Originally Posted by sailvayu
Cole Hersee PN 24117-01BP It is used to reduce the size of the power switch for the autopilot. Help reduce voltage drop to the autopilot from wire runs as well. Hope this helps
Expanding on what sailvayu said:
I'm not sure which SmartPilot you have, but we have the S3G model from Raymarine
. It has an input for a remote
switch, which accomplishes your setup without needing the external relay.
What your external relay accomplishes is reducing the length of the run of the heavy high-current carrying cables
supplying the autopilot - enabling you to use lighter-gauge cable. For example, let's say the distance from your autopilot to your breaker panel was 25 feet, the distance to your house battery
bank is only 5 feet, and your autopilot calls for 25 amps. Allowing for 3% voltgage drop and cable rated at 105 degrees C , if your SmartPilot needs 25A, then you'd need a minimum of #4AWG cable
back to the breaker panel - and that's not even taking into account the existing cable run supplying the breaker panel itself from the battery
bank. Since you really need to also account for the total cable run - from the autopilot to the breaker panel switch, and from the panel to batteries, you would probably need at least a #2AWG cable and possibly more, and worse - you may actually exceed the safe current-carrying capacity of that panel supply cable and need to replace it with larger cables.
Instead, if you use the remote
relay, then you'd only need #10AWG cable to make the 5 foot run and back to the battery bank. If the solenoid's energizing coil only needs 5A, then long run back to the breaker panel could safely use 18AWG cable, and you'd only need a 5A breaker, not a more expensive 25A breaker.
There are three minor drawbacks to think about
: (1) the relay uses some battery power, so in the overall scheme of things one would have to consider that in power budget
planning, (2) there's one more item that could possibly fail, but most importantly (3) the heavy cable from the autopilot/relay to the battery bank should be protected by a fuse - not just connected directly to the battery bank.
Sometimes, there's just no easy way to run long lengths of heavy-gauge cable, so you have to compromise a little. That's apparently what the previous owner of your boat decided to do.