General Advice on Moving Engine Battery to Console from Transom and Extending Battery Cables

Electrical and electronic topics for small boats
jimh
Posts: 11671
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

General Advice on Moving Engine Battery to Console from Transom and Extending Battery Cables

Postby jimh » Thu Dec 12, 2019 7:35 am

General Advice on Moving Engine Battery to Console from Transom and Extending Battery Cables

Don't Make Splices, Especially in the Tunnel
If you want to move the battery to the console from the transom, do not splice the cables so the splice is in the rigging tunnel and subject to chronic immersion in water. A splice in the rigging tunnel is the worse possible place to make a splice. You can't see it. You can't easily repair it. It is buried in a tunnel.

You Must Increase Conductor Size
If you want to extend a high-current circuit like the engine starting circuit, you have to increase the size of the conductors so that the total voltage drop is not excessive. Typically the original cables were just large enough to work for the original length. They will be too small for the longer length. You cannot extend them using the same gauge of cable.

The Size of the New Cables
If you want to extend the cables, you have two choices. You can add to the original cables with cable that is very much larger than necessary so that there is almost no voltage drop in the new cable segment. Or you can discard the original cables and replace them with new longer cables of the appropriate size. These new cables will be larger than the original cables but not as large as the cables needed if you try to retain the original cables and splice on more cable.

Let me give an example. Let's say the starter motor can tolerate a voltage drop in the cables of 2-volts maximum. The original cables were probably sized so that they provided no more than 1-volt drop, which gives the circuit plenty of margin. In the new installation we want to maintain that same margin, that is, not more than 1-Volt drop in the cables.

If we keep the original cables and extend them, the total voltage drop in the system will be too high. Even if we use monster cable to extend, there will be some drop in the new cable. There will be drop in the splice. The original drop of 1-volt was all we were willing to tolerate. So any addition adds more voltage drop.

OEM Recommendations
Usually the OEM outboard engine maker has advice about battery cable length. See

Recommended Battery Cable Size
http://continuouswave.com/forum/viewtopic.php?t=2153

Deigh
Posts: 6
Joined: Sun Aug 29, 2021 12:25 pm

Re: General Advice on Moving Engine Battery to Console from Transom and Extending Battery Cables

Postby Deigh » Sat Sep 04, 2021 9:02 pm

Great advice.