I often do work in the load bay of my van, so I need a source of electricity there. I use both 12v and mains equipment, so I need to install a 12v socket and also a power inverter to get mains out of it. Then I can use battery chargers, power drills, a glue gun, laptop, maybe even a TV and kettle. Funky!
The solution: Lay a line to the back of the van, and install 12v sockets. I’ll have the sockets mounted in a metal box which I’ll fasten down. I hereby christen that box the “Auxiliary Panel” for reference.
What will it all involve? The core of it will be a 12v line to the back which will power the sockets and the inverter. There were a number of solutions, but only one seemed the right choice. Some ideas were:
- Add a single wire; one end to the positive of the cigarette lighter socket, then feed the other end to the load bay. For ground, attach another wire to the chassis in the load bay. Voila, these two wires I attach to a socket that I mount to a box and fasten in the load bay.
- As with option 1, but take the positive from the battery (via a suitable fuse located near the battery) rather than the cigarette lighter socket.
- The complicated option I chose. See below.
The problem with option 1 was the cigarette lighter circuits cannot handle more than 20A. This is fine for a low power inverter, or battery chargers, but not much more.
The problem with option 2 was that my auxiliary panel would have power 24/7, so if I were to accidentally leave anything connected, the battery would drain and I’d be stranded, unable to start the van.
So option 3 takes the best of both worlds. It takes a feed from the positive terminal of the battery which gives it ample power. It also takes a feed from the cigarette lighter socket as a signal, which disconnects my box from the battery when the cigarette lighter socket has no power. (The cigarette lighter socket only has power when the key is in the ignition). All this needs is one relay (plus suitable fuses and of course a 12v socket).
And then I took this one step further. I also want my box to have power either when the rear doors are open, or when the key is in the ignition. This complicated things though. Not only does this entail an additional relay, but a number of diodes.
So, to follow up the last diagram I posted of my van’s interior rear lights, here is the modification which uses that signal from the interior lights (and the cigarette lighter socket) to energize the rear accessory sockets when the rear doors are open or when the key is in the ignition:
RLY1 is the big boy relay. When either the doors are open or the key is in the ignition, this relay will be energised, thereby giving power to the auxiliary panel socket from the battery.
Getting RLY1 energised when the key is in the ignition was the easy part: the positive from the lighter socket could have gone straight to the relay coil. The complication is the doors. I need the relay to be energised when the doors are open – so when the door switches are closed, and therefore when the wire coming from them is held at zero volts. So I need RLY2 to flip this around: when the doors are open, their switches are closed, the wire coming from there is at zero volts – this energises RLY2, which then via D2, energises RLY1 with +12v.
D1 and D2 are needed so the cigarette lighter socket signal and the signal from RLY2 don’t interfere with each other. Without D1 and D2, having doors closed would actually energise the cigarette lighter socket! Plus all sorts of weird things could then happen. In theory only D1 is needed, but D2 makes things symmetrical and gives peace of mind if I expand the circuit at a later time.
D4 is (probably) needed to stop current flowing when it shouldn’t. If the doors are shut, (so their switches are open), no current should be flowing through the ceiling lights. But if RLY2 was closed when the doors shut, the relay could pull current through the lights. Weird things could happen. So I thought I’d add it just in case.
I used a total of 4 fuses: one near the battery, one for RLY2, and one for the socket at the auxiliary panel. (The fourth is not shown on the circuit diagram: a 0.25A fuse at the cigarette lighter socket)
Voila. I now have convenient power in the back of my van.
For the sake of brevity, in the above diagram I omitted the aftermarket timer that was installed which turns the lights off after 30 minutes (when the doors have been left open). This is shown below. It doesn’t alter how the circuit above is connected.