Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

3-4A too much load for a car battery?

  1. Sep 26, 2008 #1

    So I have put into motion a plan to build an LED array that scrolls letters. I've got the hardware and coded the assembly to make it happen but I have not yet purchased things such as the LED's themselves or op-amps to drive them. I have calculated that the load will be around 3-4A worst case scenario (all 100 LED's lit, all IC's not idle). I plan on running this contraption off a car battery that also runs a car, and my question is is this an unreasonable load for it? Further, could such a load be supplied through the cigarette lighter?
  2. jcsd
  3. Sep 26, 2008 #2


    User Avatar
    Gold Member

    Should be no problem. My car consumes 10 amps of electrical power while driving with no accessories on. And my cigarette lighter didn't fail for quite some time after I plugged in a 15 amp air compressor.(It melted the thermal overload device).

    Before buying all that stuff, you may want to check with your states DMV and see if what you are planning on building is legal. I've heard that too many flashy thingy devices on a vehicle are frowned upon in some areas.
  4. Sep 26, 2008 #3
    Have to avoid discharging a car battery too much if you are not running the engine. If you are running the engine the alternator will be powering the device. Some people say don't let the battery go below 50% of its capacity. If it is a 40 amp-hour battery don't let it get down to 20 amp-hours... This is 5 hours at 4 amps. A leisure battery which can withstand deep discharge better and is used for medium current loads would be an alternative and you wont risk not being able to start the car. Should be ok via a cigar lighter socket as a cigar lighter is quite a high current device but make sure you are running through a 5 Amp fuse or circuit breaker.
  5. Sep 26, 2008 #4
    Excellent. Thanks for the advice.

    Good point. I have to figure out a creative way to make sure this contraption doesn't block my rear view.
  6. Sep 26, 2008 #5


    User Avatar
    Gold Member

    One more suggestion: Some states have restrictions on what colors can be displayed on vehicle lighting - especially flashing lights. In Maine, only cop cars can have blue lights, though some ambulances/rescue vehicles can have red and blue. Fire response vehicles (even private vehicles of volunteer fire fighters) can display forward-facing or roof-mounted flashing red lights. Almost everybody else that wants to use flashing lights is restricted to amber (construction vehicles, private snowplows, etc). Be thorough in your investigation before you spend a lot of money, because lighted displays that change or flash etc might be equated with flashing emergency lighting. Good luck.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: 3-4A too much load for a car battery?