For my test project I need to use this isolated DC converter in a vehicle, activated by relay and routed to a 6 circuit fuse block.
Can I route just the positive from relay to converter input and output to the fuse block, and hook up the negative side on the block to chassis? Or should I route negative from battery to converter input?
Here's a rough sketch of what I'm doing, if it helps;
You definitely need to connect the Dc-Dc converter negative input to the battery negative - whether directly to the battery negative or to vehicle chassis is a matter of convenience.
If you want the DCDC converter output to be isolated from the vehicle electrical system, the negative side of the DC-DC converter output and its loads must not be connected to the vehicle chassis.
In your drawing, it looks like the fuse block has an isolated negative connection section which you would use for the DC-DC converter output negative connections.
Chassis in vehicles is used for the return back to the battery all the time, so you should be OK doing that. You just need make sure you have a good, low resistance, moisture resistant connection where you tie the negative side to chassis.
Also, you need to make sure you properly fuse or otherwise protect the connection from the + side of the battery to the relay. In the event of a short, a battery can easily put out several hundred amps of current. An appropriately sized fuse or fusible link near the + terminal of the battery is needed.
You might be better coming off an unused/spare 30A connection in the existing fuse block (or panel).
