Wiring a Battery Switch for Trolling Motor?

[random_guy]

So I have a fish and ski with two batteries in it. Battery #1 is the crank and everything else battery. Battery #2 only runs a 12V trolling motor. I have a Minn Kota on board 2 bank charger that I use to charge both batteries.

I live in a lakefront subdivision with two lakes, one of which is trolling motor only. Occasionally we’ll go to that lake and I’ll burn through one whole battery and then have to get the wrenches out and move the trolling motor connections over, which is a hassle. So I’ve been thinking of installing a switch. Looking over most switches, it seems that they only have two hot lugs and a common lug. It would seem that I would put the hot terminal for the trolling motor on the common lug then run a battery cable from the two hot lugs to the two hot battery terminals. However, I would have to tie the grounds on the two batteries, together, wouldn’t I?

if not, then when I switch to the crank battery, the motor ground would be on the other battery, which is connected to nothing else, hence an open circuit, right? But if I tie these grounds together, would this cause any issues with my onboard charger, or any of the other electrical widgets?

Thanks!

 

[alldodge]

It would seem that I would put the hot terminal for the trolling motor on the common lug then run a battery cable from the two hot lugs to the two hot battery terminals.

This would not be normal for a boat with a gas motor, but in your case this is how it would work. You want the trolling and the gas motor to be connected to the common. Doing this will allow the gas or trolling motor to use either bat.

The grounds are connected together

Things to note, batteries drained to low can be damaged

 

[random_guy]

I talked this over a bit with my father-in-law, who’s a retired electrician and former boater guy. He was a little worried about putting stuff in parallel since he didn’t really know the circuit. He said if it was him, he’d probably just put a set of jumper cables on the boat and forget it. The trolling motor is kinda hidden away and out of sight on this boat, so having to get to the terminals at all is kind of a pain. So I’d like to make life a little easier in this situation.

I’m thinking about doing his jumper cable concept, but with a disconnect. I’m thinking that if I ran a DPST switch, then I would be connecting the hots on one pole and the grounds on the other, just like with jumper cables. But when it’s OFF, everything is just as it is right now. If my thinking is right, when the switch is ON, I’d have two batteries in parallel, so it would still be 12V and anything on the boat would work. This would seemingly also help a situation where my crank battery was dead and I would be able to start the engine from the combination of both batteries. Is my mind right?

Thinking of this switch. Thoughts? Thanks a bunch!

https://www.bluesea.com/products/6010200/m-Series_Mini_Dual_Circuit_Battery_Switch_-_Black

 

[alldodge]

The issue with jumper cables would be the same as placing the batteries in parallel. The issue is you don't want to do this after one has gotten low because it will drain the other. Now if they are paralleled at the beginning, then they both are drained at the same time.

You only have 12V so long as both have not been drained. Keep in mind what your doing could drain bats low, and therefore suggest dual purpose bat for gas and deep cycle for troll


The switch in your link will not do what your wanting to do. This is the drawing of how it can be wired

 

[random_guy]

Sorry it’s taken me so long to get back to this, and thanks for looking at it. The switch I linked to is a simple two pole disconnect switch. So when OFF, it’ll cut both the hot and the ground connections. But you’re totally right, when it’s ON, it won’t break the connections of the old battery, so yeah the new one would suck hard on it. Hadn’t thought of that.

As for my FIL’s jumper cable idea, I think he’s more thinking of “it’s an emergency now, we’ll head back” and I’m more thinking of “I’ve got this other full battery, we can cruise a while longer before we have to use the oars”......

The circuit you posted seems like it’ll work, but it I’m concerned about the “combined” option on that switch. If I switch to combined, the low battery will drain from the fresh battery still. I’m also concerned with the implications for my on board charger. Not sure if that device would be OK with everything having a common ground. Thoughts on this?

 

[alldodge]

The bottom pic is from the install manual of the switch in your link. If you look at the right side pic it does show that both make or break both connections.

The 1-2-ALL-OFF switch can combine both, but it can also go from Bat 1 only to Bat 2 only. Just depends on where the switch is placed.

Common grounds have no impact on the charging system so long as the switch is placed in OFF,-1 or 2, just do not place in ALL (both). Note that when the switch is placed lets say on Bat 1 there is no connection to Bat 2 + side

 

[random_guy]

Yep, that make sense. I think we're in agreement that it'll work, but it'll connect the two batteries together which will discharge the full battery to the low battery. It's a double-pole, single-throw switch, so both poles are on or both poles are off.

I had in mind a double-pole, double throw switch in mind to be wired as shown. So it'll MOVE both poles together from one side to the other. However, I haven't had much luck finding something like this.

So I went back to the m-Series switch that you referenced and found that they make a variant WITHOUT a combined option. Link here:
https://www.bluesea.com/products/6008/m-Series_Selector_3_Position_Battery_Switch_-_Red

So I called Minn-Kota about the on-board charger. Expected to be on hold forever and that they wouldn't give me advice expect to go to a dealer. Got a nice lady on the phone in just a minute or two, explained what I wanted to do, and she said there would be no issues with a two bank charger having linked grounds. So it looks like I'll be ordering this switch and some battery cables.

Thanks again for taking a look!

 

[Captain Caveman]

If you have the space, I would highly recommend installing a third battery. That way you can have your trolling motor running off of two deep-cycles connected in parallel for 12volts. Then you don't need to worry about using a switch or running down a starting battery that is not made to power a trolling motor.

If you can't add a third battery, then changing the starting battery to a dual-purpose and follow instructions such as in this link for installing a marine switch:
https://www.boatingmag.com/how-to-install-marine-dual-battery-system/