Hardwiring USB devices for power


That supplies up to 6.2A at 5 volts, so 31 watts, add on a bit for inefficiency in the conversion, say, 40 watts. Divided by 12v equals 3.33 Amps maximum load, so...

if it is worth using the existing cigarette fused connection on the fuse box

...yeah, that would be fine, you might as well split it 10 and 10 amps if you happen to have a 20A existing circuit.

running an earth then should I just tap into an existing earth

If you mean tap into an existing earth(negative) wire, then no, you could put too much current down the rest of that wire. You need to find where there are some existing negative connections to the vehicle's body, there will be lots behind the dashboard, probably one in both footwells by your ankles, one by the handbrake, etc. Put a ring terminal on the end of the wire and add it there, just how the other ones are.

somewhere on the car body in the engine bay?

The whole body is the negative, you just want the nearest convenient point, just start taking off plastic panels and you'll soon find one.

only on when the ignition is on? Plug it in and see??

Yeah, pretty much! A multimeter to check it first would be handy though. Don't get fooled by modern vehicles with their timed relays though. I.e when you first switch the ignition off, a lot of circuits may still be active on a timer, or until you lock the car, so will trick you into thinking it is a permanent feed, even though it's not, and can be used.

the cigarette lighter is redundant.

That's all well and good, until your friend/partner/whatever plugs in a laptop charger(lots of Amps) while you're not looking!! So, if you can find a spare slot and keep the cigarette's existing fuse rating, then do so.
 
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