Third Battery/House Battery

Hi All,

Looking for some advice on setting up a 3rd battery. Currently running a 2 battery set up with a 3 position isolation switch and isolate 1 for starting and then use the other for accessories when the engine is off. Currently running 2 sounders/plotters, sometimes a fridge, radios, deck wash etc off 1 battery so drains it a fair bit when the engine isn't running. When starting I switch to the other battery then to both when the engine is running to charge both batteries. etc etc.

I'm not interested in a solar panel at this stage. Would like to know peoples thoughts, opinions or suggestions on what would be the most ideal setup.

At the moment I'm considering a Deep Cycle 3rd battery connecting to a separate power cable/fuse box for the major accesories to be connected to in behind the dash via a isolation on/off switch. I would also connect a VSR or a relay of some sort between the alternator and the battery to isolate it for starting and to allow the battery to be charged once the engine is running. Someone has also suggested a DCDC charger?

Maybe someone can recommend an installer (to have it done for the right price) or a supplier who knows what are the best options. Also happy to do the install myself but want to make sure that I'm making the right decision before spending the cash.

Thanks in advance, Kuzi.

Rob H's picture

Posts: 4802

Date Joined: 18/01/12

One tip straight up :-)

Thu, 2017-09-14 21:14

 I'll give you one tip Kuzi, dont ever run your batteries on "both".

The ONLY time you should run on both is if you need it to start or for that momentary switching between 1, both and 2 (dont ever switch via "off" when running or you risk blowing diodes in the alternator).

Reason is that if a cell fails in one battery and it is on both you will not know until you have 2 flat batteries.

You havent said the size of your boat or what you use it for mainly but do you actually NEED a 3rd batter, especially if only a single outboard?
I have twin outboards, 6.85M boat and all the usual shit (sounders stereo, plotter, LED lights etc) plus 70L fridge and my boat is almost exclusively used for multi day Abrolhos trips and have no need of a 3rd battery.
I do have a 120W solar panel thats mainly for those days when we dont move but your batteries would need to be extremely flat not to start your outboard on both, depending what it is.
Most fridges also have a low voltage cutout.


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Posts: 103

Date Joined: 29/10/12

Thanks for the tip!

Thu, 2017-09-14 21:29

I have a similar sized boat running a 200 V6.

I generally do all types of fishing but will generally either drift or anchor and bottom fish. I'm just being cautious as I've had times when a sounder drops out during engine start if I've been sitting idle for an extended period. It's more about being over powered than under powered. Maybe I'm just being too paranoid? Hence my post. 

pelagicyachts's picture

Posts: 761

Date Joined: 23/02/11

if you are looking for

Fri, 2017-09-15 09:38

if you are looking for safety/back up
my wife bought me a compact starter pack - i forget the name but its orange and about the size of a large iPhone (but thicker) have not tried it but they reckon it will jump start my 200 series diesel from dead flat
It was not cheap (around 200 ish i think) but that's about the same as a 3rd battery set p will cost and you can use elsewhere

Posts: 103

Date Joined: 29/10/12

Definitely an option

Fri, 2017-09-15 10:41

 It's something I've been considering. I need something to power some electric reels as well so could be an option and could also use it when away camping as well.


gruntre69's picture

Posts: 474

Date Joined: 15/10/16

 I like the idea of the

Fri, 2017-09-15 10:54

 I like the idea of the Nickl/metal/H type compact backup units. Very versatile.
DCDC chargers are better for getting batteries up to a 100% state of charge but they are expensive and slow (usually rated at 20A so 4 or 5 hours to charge a flat battery).

If you end up going a VSR, look at the Bluesea units. Very good quality.  They come with an overide switch with LED status so you can switch to; seperate, both or auto. I have one in my 200 series and it is rated at 500A continuious so I can force it to both for winching. This can be important if you use an anchor winch and want to switch to both for winching. This can be quite high current at times and it's good to have the bigger ballast in the batteries for this.

I'd like to install one on my boat but at this stage I'm not using a fridge so I can't be bothered with the wiring changes and have found the two batteries and very good for normal day fishing. You can get quite a bit of charge into batteries with 70-90A pouring in for an hour or two running time. I usually leave mine on both and have a voltage alarm configured though my engine management system and the Simrad.

How old are your batteries? If you notice voltage drop when starting, maybe your batteries are struggling to hold a charge?


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Posts: 103

Date Joined: 29/10/12

Batteries are quite new

Fri, 2017-09-15 11:51

 I'm heading out this arvo for a snapper session. Will be anchoring for an extended period so will check what the voltage is reading before starting the engine.

snuffs's picture

Posts: 113

Date Joined: 06/08/12

Kuzi - I have been through

Fri, 2017-09-15 16:02

Kuzi - I have been through this in detail for my own setup. Give us a buzz (or PM) if you have some questions specifically to do with avoiding GPS/Sounder cutout on engine start etc.

Vinesh87's picture

Posts: 2747

Date Joined: 02/04/11

 I looked at a 3rd battery

Fri, 2017-09-15 20:51

 I looked at a 3rd battery system but decided against it. 

Ended up with dual purpose agm batteries with 300a house batteries and a 130a (1100cca) start battery. Running with ctek smartpass no need to ever switch anything. This system has a low voltage cut off which i use for my fridges. 

I run 2 fridges and a freezer offshore for weeks no troubles. 250w solar helps!

Posts: 103

Date Joined: 29/10/12

Cheers guys

Mon, 2017-09-18 08:17

 Thanks for all your replies. Definitely has given me some info to think about it.