How to Choose a High-Amp Alternator

Considering making the jump to a high-amp alternator?

There are four things you need to consider.

First, do you really need a high-output alternator?

Most factory alternators are rated at 65 to 100 amps and are capable of handling the factory power demands from things like headlights, gauges, fuel pumps, A/C, etc. These alternators also typically come with a 10 to 15 percent reserve to handle additional accessories. That being said, most of our viewers don’t have a stock vehicle so a factory alternator may not cut it.

So how do you really know if you need to upgrade your alternator?

There are a few telltale signs such as dim headlights, poor stereo system performance, or an alternator that simply wears out quickly. You can also check your electrical load using an ammeter or estimate your vehicle’s electrical load by checking the accessory fuses. The amp ratings, although slightly higher than the highest draw of each component, will give you a good estimate of your

Once you’ve determined the need for a higher-amp alternator, it’s time to move on to the next consideration: What amperage do you need?

Required alternator amperage depends on the current draw, along with any future accessories you plan to add. Each accessory will have drastically different draw…an electric fuel pump can draw anywhere from 3 to 8 amps while a high-power audio amplifier can pull as much as 70 amps. Powermaster supplies information on amperage draws here:

As you add up your accessories’ draw, keep in mind you can never have too much amperage. That’s because amperage is basically the amount of electrical current your alternator can supply. High-output alternators will not harm your components or charging system, no matter how high you go with the amps.

The third consideration: What accessories do you need?
Although a performance alternator really doesn’t require much in the way of modifications, we do recommend that you replace both the ground straps and charge wire. Keep in mind that the factory cables weren’t designed to handle the juice of a higher-output alternator, and can restrict the flow of electricity.
In the case of the charge wire, you really can’t go too large. Here is a chart with recommended cable length and gauge for your amp draw.

Finally…What is pulley ratio and why should you care?
Pulley ratio is a comparison between the crankshaft pulley diameter and alternator pulley diameter. This ratio is derived by dividing the crank pulley diameter by the alternator pulley. For example, a 6-inch crank pulley with 2-inch alternator pulley will yield a 3 to 1 pulley ratio.

Powermaster follows this common rule of thumb:
 Street use = 3 to 1 ratio or slightly higher
 Drag racing = 1.75 to 1 ratio
 Circle track = 1 to 1 ratio
Again, the ideal ratio depends on your application (street, drag racing, circle track racing), but you need to understand the effects of altering pulley ratio.

From amperage to pulley ratio, we’ve covered the key consideration for choosing a high-amp alternator. If you’ve got more questions, our experts are ready to help!

SHOP TOOLS AND DÉCOR

[averyalexander]

A much better way to find out if your current alternator is adequate is to turn on all the electrical accessories that you realistically would use at the same time such as headlights, AC, radio, electric fans if you have them, etc and watch the battery voltage with the engine idling. As long as the voltage isn't dropping below around 13-13.2 volts under a realistic heavy load at idle RPM, your alternator is keeping up just fine. What you really don't want to see is the voltage dropping below the battery's static voltage of around 12.8 volts because that means the alternator isn't keeping up and you are drawing power from the battery rather than charging it. But If the voltage is dropping too far, don't condemn the alternator right away. Check the battery cables, alternator charge cable, ground straps, etc for any damage or excessive voltage drop first. Sometimes just cleaning up and/or upgrading the main power and ground cables can make a big difference for very little or no cost. Also, make sure the belt is tight and in decent shape.

[averyalexander]

Another tip: If upgrading your alternator is expensive and/or impractical in your application, you can easily and relatively cheaply significantly reduce the load on your current alternator by replacing your headlights, taillights, fog lights, marker lights, etc with LED bulbs since they draw much less power than regular incandescent bulbs.

[brianyork]

What an awesome an informative video very helpful you answered like three questions that I've been trying to figure out 4 years in 5 minutes... Much appreciated.

[mauriciobsr.]

Just hooked up a 12” sub in my truck powered by a 1k amp. Never thought it would as much power as it does idling. I can see the battery gauge draw back. Time to upgrade the alternator on my ram. I even have LEDs all around.

[navascovideo]

Thanks for the video man! Very informational & straight to the point on most questions I’ve wondered.

