0x0BD2

Yes, the title is a joke but no, the content is damn serious. The problem: keeping a car (my car) parked for longer than a month without having the battery totally depleted. The solution (I hope): being curious.

First things first. I don’t need to explain you why I stopped driving my car every other day, but I definitely feel uncomfortable leaving it parked in the street while the battery is slowly but steadily discharging. What to do? I remembered having read an article about using a solar panel to trickle charge your car battery. The title of the embedded YouTube video says it all: Every car should have this. True, especially nowadays. Basically [Kerry] found that the OBD-II port gives you a permanent connection to the battery, something you can leverage to hook up a solar panel and get some juice into your battery.

Unluckily [Kerry D. Wong] had probably more time to plan everything so he used a so called OBD-II battery saver. This is a cable used for temporarily connecting the car OBD-II port to an external battery for just the time needed to replace the car’s own battery. This way you won’t lose all the settings of the car’s ECU (and your favourite radio stations). I could buy the same cable on eBay, but my battery would probably give up the ghost far before the postman would ring my bell. Luckily I buy tons of useless interesting stuff so I actually possess an OBD-II to bluetooth adapter. I payed 11$28 for it six years ago, now you can have it for less than 5$. I must confess I never used it because, you know, learning is good and everything but messing with my car always gave me the jitters.

I turned upside down my house to find it an finally gave it a try. No need to actually get to my car, I just hooked it to a 12V power supply and toyed for a while with the bluetooth connection. In half an hour I learned that:

• such devices use a de facto standard called ELM27 similar to the Hayes AT command set (face palm!).
• the commands are readily available in a document I found after 2 minutes of googling
• there is specifically one command (ATRV) that gives you the voltage of the battery.

I mean, I was only interested in the connector, but this was tempting enough. What’s better than querying your battery voltage simply leaning out of the window? Well, after I waited enough to manage to park my car in front of my window obviously.

So, following the advice of [Kerry] I opened the thing, drilled a hole in the case and connected a pair of cables to GND and +12V. As suggested I put a 1N5819 Schottky diode in series (quite evident in the photo under the heat shrink tubing). The picture here gives you an idea of the result. Attached is a 12V 10W panel. The Bluetooth signal is not strong enough for connecting with my PC, but the phone will do, after I downloaded the Serial Bluetooth Terminal app.

Will it work? Who knows, only time will tell. Meanwhile I can tell you that my battery is at 12.1V, not a number I like. Hoping for some sunny days ahead!