Oh gosh now were getting beyond my auto electronic knowledge.
Looking into it I think a 20amp inline fuse close to the battery may be sufficient.
My device is a CarPC running a Mini-Box M2 ATX 160w power Supply
Smart Car PC Power Supply
- 160 watts output
- Ideal car Power supply for operating Core 2 duo, Celeron, AMD systems
- ATX, 6-24V wide input range
- Intelligent shutdown controller
- ON/OFF motherboard control
- Survives vehicle engine cranks
- Battery deep discharge prevention
- High efficiency, 160 watts output
- "Anti-Thump" Amplifier remote control - 15A automotive fuse (mini-blade)
- VIA, P4 and AMD CPU support
- manufactured by mini-box.com
- ROHS compliant version
So in retrospect, I will be running 2 power wires, from the battery, one for constant and one for switched. On the switched I will use a relay.