DC to DC Buck Converter

DC-DC Converter Selection

We use a Szwengao DC-DC non-isolated converter model WG-48S13R860M rated for a 30-60VDC input and 13.8VDC output at 60A. Over-sizing the converter is recommended as an inexpensive way to improve reliability.

A non-isolated converter (the cheapest and most common type) shares a common ground between input and output, allowing 36V/48V and 12V circuits to use the same ground. This is consistent with how Duffy's were originally wired.

Converters usually have an option for either a 12.0V or a 13.8V output. 13.8V is a closer match to the actual output of a new 12V battery and allows for a power drop to your 12V loads.

The maximum input voltage must be higher than your battery voltage since the actual voltage of a 36V/48V battery is greater than 36V/48V. Your battery charger also outputs a higher voltage in order to force a charge.

Implementation

Our boat had three 12V taps on two different battery pairs feeding the key switch, the bilge pump, and the 12V accessory outlet on the main panel. The following diagram shows where our boat had battery connections to feed 12V and 36V circuits.

Add a small ST Blade fuse block, such as the Blue Sea 5025   to better organize your circuit protection devices.

Blue Sea has a good overview on wire size selection. 

12V Loads

The maximum 12V load in our boat is 33A, though the realistic load is a lot lower. Note that some devices, such as motors, will temporarily draw more current than their listed amperage rating when they are turned on.