Very few cars these days have a "proper" ammeter; they just have a single idiot light to indicate that the battery is being discharged. But when it goes out, you have no idea of how much current is going into the battery and nor, for that matter, do you ever know how much current is being pulled out.

Even when cars did have ammeters they were not what you would call a precision meter movement; they gave a very rough approximation of what was happening. Well, now you can improve on this situation with this LED ammeter. It has 10 rectangular LEDs, five green to indicate that the battery is being charged, and one yellow and four red to show discharge conditions.

Each LED covers a range of 5A, so the display indicates from -25A (discharge) to +25A (charge). We used a yellow LED for the 0-5A discharge indicator as this will most likely be the one normally illuminated when the motor is not running or at idle.

Every ammeter needs a shunt which is placed in the current path. In effect, the ammeter measures the voltage drop across the shunt which is a very low resistance. The question is "How do you install a suitable shunt in series with the battery?" The answer is that you don’t. There is already a shunt there in the form of the negative lead from the battery to the car’s chassis.

This lead will typically have a resistance of only a couple of milliohms but this is enough to produce a voltage to be measured by our circuit. It amplifies the voltage across the "shunt" and feeds it to a LED bargraph driver IC.