Hand Made Car Charger Socket

In our last holiday in Norway, we rented a van to be used as a camper; in this rough accomodation, one missing "luxury" was the absence of a USB charging point in the back of the van, i.e. in the sleeping area, powered even in key-off conditions.

Obviously there are plenty of "external" cigarette lighter sockets on the market, but we had no car accessories shop in the surroundings... so I sorted out something very primitive, but effective enough to provide energy for our cameras, phones, gps etc during the whole holiday.

Requirements:

• no permanent modifications to the vehicle
• possibility to move our only USB charger from the drive module (in the "original" cigarette lighter socket) to the back module
• safe enough to avoid short-circuit, fuse breakage etc

hardware:

• iron wire (diameter 1.5 - 2 mm)
• pliers (a nose pliers works better)
• cap from a dishwasher liquid bottle or similar (the red one in the picture), internal diameter 25-28 mm
• a plastic cylinder (optional) of similar diameter and about 10 mm height (the white one... in my case was from a milk container)
• electrical tape
• fastons (or similar connectors)

this will be the contact point of the "core" of the USB charger

• cut a 14-15 cm long piece of wire
• with the pliers, create a small (ext diam= about 8mm), 3-turn coil on one end of the wire
• align the other end of the wire with the axis of the coil

This will be the contact surface for the "external" terminal of the USB charger:

• cut a 50-60 cm long piece of wire
• with the help of a cylinder (i.e. the handle of a tool, in my case the handle of a whisk), create a 6-turn coil on one end of the wire, with an ext diameter of about 25-27 mm
• bend the other end back on the external of the coil

the purpose of this phase is to fit the negative wire in a sort of container, with the aim of insulating the coil and to avoid it floats close to the positive terminal:

• fit the plastic cylinder (the white one,in my case) on the outside of the coil, starting from the free-end (see picture), up to the bending point of the straight end; this phase could be optional, if the dishwashing bottle cap (the red one) is long enough
• insert the free end of the coil inside the cap, trying to fit the last turn inside the small edge at the bottom of the cap

• put the positive terminal inside the central hole of the cap (it could be needed to enlarge it, depending on the cap type: you can use a small screwdriver, or even the free end of the terminal, heated with a lighter)
• push it to the bottom, again fitting it in the small edge in the bottom of the cap

now that the conductors are properly placed inside the cap, with no risk of short-circuit them, it is time to guarantee that also the external parts are protected:

• slightly bend the free end of the negative terminal outwards
• with an electrical tape, insulate the free end of the positive terminal, leaving 1cm free
• create a thicker ring of tape at the interface with the plug, to avoid the terminal slips inside the cap
• with the tape, insulate the negaive terminal and bend it again inwards, keeping it parallel to the positive one
• roll the tape around the assembly, to keep it together

positive connection:

• identiy a 12V+ wire in the back of your car, that is ALWAYS powered, i.e. even with the key off the dashboard
• if not available, make a new extension from the fuse box to the back of the car (normally there are spare fuse sockets, or connect to the fuse of the doors locking system)
• identify the fuse upstream this wire, and temporarily remove it (by the way, take note of the fuse size)
• prepare a 1m long extension, with stripped ends
• cut the wire, strip the ends, and reconnect them together with the extension (the best solution is to solder them, but probably you don't have the tools, so it can be done with fastons)
• put a female faston on the other end of the extension

negative (ground) connection

• identiy a metallic bolt/screw in the back of your car, and unscrew it; be sure there is no paint insulating it
• prepare another 1m long extension, with stripped ends
• with the stripped end of the extension, create a small loop around the srew/bolt, and screw it back firmly on the car frame
• put a female faston on the other end of the extension

connect the fastons on the extension wires to the free ends of the connector on our socket:

• the 12V+ wire to the positive terminal (the central one),
• the ground wire to the negative terminal (the external one)

insulate the connections with the tape, and fix the socket on the car frame (according to your needs)

put the fuse back in place (if lower than 7.5 Amp, replace it with a 7.5Amp one)

plug the USB charger in the socket, and verify it works properly

NOTE: take into account that the car batteries are normally suited for high discharge currents during starts, and not for slow discharge, so be careful: do not use more than 10% of their capacity without recharging the battery itself (i.e. running the engine for a certain period). If you know the capacity of your devices batteries, you can calculate
the number of full recharges you can afford before running the engine again

example: if you have a 120Ah battery, you can consider to use a maximum of 10-12Ah. At 12V voltage, this means you have 120-140Wh available. If you have a 2500 mAh smartphone functioning at 5V, you would need 7.5Wh for each full recharge; taking into account an efficiency for the charger of 70%, lets' say the gross consumption id 10Wh for each recharge... Thereford you can afford 10-12 full recharges of your smartphone.. no more :) you have been warned!