How to Choose an Alternator
Slide Show
Good Thing: Kickin’ audio system with megawatt amps, big subs, zillions o’ speakers, crossovers, and enough wire to hook up your average city.
Bad Thing: Melted, smoking, fried vehicle electrical system ’cause you didn’t have a big enough alternator.
The fact is, not having enough amps to properly feed your vehicle's electrical accessories can DOA a typical OEM electrical system. While it does have some reserve power for small accessories, the extra amperage draw created by a high power audio system (or race electronics, or lighting, etc.) can cause a stock electrical system to literally melt from the inside out.
Luckily, a simple alternator upgrade can prevent such a catastrophe. We’ll guide you through the alternator selection process how to decide if need an upgrade, how to find the proper size alternator, and tips for getting the most out of your new alternator.
Do You Need a High Amp Alternator?
Deciding if you need a more powerful alternator is easy once you understand the exactly what kind of power, or amperage, you need.
Amperage is defined as the maximum capacity or maximum volume of electricity an alternator is capable of producing. If your vehicle’s electrical load exceeds the amperage—or maximum capacity—of your alternator, you’re asking for trouble.
Most factory alternators are rated at 65 to 100 amps and are capable of handling a vehicle’s basic accessories—headlights, gauges, fuel pumps, transmission, A/C, etc. While many alternators have a 10% to 15% power reserve to handle additional accessories, this is often insufficient capacity to power high end audio systems or other high-amperage items.
For example, a typical 500 watt stereo system draws upwards of 60 amps when cranked. A stock vehicle’s electrical accessories draw an additional 60 amps total. To run 120 amps’ worth of goodies with an 80 amp alternator, it will have to run at 100% capacity—and draw reserve power from the battery—with no cooldown time. The result is predictable—drastically reduced alternator life.
If you’re looking for hard evidence that you need to upgrade your alternator, take a look at your voltmeter. When you are drawing reserve power from your battery, the voltmeter will read below 12.7 VDC. If your voltmeter spends a lot of time below that figure, you are surpassing the maximum capacity of your alternator.
Choosing the Right Alternator for Your Vehicle
Selecting the right alternator comes down to figuring out your vehicle’s total electrical load. The most accurate way to determine electrical load is with an ammeter. With the engine off and the battery charged, connect an ammeter in series with the battery’s ground terminal of. Switch each electrical component on and off, noting their amperage draws. Add up the ammeter readings. Your alternator output should be 50% greater than that figure.
If you don’t have an ammeter, you can estimate electrical load by checking the accessory fuses. The amp ratings will be slightly higher than the highest draw of each component , but the sum of all fuse ratings will give you a general idea of the vehicle’s electrical load.
This chart shows the amperage draw of common electrical accessories:
Amp Draw of Some Common Accessories
Accessory: Amp Draw:
Air Conditioner 20-21
Audio Power Amplifiers 10-70
Back-up Lamps 3-4
Cigarette Lighter 10-12
CD/Tuner with amp 7-14
CD/Player/Tuner without amp 2.5-5
Clock 0.3
Dome Light 1-2
Electric Cooling Fans 6-15
Head Lamp Dimmer 2
Head Lamp (Low Beam) 8-10
Head Lamp (High Beam) 13-15
Heater Defroster 6-15
Horn 10-20
Ignition 1.5-4
Ignition (Racing) 8-36
Instrument Panel 0.7-1.5
Lamp, Gauges 1.5-3.5
Lamps, License Plate 1.5-2
Lamps, Parking 1.5-2
Lamps, Side Marker 1.3-3
Lamps, Tail 5-7
Nitrous Oxide Solenoid 5-8
Power Windows Defroster 1-30
Power Seats 25-50
Power Windows 20-30
Power Antenna 6-10
Pumps, Electric Fuel 3-8
Starter Solenoid 10-12
Voltage Regulators (1 Wire) 0.3-0.5
Once you’ve determined the electrical load of your vehicle, there are a couple of things to keep in mind as you are choosing your alternator. First, you can never have too much amperage. Again, amperage is defined as the total electrical capacity of your alternator, and it is impossible to have too much electrical capacity.
A good quality, high amp alternator can also help you gain horsepower. While most alternators are only about 75% efficient (some power is lost in the form of heat and wind resistance from the cooling fins), a higher amp alternator will recover lost horsepower by allowing your electrical system to run at maximum voltage.
Getting the Most From Your Alternator
Here are some ways you can get optimum performance from that new alternator:
1. Use the proper gauge charge wire. The charge wire is the wire that carries power from the alternator to the battery and the electrical system. Too small a wire will restrict the flow of electricity. Use the chart in Paragraph 16 to select the right charge wire.
2. Make sure the alternator belt and tensioner (if equipped) are in excellent shape. High amp alternators usually have a smaller pulley than stock to overdrive the system by 16%. The slightly heavier load will cause added stress to your belt, so it needs to be in good shape.
3. If you are strapped for space, many alternators can be run in reverse (pulley side toward the driver). The alternator will still charge properly, but cooling efficiency will be reduced and life span of the alternator may be shortened.
Balance of Power
Like most things, an alternator is a compromise.It strike s a balance between drawing power from the engine (via the drive belt) and delivering back that power by helping the ignition system perform at its peak. By using the proper size alternator, you can tip the balance of power in your favor and create a horsepower gain.
By using the following formula, you can determine the amount of power it takes to operate an alternator (where 745.7 equals one horsepower and a 25% alternator efficiency loss is assumed):
Amps x Volts + Watts
Watts / 745.7 = Electrical Horsepower Produced
Electrical HP X 25% (.25) Efficiency Loss = Horsepower Lost
Electrical HP Produced + HP Lost = Total Horsepower Used
Let’s apply the formula to an alternator that produces 57 amps at 14.9 volts:
57 x 14.9 = 849.3 Watts
849.3/745.3 = 1.14 Electrical Horsepower Produced
1.14 x .25 = .285 Horsepower Lost
1.14 + .285 = 1.425 Horsepower Used
As the formula shows, this alternator doesn’t take much horsepower to operate. And by supplying the proper voltage to your electrical/ignition system so it operates at peak efficiency, the alternator can actually help your engine produce more power—more power than it takes to operate the alternator itself.
Recommended Charging Cable Gauge Size
| Amps |
Up to 4' |
4'-7' |
7'-10' |
10'-13' |
13'-16' |
16'-19' |
19'-22' |
22'-28' |
| 0-20 |
14 |
12 |
12 |
10 |
10 |
8 |
8 |
8 |
| 20-35 |
12 |
10 |
8 |
8 |
8 |
6 |
6 |
4 |
| 35-50 |
10 |
8 |
8 |
6 |
6 |
4 |
4 |
4 |
| 50-65 |
8 |
8 |
6 |
4 |
4 |
4 |
4 |
2 |
| 65-85 |
6 |
6 |
4 |
4 |
4 |
2 |
2 |
0 |
| 85-105 |
6 |
6 |
4 |
2 |
2 |
2 |
2 |
0 |
| 105-125 |
4 |
4 |
4 |
2 |
2 |
2 |
2 |
0 |
| 125-150 |
2 |
2 |
2 |
2 |
2 |
0 |
0 |
0 |
