A Non-Isolated Converter is the lowest cost approach. The major characteristic is that one lead is common with the battery pack. The SPS Non-Isolated converter has the +12 V and the B+ (48 or 36 VDC) leads common. The Converter develops 36 VDC on the output lead so that the Red lead is 48 VDC and the Black lead is 36 VDC. The difference is 12 VDC and all the 12 V loads perform as normal.
In addition to being the lowest cost, this approach has the advantage of allowing the key switch (+48/36 VDC) to turn a radio on and off.
An Isolated converter has a transformer between the input (48/36 VDC) and the output (12 VDC). This provides galvanic isolation (not physical connection between input and output).
An Isolated converter should be used when any one of the following is true:
The vehicle will be operated in wet, dirty or corrosive (e.g. salty) environments
The installation will be by an end user or a dealer with limited electrical experience (not familiar with the difference between isolated and non isolated)
There is any pack voltage on the frame of the vehicle that can not be corrected
There is any+/- 12 VDC connection to frame that can not be corrected
The Installation Instructions for the Non-isolated converter explain how to determine if there is voltage on the frame or a 12 V connection to frame.