Simple Current Limiter

[John Weall]

I know this is a pretty basic question, but I can't remember for the life of
me what the answer is.
I've put a car stereo in an mdf box with a car battery, and I need to
recharge the thing. I was thinking I could plug it into my car cigar
lighter, and recharge it from there, but the cigar lighter is fused at 10A.
Therefore, I need to put something in the wire to stop the current rising
above...say, 9A or something. Also, if it's easy to do, i'd like to have
something to stop it charging with enough current left to start my car, so I
can leave it plugged in without the engine running.
Thanks for any help,
John

 

[René]

I know this is a pretty basic question, but I can't remember for the life of
me what the answer is.
I've put a car stereo in an mdf box with a car battery, and I need to
recharge the thing. I was thinking I could plug it into my car cigar
lighter, and recharge it from there, but the cigar lighter is fused at 10A.
Therefore, I need to put something in the wire to stop the current rising
above...say, 9A or something. Also, if it's easy to do, i'd like to have
something to stop it charging with enough current left to start my car, so I
can leave it plugged in without the engine running.
Thanks for any help,
John

12 Volt lamp (~50W? ~ 5 amp current limit )
limits current, has low impedance when cold (PTC effect), prevents
also exessive current to flow the wrong way if you start the car.
And a visual charge indication for free

 

[Walter Harley]

I've put a car stereo in an mdf box with a car battery, and I need to
recharge the thing. I was thinking I could plug it into my car cigar
lighter, and recharge it from there, but the cigar lighter is fused at 10A.
Therefore, I need to put something in the wire to stop the current rising
above...say, 9A or something. Also, if it's easy to do, i'd like to have
something to stop it charging with enough current left to start my car, so I
can leave it plugged in without the engine running.

sci.electronics.basics is a good group for questions like this, I think. In
any event:

To limit the current, you could just use a resistor. As current increases,
voltage across it increases, meaning the battery being charged sees less
voltage, meaning it draws less current. Use a value that drops 12V at 9A,
so that even if the battery being charged behaves like a short circuit, it
draws only 9A. By Ohm's law, R = E/I, so 12V / 9A is 1.25 ohms. It'll be
dissipating 12V * 9A = 108W, so you might want to use four 5-ohm 50W power
resistors in parallel with each other. It'll get pretty hot, but it's a
very simple easy solution. You can also build a fancier current limiter
from an LM317 and some power transistors in parallel, but it's going to do
functionally the same thing in this case. (And it's going to produce just
as much heat.)

As for the second point, respectfully, charging a car battery from another
car battery is probably a bad idea. You really want to be charging it from
a running engine. Your car battery is not intended to start your car with
half its charge.