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Dual Batteries

Osprey Rider

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G'day Raiders,

I am going to settup a second battery, in which I bought a battery switch. It didn't come with a wiring diagram. I found this diagram and was wondering if it is the correct way for 12V.

Also I was told never to use the 'both' position to charge the batteries as it can drain the better battery, is this true or myth.

Also what size cable should I buy '12mm'.

Edited by Osprey Rider
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first of all, it is possibly that one battery can drain into the other one, although you need to think about the duration that the switch would be set on both.. The reason dual batteries are set up, is that you always have a primary batter for cranking and a secondary one for your electronics/extras. When in motion and the switch set to both, there will be enough current from your motors charging system to fix this issue anwyay, so it's not a major concern, unless they are kept on both for an excessive period of time, therefore, not even a need for a switch then, just run the batteries in parrallel.

Alternatively, you can get an automatic switch which will join both batteries together for changing and when the motor is off isolate both batteries so youhave your primary and secondary one. Then use the manual switch as an emergency back up incase you need both batteries to crank. (touch wood that never occurs).

The size of 12mm cable is commonly refered to as ' 8gauge' or there abouts. it has an average continous draw of approx 60ampes. commonly seen in car doof doof systems. That should be quite enough to start most ooutboards without a concern..

give me a pm if you want to know anything about setting it up.


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I have a dual battery setup in my boat , I use the #1 battery on the way out , and the #2 battery on the way in. When at anchor , I select the #2 battery ( the larger capacity one) to run pumps , lights , rockbox etc. This leaves #1 battery to start the motor.

Always connect batteries +ve to +ve and -ve to-ve , this will double your available current , but volts will remain at 12. +ve to -ve connections will double your voltage , definately not where you want to go .


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  • 3 weeks later...

Hi all,

Just read this post, be careful re parallel or serial connections. Choose which is best for your application. In relation to drain on both, in this case the charge of both will balance out. Also if different sized batteries are used they will never fully charge the larger when on both.

You could use both parallel and serial, serial for a one starter battery, one/two or more in parrallel for house applications for example. your diagram doesn't look too bad but re legal reasons (man don't want to hear that your boat went up) can't say yay or not, wiring size you've heard about. Take up one of the guys on their offer if you need help. It's not hard and you've probably got it done by now, still thought i'd let you know.


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Good Morning Osprey, that is the basic way to wire uo a twin abttery set-up and as Ross said use one battery at a time. The only time it should be on the "both" setting is when either No1 or 2 battery is not strong enough to crank your outboard. If you run it on the both position the good battery will be drained by the weaker/smaller battery. A better way is to fit a battery distrubution cluster with a voltage sensitive relay. This will cost you about $150 more than the basic way but it the best way to do it and we have fitted to many boats.

Hope this helps,


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