Batteries switch and battery wiring diagrams

I am an electrial engineer so it would be appropriate for me to write about electricity on boats. I have quite some knowledege on the subject mainly becouse I DIY built two boats along with all the wireing an electrical systems. I know a lot of people who don’t have a clue how a 12 Volt system works. They don’t even know how batteries are connected with the alternator (engine). So I will now try to explain to you some of the possible wireing diagrams.

Of course there are a lot of different ways in which you can connect your batteries on to the engine. I will show you the basic three types. They are most commonly used with inboard engines but can be used with outboard engines as well.

1 engine – 2 batteries – 1 switch

Batteries switch and battery wiring diagrams

1 Engine 2 Batteries 1 Switch

In this type of batterie connection we have 1 engine (alternator), 2 batteries and an 1-2-ALL-OFF switch. You switch the C terminal between the A and B treminals and connect the two of them when the switch is in ALL position. This means that we can choose between the two batteries or use them both to start our engine. Of course this means that our engine will only charge the selected battery or both if we select ALL position on the switch. You should put an extra care in you batteries. Your start battery should be the classic lead start battery and your service battery should be a deep cycle batterie. The difference between the two is that the service battery should be capable of multiple losses of capacity without loosing its max capactiy value. So this means that if the battery will get empty you will be able to recharge it multiple times to its full power. If you do that to your lead battery you will permanetly damage your lead battery.

The common use of the ystem is that we use our start battery (switch in position 1) for starting the engine and when driving and our service battery otherwise(switch in position 2). The all position of the switch is usefull if both of your batteries are empty or we wish to charge both of our batteries.

WARNING: You must not move your switch between positions while your engine is operating becouse it will damage your alternator.

1 engine, 2 batteries and ACR

Batteries switch and battery wiring diagrams

1 Engine 2 Batteries with ACR

I prefer to use this is a type of connection on all my boats. It is more expensive than the first one but it is automatic. We have the same wireing than in the first connection diagram just in this case we use a switch that switches 2 connections. This switch is called a dual circuit switch. The switch has 3 positions (ON – OFF – COMBINE). Combine position is used only in emergency cases when both of the batteries are empty. You will mostly be using only the ON and OFF position.

We use our dual circuit switch in combination with ACR (automatic charging relay). We connect it to a positive side of the battery. It disconnects the link of both batteries when the engine is not running. Each ACR has a min and max voltage limit. When it reaches min voltage limit it connects the first battery to alternator output. When the start battery is fully charged  the voltage on the batterie increases. Whenit reaches the max voltage limit of the ACR service battery is automaticly connected to the system and starts to charge from the alternator.The best thing when using this system is that you don’t have to care which battery you will connect becouse the ACR will do that for you.

WARNING: You must not move your switch between positions while your engine is operating becouse it will damage your alternator.


2 engines, 3 batteries, 2 switches

Batteries switch and battery wiring diagrams

2 Engines 3 Batteries 2 Switches

This type of connetcion is used on bigger boats and is a little more complicated. There are two 1-2-ON-OFF type switches used to make this connection.

In normal situation the A switch shuld be in position 1 and B switch in position 2. Battery 1 would so power the starter motor of the first engine and battery 2 would power the starter motor of the second engine. All other electrical needs on our boat would be powered of the battery 1, assuming the battery 3 is not present in the system.

Let’s say that our battery 2 is too weak to start our engine. In this case we would turn our B switch to position 1 so we could supply  right engine’s starter with batetry 1. The same goes for the same situation with the battery 1. If it is weak, we simply turn our A switch and battery 2 now supplies the left engine’s starter. The ALL position does not have any value in this kind of connection. We could use the ALL position if one of the alternators breaks and so the remaining alternator would charge both of our starter batteries. We do that by turning the B switch to OFF position an the A switch on ALL position. In this case the left engine would charge both of batteries and we would use batterie 1 for starting our engines.

Battery 3 is optional and is used for supplying all other needs on our boat. To turn off battery 1 we connect another switch in the middle. This is the E switch in the picture and it is a ON – OFF type. This way we prevent discharging of battery 1 and starting problems.We can use an ACR intead of classic ON – OFF switch like in the second example.

WARNING: You must not move your switch between positions while your engine is operating becouse it will damage your alternator.

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