Kerry Wong Turns to the ODB-II Port for a Reliable, Easy-Build Solar Trickle Charger
With a single diode, an off-the-shelf 10W solar panel can be converted for use with a car's ODB-II port — trickle-charging the battery.
Software engineer Kerry Wong has created a solar trickle charger designed to keep a parked car's battery topped up — using a simple connection to the ODB-II port in place of the all-too-often disconnected-with-ignition-off cigarette lighter/accessory port.
"On a few occasions my car struggled to start when I returned from my business trip and I had to charge the battery manually later on by hooking up a charger, which was quite inconvenient," Wong writes. "So I decided to make a simple solar trickle charger that can be left inside the vehicle and charge the battery while the car is parked.
In some older vehicles, the cigarette lighter socket is connected directly to the car battery and can be used for charging purpose. Unfortunately, for most modern vehicles, the power to the 12V cigarette lighter socket is cut when the car is turned off. Fortunately, the OBD-II port on all modern vehicles provides an un-switched and fused connection to the battery positive terminal based on SAE J1962 specifications. And the power pin (pin 16) can be conveniently used for trickle charging.
"Many adapters on the market intended for powering the car computers while chaining the battery (i.e. OBD-II battery saver)," Wong continues, "can be conveniently used to connect the solar panel to the car battery."
The build is based around a low-cost 10W solar panel with a maximum output voltage of 22V and current of 0.6A — picked so that the current is low enough, given the inefficiencies of resting the panel flat on the dashboard of the car behind a glass windshield, that it won't overcharge the battery.
Wong then added a Schottky diode to prevent battery drain in the event of a failure of the solar panel, then wired it into an ODB-II battery saver connector through a cigarette lighter adapter. The result: A compact and simple device which avoids the issue of the 12V socket being disconnected from the battery when the ignition is off.
More information can be found on Wong's blog.