Switched vs Unswitched
For most gadgets, powering them only when the Model S is operating is just fine. Your phone only needs to be getting a charge while you’re in it and driving. Otherwise you’re likely taking it with you and leaving. So for those normal cases a switched power source is perfectly reasonable.
There are times when having a supply that is constant (unswitched) would be useful. One example is powering a dash cam while the car is parked to catch a hit and run. In that case you want a constant supply of power regardless of the state of the car.
Gadget Power Options
There are basically 4 ways of powering gadgets from the Model S 12V battery:
- Plug it into one of the USB outlets or into the 12V outlet. All 3 of these outlets are switched (turn off when the car turns off). This is what 99% of people use.
- Find a 12V supply line (usually unswitched) somewhere in the car and splice/tap into it. Older Model S’s (circa 2013) had an extra 12V line tucked up above the rear mirror. Newer ones don’t have that but there are other 12V lines in various places throughout the car if you dig.
- Drill through the “firewall” and tap directly into the 12V battery, preferably with a fuse somewhere in that mix!
- Hack the on-board diagnostics connector to provide unswitched 12V power.
Hacking the on-board diagnostics connector
When thinking about the kind of way I wanted to get constant power I wanted something that was 100% reversible that didn’t involve any cutting of existing Tesla wires or any other permanent modifications of the car. Enter the ODB.
The on-board diagnostics connector (ODBII) is legally required to be in every car. Its there to help diagnose engine issues etc. Tesla has much more advanced ways of diagnosing the car and can even do it remotely but they still had to provide an ODBII connector. There are 2 key requirements of interest for that connector:
- It must be easily accessible and inside the passenger compartment
- It must provide a 12V unswitched power source.
In the Model S (for US cars) the ODBII connector is on the drivers side just above where your left foot would rest if stretched out.
I saw a post on TMC that inspired me to look into using the ODBII connector for power. I think the approach was overly complex but the idea of using the ODBII connector was great. In that post there was a lot of wire tracing, splitting etc that had to be done. After some research I found a simpler way.
Before you continue reading please note that follow/do any of this at your own risk. While its intended to be non-invasive and safe it is using a component of your car for something it was not intended to do and you can possibly deplete your 12V battery, short out your ODBII connector and cause other problems.
ODBII to USB
First, if you want a constant supply for a USB device you can just buy a device that plugs into the ODBII connector and gives you USB power. They’re hard to find but they do exist.
I couldn’t find anywhere in the US that made one and the cost is high due to shipping. But for about $41 delivered you can just plug it in and get a constant USB power supply.
ODBII to 12V outlet
Unlike the USB solution these do not exist but its not too hard to make one. The solution starts with a $12 part available via Amazon. Its intended for a very different purpose — feeding power back into the ODBII connector when you’re changing batteries. They call these “ODB Memory Savers”. There are reasons people need to do that but the important part is its an ODBII connector with power and ground coming out so finding the right wires and getting the 16 pins down to 2 part has all been done for you. The part that isn’t right is the wrong end is on the cable. You want a 12V outlet not the thing that plugs into the outlet.
The next step is pretty easy. Find a 12V outlet that you can hijack. Amazon has several, I found a reasonable one for $6. Cut the ends off each and then solder the wires together — red to red and black to black.
Here’s my parts list and cost to build a completely removable, non-permanent constant 12V source for my Model S:
- SOLAR ESA30 OBD II Memory Saver Connector – $12.03
- Roadpro 12V Battery Clip-On and Cigarette Lighter Adapter – $6.11
Total Cost: $18.14
Summary and caution
With little cost i’ve got a constant 12V power supply solution that I can use for a future dash cam project. It was simple to put together and is completely removable.
Just a reminder on the warning above – you can drain your 12V battery if you’re not careful with this. It is constant power/always on so be warned.