Why you should not leave your batt switch on Both or 1+2...

LouC · **Posted:** Sun Jul 26, 2015 12:12 pm

I did some measurements to test the theory that if you leave your batt switch on BOTH or 1+2 it will pull down the stronger battery to the level of the weaker one. I was troubleshooting a battery issue (turned out to be the auto ON feature on the Garmin that was draining one of the batteries). So for the heck of it I took my Blue Seas ammeter and hooked up 2 wires with battery clamps on the end. Disconnected both ground cables at each battery. Batt #1 was fully charged at 12.7V batt #2 about 50% at 12.4V.
Hooked up the ammeter with between the neg battery post of #1 and the ground cable, switch on BOTH. You could see about 5 amps flowing OUT of the battery! Tried the same thing on batt #2 and you could see 5 amps flowing INTO the battery! So for anyone that has a simple 1, 2, 1+2 or BOTH switch, listen to those who tell you do not leave it on BOTH, use either 1 or 2 unless the engine is running in which case the alt can charge both but at a slower rate. It's also a good practice now and then to check for amperage drain between the battery ground post and the neg cable on each batt with everything turned off (but the switch on for whatever battery you're checking....

GottWhat · **Posted:** Sun Jul 26, 2015 3:48 pm

This is my first boat with two batteries and I was thinking about alternating between 1 & 2 rather than both. Thanks for the tip.

ridepate · **Joined:** Tue Apr 05, 2011 9:56 pm **Posts:** 220

I always rotate between 1 & 2. BOTH seems to put too much on the charging-system.

Thalasso · **Joined:** Tue Jan 18, 2011 8:45 pm **Posts:** 333

I have three batteries and never have i changed the switch. It's a 07 and never a issue with any battery or charger

LouC · **Posted:** Sun Jul 26, 2015 7:54 pm

Do you have a battery combiner?

rpengr · **Posted:** Mon Jul 27, 2015 6:52 am

Well, if you leave it on both, you effectively have One battery that is twice as big...they will balance out at the same voltage. That's OK, but you lose the redundancy of having Two batteries, so I operate on 1 or 2, not both.

babbot1 · **Joined:** Tue Mar 12, 2013 3:46 pm **Posts:** 570

From the Four Winns manual for 2008. I don't think I have ever notice an emergency start switch but it is recommended not to use "Both" unless one battery is not enough to start the engine.

The use of one battery at a time is recommended.
Position the battery selector switch to either the #1
or #2 position.

Under normal conditions, avoid using the “BOTH”
position. Use the “BOTH” position only when a single
battery is not capable of starting the engine.
If one of the designated engine batteries fails to start
its respective engine, engage the “EMERGENCY
START” switch at the helm and attempt to start the
engine. Current is drawn from the other batteries to
start the engine. Once engine has been started return
the “EMERGENCY START” switch to the “OFF”
position. Once back at port, be sure to have the failed
engine battery checked out for possible service or
replacement.

Four Winns recommends alternating between batteries
during boating. Depending upon boating use, this
may increase battery longevity . Use battery #1 for the
first day of a cruise and switch to battery #2 on
the second day.

It's About Time · **Posted:** Mon Jul 27, 2015 8:14 am

This is great info! Thanks for posting!!! I always run on both without much thought, but this will change that for me. It makes sense that it acts as one large battery as well as it negates the redundancy of having two batteries.

Jdpber · **Posted:** Mon Jul 27, 2015 8:23 am

THIS is why i have an AUTO Isolator switch that controls my HOUSE and Engine batteries.

GottWhat · **Posted:** Mon Aug 03, 2015 8:31 am

This post might have saved me on Saturday...

Went out with my wife and kids on Friday. I used to run the boat in "BOTH" quite often before reading this post. On the last leg of the trip we were on the #2 battery. At the end of the day, I made sure everything was off before it was lifted into rack storage.

Saturday morning we go out with my wife, kids and in-laws. After the boat is placed into the water from the rack I turn on the blower and inline switch for the GPS...nothing. Switch to battery #1 and everything is good to go. Battery #1 was strong so we didn't have any problems throughout the day but if I had left it on "BOTH", there is a good chance that both batteries would have been flat.

Picked up a pair of new batteries yesterday and will install them this week.

JeffLW · **Posted:** Mon Aug 03, 2015 8:38 am

Yup, "Both" should only be used in an emergency if neither battery has enough juice to start the motor. By combining both you may have enough to get you started.
I aways switch between 1 and 2 daily.

LouC · **Posted:** Mon Aug 03, 2015 11:23 am

It was pretty interesting to actually see this on the meter. I've used it for other things too. We do a fair amount of 4 wheeling and for that reason use 12V air pumps a lot. We have graduated from the cheap auto store ones like the Superflow (pretty good for $50 actually) to the Viair 300P (3x as much but a lot faster). The Viair draws 22 amps airing up a tire from 10 psi to about 40.
I want to get a Fluke meter that can be used to measure high amperage draw circuits, like the starter circuit. This way you can find out are your battery cables really of the right gauge to prevent voltage drop. Yes you can look in the charts on the Blue Seas website but unless you know how many amps your starter draws you are still guessing.

www.iFourWinns.com

Why you should not leave your batt switch on Both or 1+2...

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