This instructable will show you how to create a switched, permanent 12V power supply for your accessories.

My vehicle has several 12V accessory power outlets, but they power off when I turn off the ignition. I recently bought a small 220V inverter so I could charge my laptop and phone. I was frustrating that every time I stopped the engine, my devices stopped as well.

Please note:

this instructable deals with electricity, I know it's only 12VDC but it can still spark, burn and give electric shocks.
please only undertake works like this if you know what you are doing, modern cars are very reliant on technology and as such are very easy to damage with crossed wires.
incorrect wiring may cause vehicle fires so don't mess about if you are unsure.

Step 1: Identify a Permanent 12V Power Source

I didn't want to go wiring straight to the battery with an inline fuse, its messy looking, requires a lot of work to route the cables and in Ireland raises questions during mandatory yearly vehicle testing.

I decided to tap off the existing fuse board.

I consulted the user manual for my car and found which fuse was responsible for the interior lights (these come on even when the ignition is off)

I then went to the fuse board and found the relevant fuse, I used my multimeter to confirm there was 12V with ignition turned off.

Step 2: Set Up the Fuse Tap

A fuse tap is like a double adapter for fuse terminals.

You remove the fuse in question and insert it into the lower section of the tap, you then insert the appropriate fuse into the top section as this will feed your supply.

You don't need to worry too much about the size of the original load on the fuse terminal, the supply is still 12V @ full amperage, the 15A fuse will still relate to the lights and the 20A to the inverter.

I used a straight through barrel crimp to extend the tail and I routed it through the dashboard and into the center console.

CAUTION: Make sure you check the direction of power flow in the fuse terminal with your meter. If you put the fuse tap in with the tail to the incoming side there will be no protection through the fuse.

Step 3: Add the Switch

I don't want the inverter to be running all of the time as it would drain my battery even while sitting there (there are lights and fans running constantly) and the onboard fans are quite loud so I installed a power switch.

There is a small section in the center console for a switch so I popped that out with a flat head screwdriver and cut a hole to accommodate the switch.

I crimped on fully insulated spade terminals to my cable to connect onto the switch.

Step 4: Connect Power

As you can see in the image, the inverter came with a standard 12V plug but this is no good for my switched installation.

I cut this plug off and took a look inside to find which wire was positive (they are both black). The positive goes to the middle of the plug, this was connected to the switch.

The negative was fitted with an eye terminal and one of the bolts in the bottom of the recess in the center console was removed and replaced around the terminal. In most cars the negative goes to any piece of bare metal on the body.

Step 5: Mount the Inverter & Test

I fitted the inverter with some strips of double sided tape and then mounted it inside the recess in the center console.

I turned the switch on to test for power on.

Step 6: Mount the Switch

Once the unit powers on you know it's okay to reinstall the cover with the switch.

The inverter is now available to me all the time, it's mounted in a compartment with a lid so it's out of sight and with the lid closed there is no sign of modification.