Benefits of permanent automotive power outlets
This page in a nutshell: A permanent automobile power outlet, which also provides power despite engine and ignition being powered off, can provide a significant benefit of convenience to passengers and the driver, such as the ability to recharge appliances while parking or absent.
A permanent automotive power outlet is a power outlet inside a vehicle's cabin (also described as 12V outlet[note 1]) that is always supplied with power, even with engine and ignition switched off, due to being directly connected to the positive pole of the vehicular battery.
This article describes the benefits of an automotive power outlet being permanently supplied with power, rather than only with a switched-on ignition or running engine.
Powering electronics while parked
Vehicular batteries have abundant energy capacities relative to mobile phones, power banks and various portable electronic accessories. Devices such as mobile phones and power banks can conveniently be charged by the passengers even without the engine running, and low power accessories such as portable fans are able to operate.
Having to run the ignition or engine on idle just to charge a mobile phone while the car is parked is squanderous and inefficient.
An idling combustion engine wastes fuel and may pollute the surrounding air within minutes, especially indoors. For that reason, many public parking garages prohibit idling the engine.
While a vehicle is in ignition mode without the engine running, the car battery is being rapidly drained by the ignition itself and activated components such as headlights[note 2] (depending on vehicle) and on-board computers.
A permanently powered outlet also spares the passengers from the inconvenient moment of the power being disconnected when the engine is powered off.
Some vehicles, mainly family vans, have an additional power outlet in the trunk that acts as an easily accessible source of power during e.g. a picnic.
Charging devices during absence
Electronics such as a secondary mobile phone and power banks can be left charging inside the automobile's cabin during absence[note 3][note 4], so to be topped up or recharged upon return, making efficient use of that time.
Power not interrupted by engine start
During an engine start, the voltage of the vehicle's battery might drop to around 10 volts, which is a voltage most gadgets powered from an automotive power outlets, such as mobile phone chargers, can still operate on, therefore not inconveniently interrupting the power during an engine start.
The voltage might drop more or less depending on whether the combustion engine is warmed up (= less drop), the charging state (higher = less drop) and age (higher = weaker) of the car battery.
Emergency power supply
In case of a breakdown event or similar emergencies such as fuel exhaustion, the stopped car's battery will still be able to provide power to mobile phones without running engine or ignition.
Charging from inside
If the power outlets are permanently connected to the car battery, it can conveniently be charged from inside, perhaps with the help of home-made adapters.
While the electrical current supported by the outlets may be limited to around 10A to 15A (ampère), depending on vehicle and temperature, it could suffice in many situations. Many don't own a power supply that can output current at around 15 volts (elevated voltage required for charging and passing through resistance) at more than 10 A.
How to patch?
Automobiles usually have a fuse box under the steering wheel or under the foot space of one of the front passengers, where a fuse slot permanently connected to the battery's plus pole can be tapped into using a so-called piggyback connector, connecting the battery's positive pole to the power outlet(s), closing the circuit.
Battery drain argument
Some argue that it could lead to a device (e.g. a portable fridge) to drain and empty the vehicle's battery if unintentionally left unconnected and powered on. However, some devices have a bottom power cut-off voltage to prevent the battery from unintentionally discharging below it.
For that bit of required responsibility, the automobile's passengers could enjoy a significant benefit of convenience.
One should note that it may be more likely to drain a vehicle's battery by habitually leaving the ignition on just to charge one mobile phone than unintentionally leaving an appliance powered, causing the car battery to drain.
Many accessories have too low power demands (such as a mobile phone battery charge and low-power accessories such as portable fan) that they drain it very slowly only and/or don't demand as much total energy as the battery stores.
- The voltage varies depending on factors such as battery charging state and whether the engine is running nd generating power through the alternator (usually 14V to 15V). The alternator regulates the voltage down at higher temperatures to prevent battery damage.
- Vehicles usually allow turning off the headlights by enabling Park lights, but those remain on with the ignition key removed until manually powered off
- Leaving devices charging inside the car is not recommended during higher temperatures and exposure to sunlight which can cause the vehicle's cabin to heat up to temperatures that can, in addition to heat internally produced by the electronics while charging, be damaging to batteries, which are not interchangeable in most mobile phones released since 2015, therefore disabling the entire device upon expiration.
- The vehicle's battery capacity should not be over-estimated. It may be abundant relative to a few mobile phones, but a few larger power banks or laptop computers could drain the battery to dangerously low levels, i.e. it being too weak to start the engine. This convenience should be used with responsibility.
- Permanent 12V Supply for Car Accessory by Left-field-Designs on Instructables.com