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How do I connect my batteries to my engine?

2011 July 19

Dear Boat Doctor,
I am trying to connect my batteries to my new Beta 60 engine. I have my batteries configured into two banks, one for the house and another for the engine. My engine has a conventional alternator.

My first attempt was to connect my engine bank to the starter post on the engine starter and the alternator output to the house bank positive terminal. This seemed to work but had some strange side-effects: my banks were tied together and I could not stop voltage from flowing to the starter post. I next tried to connect the alternator output to the engine bank. I still could not stop voltage from flowing to the starter post.

I want to separate the banks and be to be able to turn off the battery switches when I leave the boat to prevent anyone from starting the boat from the cockpit using the engine battery switch.
Brian Flannery
Columbia, Maryland

Dear Brian,
In a normal charging situation you want the output of the alternator connected to the house bank. The theory here is that the house bank will be the most discharged and will cause the alternator's regulator to produce the maximum output, if you connect to the engine bank the regulator would see a charged battery and would just float charge it. In a case where the banks are paralleled via a charging relay you can connect the alternator to either bank since they are effectively one bank when the relay closes in the presence of charging voltage.

The Beta engine is marinized Kubota engine; essentially a large tractor engine. In a tractor there is typically just a start battery, and for simplicity sake, Kubota connects the starter post to the alternator output to allow just a single-cable hookup.

What this means is that if your alternator and starter are connected to the same battery you can do it with just one cable, from the starter post to the battery. It is important to not connect just to the alternator; the cable between the starter and alternator is not adequate to carry starting currents but is adequate for charging current. This situation will only charge correctly if you are using a charging relay to tie the banks together.

If you use a "smarter" device for charging, like a Balmar Duo-Charge, you'll need to connect the engine differently. You would connect the engine bank to the starter post, and the alternator to the house bank. To alleviate the problems that you reported initially, you would need to remove the on-engine cable between the alternator and starter post.

A simple way to visualize this is to draw the circuit that you expected and then add a connection between the alternator and starter.