Hey all,

35ft Fountain Lightning, currently boat has (2) batteries, and (2) Perko switches under the rear seat. I want to add a house battery and have no idea how I should go about this. Do I need to tie it in to the existing (2) perko switches or would I add another switch, and then move all the stereo electronics etc to the new battery and somehow tie it to the existing (2) batteries?

Thanks for any suggestions!

Typically a house batterie or one that run a stereo gets charged by the alternator the decision is how are you going too electronically separate it from the starting batteries. The two common ways are to either use a high amperage relay to join all the batteries together when the engine is running to charge all the batteries , when the engine is off the relay opens up allowing you to draw down the house batteries but not the starting batterie. The second option is a batterie isolator, an isolator is a diode pack that acts as a one way valve keeping banks of batteries separate without using a mechanical relay. The down side is the diodes use some power and the fully charged voltage after the isolator is lower than the alternator output. usually not a big deal if your running light but a high power stereo will not make rated power without full voltage . A relay would charge all batteries at the alternator output voltage and with a momentary push button switch you could temporarily combine a charged house battery with a dead starting battery to jump yourself in a pinch.

Several ways to do it. I have the same set up with 2 switches and added a 3rd deep cycle battery for the stereo.
You could use a Battery Isolater and connect it to one of the other batteries
You could just make it Battery 2 on one of the Perko switches.

The way I did mine......
I have my 3rd battery wired so it is # 2 on both of my Perko switches. I can charge Battery #3 off either engine this way and when both switches are turned to ALL, all 3 batteries are connected. This allows me to start both engines as long as I have 1 battery that is charged.

Thanks for this info. I am curious as how you set up the 3rd battery to be #2 on the switch. Does the switch need to be set to #2 while running? Currently I just turn both switches to ALL and run.

Thanks again!

Found this as a relay setup?

Running on ALL is not really the best way to run all the time. Better to have each alternator just charge one battery at a time.
I can charge the 3rd battery when running on ALL or 2 on either engine. If I go park/anchor for a long time and plan on the running the stereo for a long time, I switch both switches to 2 so only the 3rd battery gets drawn from. Both of my stereo amps are hooked to the 3rd battery as well. My 3rd battery is a group 29M deep cycle, so it is designed to be run down and then recharged frequently unlike traditional batteries that are designed to be maintained at near max volts.

Yup, when a two engine boat is on all, it’s possible only one alternator is charging all the batteries, vs both the alternators doing the charging.

Ahh ok, never realized this, have always ran on ALL, so should one normally be on 1 and the other on 2? I never understood how this works. I would definitely run a 29M deep cycle for the audio. I just want to isolate the 2 massive amps and stereo stuff for raft ups etc and not worry about it affecting my running batteries. I changed all the lighting in the boat to LED so I dont think that draws much of anything.

Very interesting, never realized this and have been running a 2 engine boat for many years lol. So I guess I need to get at my switches and see what switch controls each engine or is hooked to what alternator?

Do a google search as my answer isn’t very exacting, however there are nuances to combining multiple alternators not designed to charge with another at the same time.