How To Use Battery 1 and 2 - Conflicting Info - Please help!

Folsom Dan

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Just add a charging relay to your system. I have two batteries and a single “on / off” switch. One battery is connected to my engine, the other connected to the house fuse panel. They are completely separated.

When the charging relay senses a charge (from the alternator), it closes the circuit and allows the house battery to charge as well. When the motors shuts off, the relay senses there is no longer a charge and the relay opens to isolate the batteries.

So - all I do when I’m at the ramp is turn the battery switch on. I can run my stereo all day - until the battery drains completely if I want - and my starting battery will be fresh.

Very simple - no worries - virtually idiot proof. No more thinking about 1-2-or both - no more worrying about draining your starting battery.



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North Carolina
Ha! I had 4 big batteries and still brought my booster battery!! Now I will add that it was mostly for filling the tubes and if I needed to boost a fellow boater!

As for the switch, I will add this, the motor does NOT charge both banks regardless of switch selection, if it does, then it's wired wrong. I know this because I had a Victron battery monitor that I could see voltage levels of both banks. As for burning out the alternator, it was told to me that it is possible, but ONLY in special circumstances. For example, I had 3 group 31 Northstar batteries for the stereo, and a group 24 for my starting / house battery. These are TPPL batteries that can absorb huge amounts of current, fast! Problem being that (to go with the pump analogy) it would be like a garden hose feeding into a sewer pipe. Normal batteries simply cannot absorb the charge as fast so you will never have a problem with having the switch on 1+2. I had my stereo isolated to battery bank 2 completely for this reason, with a separate shore charger for this reason. I DID however wire it into the switch in case my starting battery ever failed I could put to combined and start.

I will add that at first start up (in normal boats applications) I would never put the switch to 1+2, reason being if you have one dead or bad battery, they will equalize and you will then have TWO dead batteries. Far better to select either one OR two to start, then once running switch to 1+2 (never going through OFF position to get there while motor running) to charge both banks. Just my opinion, but I did spend a lot of time thinking about it!
Couldn't imagine 4 batteries. The 2 I have now already take up valuable storage space under one of our seats.