June 2014
My truck only has a voltmeter for battery condition indication. I don't have room for an ammeter. Is there a circuit I could build with a red and green LED to indicate if the battery is charging or discharging?
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If you want to know if the charging circuit is working, note the voltage with the switch on, motor off, then start the motor; the voltage should go up about 1 volt. If it goes into the red, the regulator is defective.
I suspect that your truck already has the circuit installed. If it is less than 20 years old it almost certainly does. It is called the "battery indicator light". That light does exactly what you want, except it is dark when you want an illuminated green LED.
Put the keys in the ignition and switch to the ON position without starting the engine. All the warning lights on the dash should light up (this is a bulb check routine to identify burned out indicator bulbs or LEDs). After a couple seconds most of the warning lights will go off, except probably 2. One is the oil pressure warning light. The engine isn't running, so the oil pressure is zero, so the light is on. The other light still on will be the battery light. The engine isn't running so the battery is supplying all the power to the truck’s electrics.
Now start the engine. Those two lights should go off. The battery light is off which means the alternator is supplying all the power to the trucks electrical systems. That little indicator light basically just looks at the electrical system's voltage. A good, fully charged car lead acid battery will read 12.6V without any load. The charging system of a car runs at about 13.5V to 14V.
You can check the voltages with a voltmeter at the battery posts with the engine on and off. If the electrical system is at a voltage of 12.6V or less than the battery is carrying the electrical load. If the electrical system is at more than about 13V then the alternator is supplying all the electrical power.
If the battery needs charging then the alternator is doing so, as long as the battery indicator light is off. If the battery is not charging because it doesn’t need to be, then the light is also off. What the battery light won’t tell is the charge/discharge rate, which you would need an ammeter for.
