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Not exactly sure if the device / component or setup I'm asking about even exists. But, here is my naive question (I am bad with electrical in general):

Is there something built into modern vehicle electrical systems which prevents reverse flow of power from the components plugged into the accessory outlets (cigar lighters)? Can that device go bad in such a way that the vehicle actually draws power from say my tablet plugged into the 12v receptacle in the event of lower voltage?

Vehicle: 2002, Excursion with the 7.3 PSD

Reason for question: We seemed to experience a situation recently where this vehicle was draining several Apple devices that were plugged in while driving. In fact, the only thing that caused me to pull over and plug in my diagnostic tester was my spouse telling me something was wrong beyond just not-charging. Voltage was noticeably low and continuously dropping. Previous owner did a poor install on new alternator cable (didn't solder it to the copper eyelet, but just crimped and it had come loose). Fixed, on our way and no big deal but when I told this story to a diesel mechanic friend of mine, he stated that it was impossible the vehicle was draining the devices and that they were just not-charging. Maybe that's the case, but sure seemed like they were being drained in short order. Plausible or no?

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  • Soldering connections in cars is bad practice. Always use crimp terminals, or screwed ones. Cars are a highly dynamic environment, and as such, connections in wires always experience wiggling, yanking etc. If you solder a connection, you create one solid brittle wire which easily breaks at the spot where the solder stops. At first, only a few strands may break, introducing frustrating intermittent problems.
    – Bart
    Commented Feb 11, 2018 at 12:51
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    @Bart - What you are suggesting seems counter-intuitive. Do you have any references backing up that claim? What the OP is talking about is soldering the end of the cable into the eyelet which is pretty standard procedure. This creates a very good connection to the eyelet, where as all of the wiggling you talk about would pull it out of the eyelet if it wasn't. I'd be interested in seeing any industry standard which explains your statement.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Feb 11, 2018 at 13:20
  • @Pᴀᴜʟsᴛᴇʀ2 A soldered connection gives a better electrical connection, but it's a poor physical connection. If it's deemed neccessary to solder something, a safety for yanking and wiggling should always be included, to relieve all stress from the soldered part. Corded household appliances use one all the time. Naked crimp terminals have two sets of clamping arms, so it's less of a concern if only the second one is soldered. Still caution is advised, since fluid solder travels up the wire by capillary working. With those big eyelets for alternators, a hexagon clamping tool should be used.
    – Bart
    Commented Feb 11, 2018 at 15:16
  • Lots of people soldering stuff like that is not a reason to deem it good practice. Where I work, we always use a tool like this to crimp large connectors.
    – Bart
    Commented Feb 11, 2018 at 15:18
  • @Bart Factory / OEM is soldered for a reason on that specific connection. While you make a couple of interesting points... I don't think what you're suggesting is very standard and I'm not sure your comparison to household cords is applicable.
    – maplemale
    Commented Feb 12, 2018 at 20:34

2 Answers 2

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The vehicle wasn't draining power from the devices as that's impossible due to the voltage regulators inside of the devices.

One reason your Apple (or other) devices could have been draining is because they where using the battery faster than it was being charged. Modern Apple devices will only charge at a fast rate if being charged by a genuine or approved charger and cable. So if you had a lot of services running on the device such as GPS, WiFi, internet, music etc then it may have been draining the battery faster than it was being charged.

Although there is technically nothing in most cars that stops it being charged through the accessory socket. If you hook a charger up to the accessory port providing it provides enough current it will charge the battery. You may need to turn the key to the accessory or ignition position, if so your charger will need to provide enough current to supply everything that turns on. But in very modern cars the accessory port maybe supplied directly from a circuit in the BCM and therefore it will not charge the battery.

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  • This answers the question directly. Thanks! Debate solved - that makes sense.
    – maplemale
    Commented Feb 12, 2018 at 1:49
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The devices will not charge the vehicle - your mate was correct.

You may want to consider having a second battery with a split charge system - a common thing for rv’ers and vehicles with winches etc.

Here is a link to one and there are many many... No affiliation just one picked from the search result : Split charge relay link

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  • You mean a 3rd battery? LOL
    – maplemale
    Commented Feb 12, 2018 at 1:49

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