Car Battery Charger From Spare Parts in Mint Tin

I needed a trickle charger for a seldom-used vehicle. Not wanting to spend money, I slapped this one together in about 30 minutes from parts lying around:

Bill of materials:
- Mint tin box or other enclosure
- LM317T regulator in TO220 package
- Heatsink for TO220
- Spare Notebook charger with 14V or more output
- 1 ohm, 2W resistor (I used 5W because that is what I had)
- #4-40 x 3/8" screws and 2 #4 nuts
- Wire
- Cigarette Lighter plug
Tools
- Solder
- Hot Glue Gun
- Soldering iron
- Dremel tool with abrasive cutoff wheel
- Drill with 1/8" bit
- Wire cutters n strippers

Step 1: Prepare the Box

Drill at least 2 holes in the lid to attach the heatsink and LM317T regulator

Gently bend the LM317 leads to a right angle

Align the mounting hole on the LM317 to one of the mounting holes for the heatsink

Mark a slot for the leads to pass through the lid. Cut this slot out with an abrasive cutoff wheel in a Dremel tool (High Speed rotary)

Use the rotary tol to cut a slot in both ends of the box bottom, and fold down. Close lid and make sure there is enough clearance for the cables to pass through the slots when the lid is indeed closed.

Step 2: Assemble and Wire

Use the two # 4 screws and nuts to fasten the heatsink and LM317 to the top of the box, with the LM317 leads to pass through to the inside of the box.

Cut the power jack from the notebook power supply. This is usually a round ( barrel) plug which plugs into notebook). Strip and tim the leads.

Usually, the center wire is positive, but use a voltmeter to ensure this is true

Strip and tim the wires which will connect the regulator box to the auto power plug. I used 10 feet of 16 guage lamp cord because it was handly. long is good, as it allows the electronics to be placed out of the weather.

Wire the box per the schematic. Make sure to not have either of the pairs of wires to touch the metal box. The metal box is already connected to one of the pins of the LM317. Repeat, DO NOT WIRE ANYTHING TO THE METAL BOX.

Use generous amounts of hot glue to fasten down the resistor, the wire leads, the leads of the LM317, and the points where the leads pass in and out of the box.

Plug in the notebook power supply, and measure the voltage on the auto power plug. This needs to be 13.7V or more for proper operation. In the below pic, it is 16.16V, a relatively optimal voltage.

Step 3: Usage

This is a relatively crude charger. As it does not have a precise termination voltage, it should not be left on the batter indefinitely. An overnight charge should be about the limit.

Plug the power supply in, and plug the auto power plug into a cigarette lighter outlet in the car. YOu should notice the dome lights and everything become a bit brighter. Make sure everything is turned off, and leave the car for 2 to 12 hours, longer maybe for a totally dead battery

This power supply provides a contant current of 1.2A. Depending on the power supply voltage, this peak current will diminish somehwat as the battery approaches full charge

Step 4: Theory of Operation

The LM317 is normally used to create a constant voltage. In this application its behavior is exploited to create a contant current source. The LM317 regualtes int he following manner:

The device does whatever it can to keep the OUT pin exactly 1.2V higher than the ADJ pin. For example, if ADJ is shorted to ground, then OUT is 1.2V. In this application, we have connected a 1 ohm resistor from OUT to ADJ, and connected ADJ to the car battery. If the battery is 12V, then OUT pin of the LM317 is 1.2V greater, or 14.2V. Because there is a 1 ohm resistor from out to ADJ (and car battery), and there is 1.2V across this resistor, therefore 1.2A flows per ohms law

V = I * R ===> 1.2V / 1 ohm = 1.2A ( a bit of algebra here)

The LM317 will perform exactly as described as long as the following conditions are met:
- the input voltage of the LM317 must be about 1.5V greater than the OUT pin
- The LM317 does not overheat and go into self protection mode (hence the heatsink)