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In some cars, the power line from the battery to the accessory outlet (aka cigarette lighter socket) is always live. In others, it's off until ignition.

It seems pretty much random which cars are wired which way, and some cars are wired differently between models in the same series.

Assuming I'm not going to do anything stupid like leave a power-draining accessory plugged in over the weekend, are there any side-effects, disadvantages or risks to rewiring an ignition-only socket so that it has a (fused!) direct connection to the battery, bypassing ignition?

Why I'm asking, if anyone is interested: I want to connect a trickle-charge solar panel to my car battery in a car (Toyota RAV4) that doesn't have an always-hot accessory socket (I mostly use the car for short journeys at night, sometimes with days/weeks between journeys and usually 3+ weeks between long journeys, which is terrible for draining my battery - this panel is designed for this purpose and doesn't draw any power when there's no light).

For obvious reasons, I'd like to keep the panel on the dashboard inside the car connected via a (fused) accessory socket, rather than clipping and unclipping it under the bonnet directly (unfused) to the battery every journey with the panel outside the car exposed to weather and passers-by; but before I rewire my socket, I want to know if there are any negative consequences.

The main use of the socket will be charging via the panel, which is why I'm not worried about the most obvious downside of always-on sockets which is that someone could cause a flat battery by forgetting to unplug an accessory.

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  • Some cars (well, I only remember seeing it on minivans) have one or two "always-on" cigarette lighter outlets, so it won't hurt anything to have yours always on. Be careful what you leave plugged in, I once left an inverter plugged in overnight and it drained the battery even though nothing was plugged in to the inverter.
    – Johnny
    Commented Jan 6, 2016 at 6:18
  • Probably not, but you WILL forget to disconnect something one day if you use your sockets frequently. Also, forget about dashcams and similar devices that need to turn on only when driving (unless you wire it manually without using the sockets, then you're good).
    – I have no idea what I'm doing
    Commented Jan 6, 2016 at 8:42

2 Answers 2

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If there is nothing connected to it (which can draw power), no power will be drawn. To me that means there are no drawbacks. I would not have an issue of making the socket as always hot for just the purpose you suggest.

NOTE: I don't know what else I can add here.

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  • Great, I basically just want to be sure I've not overlooked anything, so that's perfect. I don't suppose you'd happen to know why some cars are set up to only have live sockets when ignition is on?
    – user56reinstatemonica8
    Commented Jan 5, 2016 at 22:55
  • It basically comes down to manufacturer/designer/engineer choice. No real clue ... it's hit or miss, really.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Jan 5, 2016 at 22:56
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    It's the same for headlights. Some cars switch them off when turning off the motor, others don't. And my car switches off ANY lights when pulling the key. May be, it depends on the battery? I have a cute 40Ah battery, and it's a good idea to save as much power as possible.
    – sweber
    Commented Jan 6, 2016 at 0:00
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    This is just "my personal experience", but older cars (80's and older) only had always-live sockets. The socket switched with a key is a newer development from what I've seen and probably results from more people using it as something other than a cigarette lighter and killing their batteries.
    – JPhi1618
    Commented Jan 6, 2016 at 14:13
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This is an answer to your problem, rather than an answer to your specific question.

It is not necessary to rewire your accessory socket. You can get a power cable that connects to the diagnostic port, and connect your solar panel to that.

The specific item I use in my Toyota Sienna with a solar panel (which has the same situation as yours regarding the accessory sockets) is:

Battery Tender 081-0154-DL OBDII Connector for Battery Chargers and and Maintainers

Cost is about $16. The only tricky part is locating your diagnostic port, which may be somewhat hidden; mine is underneath the dash near the gas pedal.

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