[hausboxes]

Hahahaha"as much as 70 amps for a high powered amplifier" lool

[murrayshekelberg]

Was going to laugh about "as much as 70 amps" for a high power car amp but looks like everyone else took care of it. 300a here

[deankay]

Use an amp clamp with direct readout or coupled with a good digital multimeter. Fuses are thermal devices at the location changes the rate at which they open due to current increases in a circuit. They are chosen at a rating 20% higher than the circuit or device they protect. They heat up during turning the circuit on to handle cold coopers low resistance known as amperage in-rush. The amperage is the intensity of the pressure of electron flow thru a circuit. Voltage is the potential for electron to move from an area of abundance to an area of deficiency. A battery in good continental uses less the 0.5 amps or less to charge it. The circuits are generally all series circuits and the device is designed to consume all of the voltage in that circuit, minus the voltage loss across connectors, switches, fuse and all wires on the positive & negative side. Clean large gauge wire it recommended for the frame to all point electrical devices are used. Commonly solder, shrink tubing, Star-washers and the use of dielectric grease keeps out moisture and oxygen to reduce or eliminate corrosion. Remember, the end frame of the alternator it where diodes allow the negative half of the alternating current is produced. Providing a large ground wire to bolt directly to it then to the battery, engine and frame will allow more current thru less resistance allowing the alternator to work less hard and last longer. High output alternators when needed are expensive but bolting one on is just the beginning of upgrading the vehicles electrical system. Please, always use rubber grommets thru holes if adding components under the seat or trunk or even underneath the vehicle, like a automatic trans cooler with a fan. Keep wires clean away from moving or hot parts or add a simple heat shield to leave an air gap and reflect radiated heat away. Add a ground next to wires that carry low current A/C signals like a speed generator, wrap with foil tape, route away from spark plug wires then terminate the ground at the end of this circuit to help with shielding. Learn voltage drop testing dc circuits to accurately and quickly locate high resistance before purchasing expensive unnecessary components. Hope this helps

[autonomous_collective]

Thanks for sharing. I learned something today.

[congojackshootsu]

“As much as 70 amps” 😂😂😂 me sitting on 150amp 0 awg and 120amp 4awg 😂😂😂😬😬😬

[ericmcginnis]

He said an upgraded amplifier can pull up to 70 amps ! What world does he come from! I've seen amps pulling 340 amps !

[CarsSimplified]

9 amps! Almost blew the fuse in that multimeter! (After 1:56 )

[pedritoc.c.t.]

So i cant instaló a370 amp. And change the polley "if needed" to my shock truck

[Raymonkey]

I put a 320 in place of a 63 amp in my 91 Ford pickup. I deleted the mechanical fan and replaced them with 2 electric cooling fans, also used a 2000 watt inverter often. Had a manuals throttle cable to increase the RPM when using the inverter. I also swapped the pulleys to increase the alternator RPM. I installed a 2 gauge wire from the alternator to the battery and added two grounds both 2 gauge, negative post of the battery to the body and same to the alternator bracket of the alternator. The Ambulance alternator was rated at a 90% duty cycle. Overkill or should I have done more?

[tmoore]

1995 z28 with a 2, 000 watts music system . What alternator do you say get

[vernondavis]

Have you ever installed a 200 amp one wire Powermaster Alt on a Vortech supercharged 302 block Mustang? Alternator is on passenger bottom location.

[Rosetti.]

My 79 Lincoln Mark V died on the road. It is an alternator issue. The alternator is just not charging the battery. I believe it is a 60 or 70 amp alternator. I have an aftermarket sound system installed, but it's only like an 800 w amp that powers 2, 12-In subs.

My question is: is it just a bad alternator, or should I go for something with more juice, like a 100 amp alternator? I have heard that using an alternator that is too big can just fry the cells in a battery. Since my sound system is not huge, should I just replace the alternator with something similar?

[garrykellogg]

My car ('Hetty') has a 70amp Alternator (factory), and I want to add a 2nd battery to exclusively power Accessories (Phone, LapTop, Front/Rear Camera, etc...) do I need to upgrade the Alternator to a 80amp? 145amp? etc?

[antonioguerrero]

Thanks for you enlighten..m so grateful..

[socaliguy]

Singer Alternators are the best, period.