Head units aren’t always-on. Typically they go into a low power standby 2-5 minutes after ignition / accessory mode turns off, and go completely power-off 30-ish minutes later.
replyOtherwise any car sitting unused for a week or two would have a dead battery.
I learned that not all electronics goes into low power mode even when designed to run off a car battery, from using a cheap Bluetooth OBDII dongle.
replyA lot of older cars didn't turn off their OBD port, have their headunits go into standby, or even turn off the cigarette lighter port. Early OBD ports connected to dealer computers for a few minutes, not an always on dongle. Plain headunits just play music, what could they possibly accomplish by staying on when you turn off the car? It was a convenience having the cigarette outlet left powered so you could light a cigarette without turning on the car. Other than maybe a bag phone, what would you possibly plug into that?
replyIf that was one of those ELM327 dongles, yes they have 12V and are known to drain your battery. They're only meant for short diagnostic runs.
replyThey are always wired to battery power though. The point is that it could look powered off, and still be running a proxy.
replyYou would hope that the ignition switch really cuts the power to the head unit when it is switched to off.
replyNo you wouldn’t, because then you always have a cold boot of the headunit, even if you just accidentally hit the ignition. Users want the head unit to resume within a few seconds. Just like their phone.
replySome of these Android units also double as DVRs and dashcam recorders (parking mode!) as well so may be hooked onto the normal +12v rail.
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