If you’ve got a basic, stock vehicle, chances are you don’t need a higher-output alternator. Most factory alternators in older vehicles are rated around 65A-100A, and newer vehicles upwards of 200A. These are typically capable of handling your vehicle’s basic necessities, such as headlights, gauges, fuel pumps, A/C, etc. These alternators also typically come with a 10 to 15 percent reserve to handle additional accessories.
However, many of our readers don’t have a stock vehicle. For example, you may have a custom-built street rod with a unique combination of accessories. Or you may have a high-end stereo system or a race vehicle with an array of on-board electronics, bigger fuel system, fans, and intercooler pumps turning high RPM. As the electrical load of all these accessories add up, you may find yourself in need of a higher-amperage alternator.
But how do you know?
There are a few ways to figure out whether you need to upgrade your alternator. A few telltale signs are dim headlights, poor stereo system performance, lean conditions, or an alternator that simply wears out quickly. You can also check your electrical load using an ammeter. Simply connect the ammeter in series with the battery’s ground terminal (with the engine turned off), switch each electrical component on and off, and note their amperage draws. Add up the total electrical draw and compare with your alternator’s rated output. The output should be 50 percent greater than the draw.
Don’t have access to a DC ammeter? Simply use a voltmeter and if your voltage is dropping below 14V during operation, the alternator is starting to be overrun.
One final way to estimate your vehicle’s electrical load is to check the accessory fuses. The amp ratings, although slightly higher than the highest draw of each component, will give you a good estimate of your vehicle’s electrical load.