running long enough to do much, it will help but probably not much

 

[NYBo]

You could but I don't see it being worth it. Your probably not going to be running long enough to do much, it will help but probably not much

I agree.

 

[airshot]

Be putting an awfull lot of load on that OB, probably shorten the life of its electrical system.

 

[Silvertip]

Why feel silly over shore power for charging. It is, after all, the most practical.

 

[Trimitup]

Why feel silly over shore power for charging. It is, after all, the most practical.

The reason is we only really fish when we go camping. When we go camping it's usually for a few weeks and our sites don't always have power. I have a 160 watt solar system on the 5th wheel and we do bring a little 2000 watt generator but I don't love running it mostly for disturbing the peace and quiet. The solar would be fine but as you can guess only works during the day (when we be fishing).

I agree with you guys that it would probably would be an exercise in futility trying to get any charge into the batteries while the motor is running. The math kinda says otherwise though...

Assuming 50% to 70% average load on the 35 amp alt @ maybe 2 hours total run time per day is 17.5 to 24.5 amps split amongst 3 batteries is 5.8 to 8.2 amps per battery. Correct me if I'm wrong but that's a pretty reasonable charge rate. You could also assume the starting battery wouldn't take its full third as it will be pretty topped up considering it's not used much.

I'm 2 beer in but does that hold any water?

 

[lmuss53]

I think the run out time is the key here. If you are running an hour to go fish I would by all means put a Perko switch or combiner in and tap the big motor to top off the trolling batteries. Once the cranking battery is topped off, the alternator is just bleeding off any extra current through the regulator anyway isn't it?

If you are running 10 minurtes to your fishing spots it might not be worth the trouble.

I have a Yandina combiner on my pontoon, if the cranking battery is over 13.1 volts the combiner kicks in and puts the outboad charge to a second "house" battery. If the combined batteries fall below the 13.1 number the combiner kicks out and keeps the second battery from pulling the cranking battery too low. I think something like this would be perfect for your setup, it would let you tap the extra voltage from your big motor to charge the trolling bank without putting the cranking battery level at risk. One thing about the combiner, the second battery has to be within about 6 feet of the cranking battery, you can't make the wire any longer. I do think you can special order a longer wired unit.

 

[Silvertip]

A 50# troller will suck about 40 amps in one hour at full speed and something less than that at slower speeds. But lets use just one hour run time as a factor. The 40 amps is sucked from the batteries means you need to run five hours if you are charging at an 8 amp rate. One hour or more at wide open throttle. People tend to way over think the starting battery. Starting an engine that is properly tuned and an operator that knows how to start it without killing the battery each time requires very little demand from the starting battery. If it wasn't being charged at all you could probably run all summer charging it manually a couple times. But back to the real issue is that you have two troller batteries. Leave one in the boat and leave the other at camp on the solar panel or in the vehicle equipped with a quick connect system so whoever goes to town for supplies or shopping can charge that battery. You should be able to get a nearly full day on a good troller battery anyway so doubling up is not really necessary unless you tend to run the troller full tilt all day long and that is not likely. They are called trollers and not prime movers.