Piston Slap: The Cigarette Lighter’s Enduring Appeal

Roger writes:

How can I make the USB ports hot when the vehicle is shut off?

Sajeev answers:

The easiest answer is to not do that. Instead, consider adding USB charging ports to your cigarette lighter. Re-wiring any part of a vehicle carries inherent risks, especially in modern times of multiplexes and electronic modules controlling everything. Buying a USB port for a standard automotive, 12-volt cigarette lighter is cheap, easy to install and gets the job done, as you can see here in my lovely Lincoln Continental. (Grabs phone, scurries to the garage…)

Cigarette USB chargers are available in a million different designs, some with extra features like a voltmeter, or bluetooth-to-FM modulation. Some also mount nearly flush, ensuring they fit behind an ashtray door or modesty panel. (I both have and recommend the flush ones, but the USB stick pictured here was free and this car was currently at my house.)

But perhaps your application doesn't have a cigarette lighter. Or you don't want anything else inside your cigarette lighter. Fine, I guess we need to continue this thought experiment!

I have my reservations publishing everything you shall see going forward, as you must find a way to run down a 12-volt wire down to 5-volts for a USB port. Or maybe not, but I am not electrically savvy enough to commit to either side of this discussion. Perhaps it's worth a try, especially if you are comfortable replacing a USB port if it gets toasted in the process. So get your car's wiring diagrams, disconnect your battery, throw out your factory warranty (if applicable), and proceed!

Once you bust out the factory schematics, find a power wire that is "always hot" from the fuse box, one that isn't controlled by the ignition key (or push-button system). USB charging for smartphones is usually around 1-3 amps, so just about any source of fusebox power could work without blowing fuses, or damaging your fuse box.

Once the appropriate wire is found, splice into it and run a fresh wire to the USB. Before adding it to the USB charging port's wiring harness (again, look at the wiring diagrams to find the correct wire!) consider installing an in-line fuse holder as close to the USB as possible and pop in a 3 amp fuse in it. Or perhaps this is mandatory? I'll let the comments section handle that.

With the USB's wiring harness at hand, cut the wire providing the current source of power and replace it with your new wire. Plug in the USB port to its harness, and reconnect the battery to test. Once you are confident the USB port is charging your phone and not cooking a wiring harness at the same time, carefully reinstall everything and hope you never have to go back in there.

Or, you know, just get a USB adapter for your cigarette lighter. It's the right move.