How stable is voltage in a 12V car outlet?

Question

I want to use a Mini-ITX PC board in a car. I plan to use the 12V "cigar" outlet or take the 12V elsewhere, but from the same 12V source that is dedicated to be used for user devices within a car's cabin. How stable is the 12V voltage?

My PC will consume about 30W. I plan to use a picoPSU power source. The cheapest one I found is this $25 thing: picoPSU-80, (80W). It seems it needs a stable 12V - its manual says:

12V regulated, min=1A, max=10A (load dependent). Over-voltage shutdown will occur at ~13-13.5V.

I dont know what that really means... A comment on other site says it needs 12V+-10%. Will I be OK with this power supply?

In case those 12V+-10% is not enough there are of course more expensive PSUs like this one: M3-ATX (125W). It accepts 6-24V input and costs $69. However, I would like to avoid spending extra in case it is not necessary.

Answer 1

Don't go with that model for automotive use. It passes through the "12v" input directly to the motherboard and drives, meaning you'll be delivering ~14v with the engine running, and all sorts of ugly voltages during cranking, near-stall, etc. situations.

I would highly recommend this more expensive model:

http://www.mini-box.com/M3-ATX-DC-DC-ATX-Automotive-Computer-car-PC-Power-Supply?sc=8&category=981

I've used both, and even in my non-automotive application, I was much happier with the latter.

Answer 2

ANSWER: Pretty unstable.

Connect a voltmeter to your ciggy lighter socket, and watch it while starting and running the car. It will be ~12.5V when the motor is off, up to 14.5V while just started, and once the battery is topped off by the alternator the voltage will drop to around 12.5-13.0V for warm running.

Also try turning your high beams and AC on to see what the voltages do. At worst case it will be as low as 11V and could drop lower as your battery approaches its end of service life.

You may need a 12V/12V converter like this http://www.powerstream.com/dc12-12-8A-isolated.htm which will smooth out all those nasty voltages.

Wiring the socket straight to the battery helps but won't make it a perfect 12V input. If your vehicle is dual 12V battery then wiring to the accessory battery can also help, but actions like winching will sag the voltage massively.

Your other option is a 12V SLA battery and a true-online UPS circuit, so the computer is always running off the UPS, and its 12V battery is charged by the car. Downside is the weight penalty and the extra size.

Answer 3

The voltage available on the "Accessory" circuits on some modern (last 10 years, at least) can be pretty stable at 12-12.5V. Ignition circuits, not so much (9-14V).

To be safe, however, you should measure with a fast acting (ie, analog) voltmeter on the cigarette lighter in the following conditions:

• Key on, engine off
• Key on, engine cranking (likely it's going to be off in this state*)
• Key on, engine running

A digital one would probably work as well (the big deal is getting a reading during cranking). If it's within the range there, you're good to go.

*Some cars have the cigarrette lighter outlet connected directly the battery, so it's on with the key off. Some do that with a timer, some are on the accessory circuit, etc. It varies a lot between years, makes and even models.

Answer 4

I would go with the more expensive one in this case. Typical output of a "good" system is in the neighborhood of 13.1vdc. Depending on the alternator and how much it needs to put out to recharge the battery, it could be upwards of 14vdc (or maybe a touch higher). The power output from the 12v outlet source is going to reflect what the alternator is putting out. I think the little pico you showed is going to have issues with the power. The second seems it will accept a much broader array of power input without shutting down.

Answer 5

There is another issue with a vehicle's electrical power. To keep vehicles as light, read, fuel efficient as possible, they scrimp on wire gauges. Costs less too. 14 gauge is fused at 20 amps, try running that past your friendly building inspector. Consider measuring the voltage at the socket you plan to use and hook up a light, say a 5 amp headlight and see what the drop is. Years ago, I was setting up some 2-way radios at a remote location and used the company vehicle's lighter socket for power. The radio, a 45 watt guy, draws 9.5 amps on transmit, would beep "lo voltage" when I keyed it up for test. The voltage under load, even with the engine running dropped to less than 9.5 volts, the radio's minimum.