Switched Solar?


My marine electrical includes an Automatic Smart Battery Switch and was wondering how to install a trickle solar charger to it.

Should I bypass the switch, or should it be installed just like a Guest battery charger to the “house” battery? If so, do I still need a charge controller, and where should it be located (between battery and panel, or battery and smart battery switch)?solar panel

Thanks for all of your help.


Hi Greg,

In my opinion, the best location to connect a solar charger is to the house battery, but at the battery switch end of the boat battery cables.

Connect the positive lead of the charger to the battery switch stud with red cable from your house battery and connect the negative lead of the charger to the ground bus. Connecting the charger to the switch will help reduce the amount of clutter on the battery.

If you want the solar charger to charge the starting battery also, turn the switch to either ON (VSR will control the charge) or COMBINE (batteries are mechanically paralleled with the switch).

Hope this helps,


Close Call. Go Small.


I have a 12 volt 55 pound thrust trolling motor on my boat’s marine electrical system.

What gauge of boat wiring is recommended to run 25 feet of wire from the motor to the boat battery?

Thanks,Chalkboard math


Hi Kurt,

Your boat’s trolling motor will draw 15-20 amps.

To maintain a 3% or less voltage drop at 20 amps and a 50 foot total run, you would need to run 4 AWG marine wiring. If you run 6 AWG, your voltage drop will be 4% or less and the wire will weigh and cost 2/3 as much as the 4 AWG. It is common practice to run 6 AWG with your application.

Hope this helps,


Boat Wiring Size


Thank you for your answers to so many electrical questions. I plan to redo my fishing boat wiring and have a couple of questions.

My 1977 18 foot Starcraft has an outboard motor with it’s own circuit and will not be involved in the rewire.

My current marine electrical has a battery in the back of the boat, approximately 16 feet from the fused, marine electrical switch panel.Click to download Kevin's boat wiring size calculator

The old boat wiring is a rat’s nest with each circuit wired individually.

I will be using the standard small boat items: a depth finder, FM radio, bilge pump, navigation lights (bow and stern), gauges and a rear-mounted gas tank.

I would like to run a duplex cable from the rear battery to the switch panel. The total duplex run would be 32 feet and the total amp draw should be no more than 30.

I think I need a 10 AWG duplex cable for the 30 foot run and 16 AWG boat wiring for leads from the switches to the instruments (+) and bus bars (-)?

Does this sound right?

Thank you for any help you can provide.


Hi Tom,

I would recommend using my boat wiring size calculator to answer your wire size questions.

  • Leave the voltage drop percentages in column B and G alone.
  • Enter the current draw of you switch panel devices in column C and H.
  • Enter you total wire length run in inches for each device (switch to device and back to ground) in column D and I
  • Enter your total wire length run in inches for the panel feed (battery to panel and back to battery) in cell 25D – 384 is your guess.
  • Your wire sizes will be in Column E and J.

Let me know if you have any questions,


Mercury Optimax – Boat Battery Wiring

Hello Kevin,

I have a 17 foot fishing boat with a 2004 115 Mercury Optimax and a 2007 9.9 Mercury kicker. I only have a single cranking battery for both motors. Mercury 9.9 Kicker

I was told that my marine electrical setup should have two cranking batteries, one for each motor hooked up to a boat battery switch.

The guy said I could damage the computer in my Optimax if by accident one motor was started while another was running. Would the automatic smart boat battery switch (the one you designed for your sister marine electrical site) prevent that if each motor was hooked up to separate batteries with this switch? How detailed is the marine wiring for this switch?

I really like the idea of this switch. My old boat had a Perko battery switch and I always forgot to switch it back and forth.



Hi Corey,

Your system is very common and I have not heard of people damaging computers in this manner. I have, however, heard of people damaging computers by turning the battery switch to the OFF position while the engine is running.

With our smart battery switch system, when either battery is above 13.7 volts, the batteries are connected together. When it drops below 12.8 volts, the switch opens and the batteries are not connected.

If you had your kicker connected one battery and your main connected to the second battery, while charging, the batteries are connected together and they would not be isolated to prevent the situation that you are describing.

The connection is fairly simple and we supply all of the boat battery wiring required with the Smart VSR Battery Switch System.

Please let me know if you have any questions,