Introduction: Modify Your OBD-II Cable to Not Discharge the Car's Battery
Since a long while now all vehicles are equipped with an on-board diagnostics port. Very often this port is available as an OBD-II connector. There are multiple devices which are capable of communicating using this connector, many of them are based on the original ELM327 chip (or it's clones). When the OBD-II interface was introduced the cables used RS-232 but nowadays USB or Bluetooth are utilised. On the photo you can find two inexpensive USB OBD-II cables.
Most of these interface cables have one flaw however. Their internal board is often powered from the car battery (+12V pin 16) when operating. This creates a problem when having the cable constantly connected to the OBD-II (for example as part of a data-logging system) as the car starter battery can discharge.
This howto will show you how to fix this problem.
ELM327-based OBD-II <-> USB interface cable
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Step 1: Open the Case and Find the 5V Regulator
Fortunately, there is usually a simple way to rewire the internals of the interface board so that the power comes from the USB port not from the car's battery. This means that the interface can be left connected without discharging the car. The basic idea is to rewire the output of a 5V linear regulator used on the boards to power the interface chip. On both boards you can find an identical regulator marked "5V regulator".
Step 2: The Wiring As It Is Now
The circuit on the interface boards will be very similar with the +12V
from the battery being fed into the input of the regulator and the output powering the ELM327 (or equivalent) interface. There is a very crude schematic of this provided.
Step 3: Find the Regulator Datasheet and Pinout
We easily find the datasheet for this regulator on Alldatasheets and by checking the pinout of the HSOP case for this chip (marked in red) we can see that the output pin is pin number 2.
Step 4: Remove the Regulator From the Board
It's best to remove the regulator from the board alltogether. Don't worry if you will want to change the wiring back to what it was later. The regulator is a standard part and there should not be a problem purchasing it later if a need arises. Unsolder the regulator and cleanup the pads.
Step 5: Wire the New 5V Supply
Now you just need to take the 5V wire from the USB cable (usually red but check with a voltmeter) and run it to pad number 2 of the previously unsoldered 5V regulator. Remeber to run a wire back to the original point as the RS232<->USB FTDI interface chip is likely powered from USB directly. Now try if the interface is still detected by the USB host after being plugged in into a PC, put the case back together and check if it works!