The following circuit contains an Uninteruptable Backup System (UPS). So if the main battery fails, the UPS auto-switches on to power the essential instruments. (The UPS senses the voltage drop.)
The dual-pole switch connects the UPS to the main battery, and switches it on at the same time. When switched off, this dual-pole switch by-passes the UPS, via the bypass line, and powers the essential instruments from the main battery. This is in case of UPS failure, so the essential instruments can still get power.
The diode protects the UPS from reverse current, when the UPS is switched off.
This is all very logical (my circuit design).
However, what should I do with the negative cable on the bypass line? Will that need a diode in it too, or can I connect it direct to the main battery?
I would be grateful for any advice.