How to Choose a High Output Alternator

Updated March 23, 2017

High output alternators are needed to power the energy hungry accessories people often add to vehicles and boats. They are practically a necessity for the amplified sound systems many are so fond of. Standard alternators are not built to supply the increased power demands of such electrical system additions. Consequently, high output alternator switch-outs are the order of the day. These alternator swaps, however, are not a matter of simply bolting on replacements.

Check to see if your vehicle warranty is still in affect. Review the section about changing out or increasing alternator size to see if doing so will void your warranty. Consider waiting for the warranty to expire before changing out your alternator. This will save you from voiding your vehicle warranty prematurely.

Examine the stock alternator that came with your vehicle. Measure it and write down its dimensions. Determine if additional room is available around it that can be used for accommodating a larger, higher-amperage alternator. Be realistic about what you can make to fit and recognise that some components simply will not fit. Do not buy a "rebuilt" same size unit which claims to produce more power; often they do not.

Check the mounting bracket to see if it is a single or dual post mount. You need to be ready to swap out to a conversion bracket. This is often required to accommodate the new alternator. Look for a workable drive pulley and see if or how much it will have to be modified to make it fit. Determine the ratio between the alternator drive pulley and the crank pulley. Confirm with alternator spec charts that maximum alternator output is available at the desired engine rpm level and that alternator over-speeding will not happen with higher engine speeds.

Next, consider your fuel type diesel or gasoline. Keep in mind diesel generators generally run at lower RPM and produce more torque at lower speeds. For example, a 7500-watt gas run generator has 2571 RPM speed, while a diesel of the same wattage will have a little more than half the RPM speed, at 1950 RPM. Therefore, fuel type has a direct bearing on the generator purchased. Verify vehicle engine age and engine model, because they all have a bearing on high output alternator selection. Consider that as vehicles age, so do their wiring harnesses; also, certain makes and models of vehicles, such as work and recreation vehicles and vehicles with more accessory power demand will have greater alternator current demands than others.

Match the alternator's amperage capacity to the size of your drive belt. For instance, a 3/8" drive belt needs a 75-amp alternator, a ½" drive belt needs a 100-amp alternator, and a dual ½" drive belt needs a 150-amp alternator. Check your owner's manual, the local dealership or parts store if you are unsure of your belt size.

Do not forget the role that battery power plays in alternator selection. Keep in mind that alternator output is generally 25 per cent of battery capacity. For example, a 400-amp hour battery will usually have a 100-amp alternator. Check this with Step 5 information to confirm your most appropriate alternator choice.

Take your specs with you to the dealership or parts store and have them show you what is available for your specific situation. If you are charging two battery banks, look at the dual output alternators.


The key for selecting the right alternator lies in what you are asking it to power. You can purchase these high output alternators online at or at many local automotive parts stores.


Buy all car parts only from reputable sources with good return and core policies.

Things You'll Need

  • Engine size and type
  • Your old alternator
  • List of what you want the alternator to power
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About the Author

Chuck Brown is a freelance writer and former teacher and athletic coach. He has held professional stints as a business owner, personal fitness trainer, curriculum designer, website designer, market trader and real estate investor. Brown holds a bachelor's degree in English and a master's degree in Christian counseling.