If I want to install a 24v cig plug using 48v battery do I need a 48v to 24v step down converter or can I use the 48v to 12v and just use 2 poles on a fuse block?
Yeah, you need the 48v to 24v step down converter. 12v doesn't work like house AC to use 2 poles, on 12v DC you would just get 0v across 2 fuse poles positive leads.
May I suggest anderson powerpole connections? Powerwerx makes some nice kits. Standard cig lighter type plugs are terrible for conducting and have durability and resistance issues.
