How to Choose the Right Alternator for Your Vehicle

David Beckam

When choosing an alternator for your vehicle, it’s essential to make sure that you have enough amperage to keep your electrical system charged. When installing power-hungry aftermarket components, a stock factory-style alternator will often be right up against the ragged edge of what it can deliver. To ensure that your alternator can handle the demands of your upgrades, start by calculating the current requirements for all your equipment and adding in a reserve. From there, select the correct alternator.

Pulley Diameter

If upgrading your vehicle, you must know the make, model, and year to find alternators Winter Garden FL, with a compatible mounting style. It’s also necessary to understand the amperage output requirements of your electrical system so that you can choose an appropriate alternator for your specific needs. If your car uses a belt-driven system, you’ll want to pay special attention to the pulley diameters to avoid over or under-driving the belt. It can damage the belt and shorten its lifespan, reducing its efficiency. First, to determine the shaft diameter of a V-belt pulley, measure its outside diameter (OD). Then, to calculate its pitch diameter (PD), place a straight edge across the flat side of the pulley where the belt rides. Note that the pitch diameter can vary slightly from pulley to pulley because of a groove angle and other factors. Then, divide the PD by the OD to get the pulley ratio.

Amperage Requirements

A standard alternator for a stock factory vehicle will provide enough power for all the OEM electrical components to operate. However, when you add power-hungry aftermarket equipment, your stock alternator may need to meet the demand. With sufficient volts, your components can experience success (or worse). You can use a calculator to determine the alternator size you need for your vehicle. To do this, you need to know the total current draw of your entire electrical system, including a reserve for battery charging. It’s essential to look at the output of an alternator in terms of both volts and amps. Some manufacturers publish this information so you can see what the alternator produces at idle and cruising speeds. Look for the type of mount – a long hinge or pad mounting – and whether it has an internal voltage regulator or an external one.

Voltage Regulator

One of the most common mistakes people make is selecting an alternator that is too big for their needs. A higher-capacity alternator may offer more amperage at maximum output, but that extra power will only be used when the electrical system demands it. To determine your electrical load, connect an ammeter in series with the battery and turn on each component, noting their current draw. Then, add them up to get an idea of the total load. If you don’t have an ammeter, check the fuse ratings for a general idea of your load. When selecting a new alternator, replace the charge wire, which leads from the alternator to the battery. Choose a larger gauge to handle the new alternator’s increased amperage capability. Also, consider replacing the alternator belt and tensioner with a larger-than-stock size. Finally, if using a serpentine belt system, consider the pulley diameter of the crank pulley.


Choosing a suitable alternator can make or break your electrical system. Remember to upgrade the charge wire, too – it must be of a proper gauge for current requirements and run length. Using an ammeter is the best way to determine the current load. Connect each in series with the battery ground terminal and switch each component on/off, noting their amperage draws. Add them up and choose an alternator rated 50 percent greater. You can also use your accessory fuse amp ratings to estimate electrical load. Select top-quality battery terminals (crimp style is preferred for automotive applications). Ideally, use fine-stranded stereo wire for battery cables as it will easily pass the extra amperage of a modern high-output alternator. It ensures that your newly installed alternator and battery work together correctly for years. After that, you can concentrate on reaping the rewards of your car’s freshly installed power system.

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