Electronics > Beginners
Car battery confusion.
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andrewlapham:
Hey,

I've been drawing up plans for an led light bar I'm designing for my car. the only thing that is bugging me is the source voltage. I'm yet to take a multi-meter to the car to test the output but is it really necessary if it fluctuates so much?

i know that lead acid batteries are pretty much dead at 12v and fully charged at 14v ish.

I'm currently using sets of 4 LED's in series with a 10ohm resistor. is calculation is running of the idea that my car (on average) will be outputting 13v.

is assuming 13v as a standard voltage the right way to go about this? 

cheers.
cur8xgo:

--- Quote from: andrewlapham on June 21, 2019, 03:42:20 pm ---Hey,

I've been drawing up plans for an led light bar I'm designing for my car. the only thing that is bugging me is the source voltage. I'm yet to take a multi-meter to the car to test the output but is it really necessary if it fluctuates so much?

i know that lead acid batteries are pretty much dead at 12v and fully charged at 14v ish.

I'm currently using sets of 4 LED's in series with a 10ohm resistor. is calculation is running of the idea that my car (on average) will be outputting 13v.

is assuming 13v as a standard voltage the right way to go about this? 

cheers.

--- End quote ---

Battery voltage in a car swings from say 8.5V during cranking to maybe 14.5V charging. If you use just a resistor you will have to deal with that entire range and make sure nothing breaks. So resistor will be oversized at low voltage and undersized at high voltage.

Could use a voltage regulator to protect it from the high voltage swings and some kind of low voltage cutoff circuit to prevent it from operating at too low a voltage (presumably thats non-ideal as well)

rstofer:
I think the float voltage supplied by the alternator is 13.8V
It will be up around 15V when charging.
Maybe you should look into constant current circuits.
Nerull:
Any device meant for '12V' automotive use needs to be able to deal with voltage drops to almost nothing during cranking, widely variable 'running' voltage, and spikes up to 24V+.
Benta:

--- Quote from: Nerull on June 21, 2019, 04:31:32 pm ---Any device meant for '12V' automotive use needs to be able to deal with voltage drops to almost nothing during cranking, widely variable 'running' voltage, and spikes up to 24V+.

--- End quote ---

Actually, an automotive circuit should be able to withstand +/- 60 V, this can happen during a load dump.
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