CHOOSING YOUR ALTERNATOR
(an amalgam of web info, experience and editing by GoMoG)

Over time,  we all seem to add electrical goodies to our Morgan. Over that time, we ultimately Peter Principle the original alternator..or hurry its demise by overloading it.  

To keep this simple:

Good: A great audio system with megawatt amps, big subs, zillions of speakers, crossovers, and enough wire to hook up your average city along with power plugs, EFI, GPS, heated seats, an electrical fuel pump upgraded rad fans and all sorts of lovely nonsense, supported by our alternator and charing system without a problem. .

Bad: All the above with melted, smoking, fried vehicle electrical system because you didn’t have a big enough alternator.

The fact is, not having enough amps to properly feed your vehicle’s electrical accessories and wonderful addons make a typical electrical system a ticking bomb. While it does have some reserve power for small accessories, the extra amperage draw can cause a stock electrical system to literally melt from the inside out. Luckily, a simple alternator upgrade can prevent such sadness.

Do You Need a Higher Amp Alternator?

Deciding if you need a more-powerful alternator is easy once you understand 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-percent to 15-percent power reserve to handle additional accessories, this is often insufficient capacity to power  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-percent capacity—and draw reserve power from the battery—with no cool-down 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 or multimeter. With the engine off and the battery charged, connect an ammeter in series with the battery’s ground terminal. Switch each electrical component on and off, noting their amperage draws. Add up the total ammeter readings. Your alternator output should be 50 percent 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.

Amperage Draw of Common Electrical Accessories
Air Conditioner 20-21
Audio Power Amplifiers 10-70
Cigarette Lighter 10-12
CD/Tuner with amp 7-14
CD/Player/Tuner without amp 2.5-5
Clock .03
Electric Cooling Fans 6-15
Electic Fuel Pump 3-8
Headlights (Low Beam) 8-10
Headlights (High Beam) 13-15
Heated Seats 8-12
Heated Windscreen 14-16
Horn 10-20
Ignition (Distributor type) 1.5-4
Ignition (multipoint) 12-32
Instrument Panel Lights .07-1.5
Lamps (Fog or Spot -Front) 8-10
Lamp (Fog or Spot -Rear) 4-5)
Lamps (Gauges) 1.5-3.5
Lamps (License Plate) 1.5-2.0
Lamps (Parking) 1.5-2.0
Lamps (Reverse) 3-4
Lamps (Side) 1.3-3.0
Power Antenna 6-10
Starter Solenoid 10-12
Voltage Regulator .03-.05
Windscreen Wiper Draw 1-4
Windscreen Wiper Draw (when seized) 12-15

Once you’ve determined the maximum 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.  Second, though one rarely uses all electrical components at the same time, this can happen. Driving at night, heater fan, music....

A good-quality, high-amp alternator can also help you gain horsepower. While most alternators are only about 75-percent 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.

Wiring

Use and/or install the proper gauge charge wire. These are the wires that carry power from the alternator to the battery and the electrical system. Too small a wire will restrict the flow of electricity. Use the chart below.
 
WATCHPOINT    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.
   
Wiring Gauge For Alternator 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