Hi All,

The title pretty much says it all. I've got a system in the car which charges a spare battery. I'd like to wire up a 12V LED that activates when the charge coming into the second battery is above a certain voltage (probably 13.8V). This would let me know that the battery is currently charging.

Looking to keep it as simple as possible as I only have basic knowledge of circuit design etc.

Cheers!

 

I don't have time to figure out a design, but the LT6700-x or the LT6703-x where x needs to be determined.
if you pick the 6700-1, if the light comes on the wrong way, you just use the other comparitor.

If you set both thresholds to 13.8V, one comparator will turn on when Vin < 13.8 and the other when its > 13.8

A LED in series with a resistor would be connected to the positive supply and the negative of that mess would go to one of the comparator outputs.

R would be sized from R<= (13.8-Vf(led)+Vsat)/(Operating current of LED). You can't exceed the capability of the sink current. Most LEDS would operate at 10 mA, but there are some that will operate on 2 mA. The color of the LED changes the value of Vf.

You'll have to sprinkle in some bypass caps and possibly a TVS diode for the automobile environment.

 

Easy with a Tl431 zener, make R2 4.7K preset, set the voltage to make the led on at 13.8V.

 

If R1 is a preset then when it is set to zero, the full battery voltage will be applied to the TL431 reference input. I think this will damage it.
It would be safer to make R1 a fixed 10k and R2 a 4.7k preset.