The Basics of Alternator Ratings:


I often hear enthusiasts indicate that they’ve “upgraded” their Alternator from the 90 Amp* stock OEM unit, to a new High Output 200 Amp* alternator (* An arbitrary choice of ratings). Unfortunately, rated* Amperage Output seems to be all that most of us know (or care) about Alternators. (*SAE Alternator “ratings” are for Amps output, under Cold (start-up) operating temperatures, at Design Rotor speed - full RPM).

Here’s the rub - Cars often re-charge their batteries at lower than full RPM, and almost always at higher operating temperatures. These two conditions both reduce actual Alternator output. Hence a truly useful Alternator specification should present the Amperage output at varying speeds and temperatures (usually as curves, but sometimes charted).

Output vs Rotor Speed:
An alternator’s output is dependent on it’s rotor speed, but this can be deceiving because the output is not linear - it follows a curve. Each alternator has a unique curve, and at lower speeds, small changes in the alternator’s speed can make a big difference in its output capacity. [see chart below]
Alternator Rotor Speed is determined from engine speed and the pulley ratio:
Ratio = Crankshaft Pulley Diameter ÷ Alternator Pulley Diameter
Rotor RPM = Pulley Ratio x Engine Speed
(eg: A 2:1 Pulley Ratio x 1200 Engine RPM = 2400 Rotor RPM)

There are two simple ways to increase rotor speed: Increase engine RPM, or increase the pulley (crankshaft:alternator) ratio.

If you are using V-belts, be sure that belt wrap, pulley diameter, and belt specifications are up to the task. Also be certain that the alignment of the pulleys is perfect. Any dusting or fraying of the belts is an indication of alignment problems. Belt slippage seriously reduces rotor speed (hence output), and creates huge quantities of additional heat, quickly destroying them. While belts need to be tensioned properly, a belt that is too tight will cause the rear bearing in the alternator (or the crankshaft bearing) to fail prematurely.

Output vs Temperature:
Operating temperature also affects output - higher temperatures resulting in lower outputs. Heat reduces output, and shortens alternator life.
“Cold” Amps rating is for an ambient air temperature of 77 degrees Fahrenheit (25 C), such as when an engine is first started, and will often be 10 - 25% higher than the hot rating.
“Hot” Amps rating is for an ambient air temperature of 200 deg. Fahrenheit (93 C), such as when an engine has been operating for some time.
[see chart below]

An Alternator Specification should describe the Output (Amps) at specific Rotor Speeds, and Temperatures. This is usually presented on a curve, but here’s an example of 2 Alternators in chart form:

Alternator RPM = Alt. “A” Cold* / Hot* ~~ Alt. “B” Cold / Hot

2000 RPM = (A)51A Cold / 41A Hot ~~ (B) 42A Cold / 31A Hot

2500 = 78/63 ~~ 73/58
3000 = 92/76 ~~ 93/76
3500 = 102/85 ~~ 107/87
4000 = 108/92 ~~ 117/96
4500 = 114/96 ~~ 123/102
5000 = 118/99 ~~ 127/107
5500 = 121/102 ~~ 131/110
6000 = 123/104 ~~ 134/113

6500 = 124/106 ~~ 136/116

Alternator “A” might be “rated” at 124 Amps, but will rarely achieve anywhere near that output in actual use. The 124A rating is “cold”, at 6500 rotor RPM (perhaps 3250 Engine RPM). Re-charging at anchor, you might realistically expect something on the order of 63A output (about half it’s rating).

Some desirable features in Automotive Alternators:
- "Hot" Rated - SAE continuous rated output for high temperature bulk charging applications
- Isolated ground terminal (2-wire)
- High amperage, high voltage diodes mounted on cast heat sink
- Heavy-guage stator windings
- Precision balanced rotor
- Extended life copper composite brushes
- Heavy duty bearings with high temperature grease
- Corrosion resistant materials & coationgs