How to properly use 2 batteries -2016 Sundeck

Bob from Kamp Kozy

New Member
Sep 6, 2019
22
Marblehead Ohio
Boat Info
2016 240 Sundeck, 4.5L
Engines
4.5L/250hp Mercruiser w/Bravo III
Just bought our first Searay in Catawaba Ohio. Getting conflicting opionions how to properly run and use 2 bat set up in this boat. Have been told by Mercruiser Dealer to run on ALL always, told by most others to use just switch back and forth alternating days.
1. Will both batteries on THIS boat charge while only 1 or 2 is selected?
2. If I am running say #1 for the day, anchoring using #1 for the radio, then if in trouble should be able to start with #2 assuming it charged also while running or still charged from last run.
3. If only one battery charges while running, should we run on ALL and then switch to #1 or # when starting or sitting?
4. Also, does all the electronics and bilge pump run off one House battery?
I will NOT have shore power on this.
 
I've had two day boats with dual batteries and a switch. I always ran the switch on "both" when the engine was running. I always parked, anchored out, spent the night with the switch to 1 or 2 (alternating). Never had any issues...

From my recollection you're only charging the batteries that are selected when the boat is running.

Your bilge pump is likely wired directly to one of the batteries and is not affected by the changing of the switch. Everything else on the boat should run through the switch (I believe).
 
One absolute, DO NOT anchor with the switch on BOTH/1&2

1) Depends on if there is some sort of isolator/combiner that allows the alternator to supply both banks with engine running, regardless of switch position. Without, then the answer is no
2) Yes, providing nothing is wired direct to that #2, drawing it down while running, and there is no alternator contribution through an Iso/combiner.
3) Yes, thats a good practice
4) with a 1/2/BOTH switch, the ideal scheme is to have ALL loads go through the common post of the switch, with the exception of some essential loads such as auto bilge, CO, etc. Loads will draw from which ever battery you have the switch turned to
 
To confirm for yourself, test battery voltage with the switch on 1 then on 2.

You will be able to tell if only one or both batteries are charging.
 
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My owner's manual says that to charge both batteries, the switch must be set to "BOTH". It recommends running on both for charging purposes. I switch to one battery to run the radio when stopped, switch back to "BOTH" to start.
 
I assume you have the Merc DTS system on your engine based on year and model. That system is hardwired to just one battery so that it must be selected while operating the engine (select that battery or both). The other battery is your "house" or backup battery and should be used while on the hook. Do not operate the boat with just one battery selected that is not the battery with the direct DTS connection.

And yes, the engine only charges the battery(s) selected on the switch.
 
1) The alternator will only charge the connected battery with a normal 1+2 switch. There is an inexpensive electrical widget called an Automatic Charging Relay (ACR) that will charge both batteries whenever the engine is running. It senses charge voltage and opens up a connection to both batteries, then disconnects when the system is discharging the batteries. This way you can run on 1 or 2 and use the other as a true backup. Blue Sea Add A Battery and Add A Battery Plus kits are popular and well regarded. I installed a plus kit so that I can plug it in at home the night before we go out, one of the best improvements I have made to the boat.

2) If you are running on #1, then you should anchor on #2. That way if your radio goes dead because you didn't have a full charge on #2, you still have your starting battery fully charged.

3) I am a relative newby, but I got stranded using the 1+2 switch in both. I am not sure how, but one of my batteries failed in a manner that kept the other from charging properly so that I didn't have enough juice to start the boat. Ended up buying two new batteries (because if they are interconnected you want to keep the same type and age) and the ACR. I always run on one battery now.

4) The bilge pump is wired directly to battery 1 as well as the bilge switch at the helm through 1 or 2, whichever is currently selected. The electronics will run off whatever battery is selected as well. The small bowriders don't have house batteries, although you can set one up using the blue sea kit. I didn't install the blue sea switch, but plan to create a house battery for the radio this winter because starting the boat interrupts the bluetooth connection.
 
Thank you all for taking the time to educate the newbie. This is great info and much appreciated. I will take a look to confirm how things are wired, throw a voltmeter on it to check it and then figure out how to explain this to the family!
 
I never run with both on. The entire purpose of having a backup battery is to always have a spare charged battery in case you run the other down. Switch back and forth.
 
I also start and run on both batteries. This keeps them charged. When on the hook or at the beach, will alternate between #1 and #2 when playing the radio. I was able to get 12+ seasons on my original set of batteries doing it this way. I also store them in the basement of the house during the winter.
 
I dedicate one for house and the other for start. I switch to both while underway.
 
As mentioned in several posts, it charges the batteries selected on the switch - so if "both" is selected, both receive charge voltage. Will both charge - that is a whole other thread about mixing batteries, sizes, ages, capacities, etc.
alternators are dumb - they just put out voltage and current.
 
On a small boat like a Sundeck, there really is no need to have a dedicated "house" battery, all you are really running at anchor is the stereo, a few electronics, maybe flushing a head. I have two batteries in my 185 and this is what I have always done.

  • I alternate between Bat 1 and Bat 2.
  • If I run the battery down while at anchor (rare/never), I switch to the other batter to start the engine, then switch back to the dead battery to let it charge.
  • NEVER turn the batter switch to OFF with the engine running. It can damage the alternator.
  • At the end of each day on the boat, part of my closing up routing is I switch to the other battery, so I am ready to go next time and won't forget to switch batteries.
  • Occasionally I will run on "Both" if it is a long run and I haven't had the boat out for awhile to charge both batteries. But it is rare and I have really had no battery problems doing this. I use two starting batteries exactly the same.
 
I had an 05 260 with the same switch

Always on “both” except at anchor when I’d select “1” if it was an odd date or “2” if it was an even date.

A ProNautic charger connects to the batteries (individually) directly so your switch has no effect on shore power charging. Underway, with the switch on “both” your alternator is charging both batteries.
 
I have a question. This is my fourth season with my 215 express cruiser. When I bought the boat the broker told me to never move the switch at all while the engine is running. Someone above said to never turn it to off which I agree but what about switching between 1 and 2? I was always under the impression not to move it all while running
 
Switching from 1 to 2 to both is fine, just don’t switch to or past off.
 

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