- This topic has 6 replies, 3 voices, and was last updated April 8, 2022 at 9:20 pm by
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Is it safe to charge a cell phone using a modern USB adaptor plugged into a cigarette lighter on a positive earth car? Will it damage the cell phone? In other words is polarity important? The same question could apply to a GPS such as Garmin.
If polarity is important would switching the wires on the cigarette lighter work?
There seems to be no clear answer to be found on the internet despite the fact that many older British cars are positive earth. -
Yes polarity matters, and swapping polarity on the lighter socket should work (provided the case isn’t grounded though a metal dash) but I would suggest getting a USB outlet that fits the lighter hole in the dash, hooking it up to positive and negative as indicated.
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I like Joe’s idea. It’s a slick setup. Find a round USB adapter, remove lighter socket, insert adaptor and secure. Use McGyver skills to secure. i.e, a small hose clamp behind the dash should hold it in place.Don’t let it touch any other metal parts. A plus is that it will likely clamp onto the electrical contact. That will provide an opportunity to secure a wire to connect your “hot” negative lead. Connect a wire from the car chassis to the center terminal. Solder the wire or in some adapters, the end unscrews and this would allow insertion of a wire.
Be sure to use an inline fuse(5AMP) directly behind the -12 lead that connects to your -12V source.
Keep in mind that any device connected to a lighter or adapter in this arrangement that has metal parts normally connected to ground in a normal 12V application (+12 and -12 gnd like most cars) will now be a short if it were to touch any part of the cars body. An example would be the metal ring of a 1/8″ audio jack may short out if allowed to touch a metal part of the cars chassis. There’s is a reason for the 5 AMP fuse. And to scare you even more. It may wipe out your devices electronics.
All in all, being aware of these factors should mitigate any issues and you’ll live happily ever after. TT
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John,
Most USB adapters like this are in fully insulated plastic cases and have + and – terminals on the back. For a positive earth car just hook the + to ground and the – to a hot wither through a fuse as Tom suggests. This could be switched or un-switched, your choice. The device drops the 12-14v generator/battery voltage down to the 5v USB standard. You can search for these things on the web. There are even some with a built in voltmeter.
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Joe and Tom, I very much appreciate your advice and the discussion.
The car does not have a cigarette lighter but it does have a spot on the dash for one. I think just installing a USB charger would be a good substitute rather than installing a lighter and using a plug in USB charger which would require the polarity being reversed somewhere and maybe the risk of possible shorts as Tom mentioned.
I saw the type of USB adaptors that you mention on line and it seemed to me all that was necessary was to reverse the connections as you say. As I have a new wiring harness that has extra wires for an accessory I thought this might be a simple solution. The fact that the USB chargers are plastic and insulated is a bonus.
My remaining concern is the reverse polarity to the USB charger. I am guessing it will not damage the charger, though I don’t know, and what about damaging the phone and its battery? Perhaps this is not a concern with DC current, but being a rank amateur when it comes to electronics I would appreciate your advice.
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John,
The USB adapter should identify a positive and negative lead or terminal. In your case with positive ground, tie the positive lead to a ground point and the negative lead to a hot line. In the series V, and also Series IV the switched hot wire is green. Your harness should have the same convention. You should put a fuse in this line.
There are several events coming up where you can meet other owners and get some hands on advice. The United will be in Ontario if you can wait that long. We may make it to the show in June at Clayton NY, depending on which car is in the best shape for the trip.
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