To my understanding, a simple first-step for testing the alternator and related components on a vehicle is to monitor the voltage (using a multi-meter) at the battery while the vehicle is running. According to my research, one should get around 13 to 14+ volts; however, what is "proper" seems to vary by vehicle.

With that in mind, how does on accurately determine what the voltage should actually be for a particular vehicle. For example, is there a relationship between engine displacement, number of cylinders, etc. and the expected reading?

To provide some context, my sister has a 1991 Cadillac Deville with a 4.5L engine. She had the electrical system tested (at a parts/battery store) and was told that the voltage was "low" for that particular vehicle. I tested it myself, and received readings in lower to mid 13.x range. While I believe such would be acceptable for some vehicles, I am not certain if it is acceptable for that particular model and year of vehicle. I could not find the satisfactory range within the Chilton's manual for that car.

How does one determine what the appropriate voltage (at idle) is for a specific vehicle?



No, there is no relationship between voltage and engine displacement or number of cylinders, except a larger engine may lead to more heat in the engine bay, necessitating lower voltage.

A voltage of low-to-mid 13.x range is not acceptable. Even float charge requires 13.5V-13.8V. In cars, the battery power is used for starting the engine, so it should be charged at voltage somewhat higher than the ideal float charge voltage. Just the float charge voltage may not provide enough charge.

What is "proper" depends more on the temperature of the battery rather than the type of the car.

You can see the float charge voltage per cell (which is the absolute minimum charging voltage) and the charge voltage limit at various temperatures here in Figure 2: http://batteryuniversity.com/learn/article/charging_at_high_and_low_temperatures

You need to multiply by the number of cells, which is 6 in all nominally 12 volt systems.

So, for example at 25 degrees Celsius 2.28 V * 6 = 13.68 V is the absolute minimum charging voltage, and 2.43 V * 6 = 14.58 V is the absolute maximum charging voltage. Your voltage should ideally be halfway between those values.

I recommend an infrared thermometer to determine the battery temperature and then look at the figure to see charging voltage limits. Be sure to use a high-quality multimeter, as the cheapest multimeters may be poorly calibrated and may work accurately only at room temperature.

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  • That max voltage you quote is not correct - just over 15 is given for my car... – Solar Mike Apr 28 '18 at 22:01
  • Hmm? I would tend to believe what Battery University says is true... @SolarMike, are you sure the 15V is at the battery terminals, not at the output terminals of a two-wire alternator with no separate third sensing wire? – juhist Apr 28 '18 at 22:11
  • I would not rate Battery University anywhere close compared to Jaguar especially when I have to check things. My car is 14 years old and I have the workshop manual and electrical diagrams. Just for info the alternator has more than just a third sensing wire, one is the voltage regulator request, another is the pwm signal to the eco for monitoring the load on the engine, third is the warning light. – Solar Mike Apr 29 '18 at 5:53

There are slightly different methods of measuring voltages defined for each car / manufacturer. The one I show below is from the Jaguar X type manual page 2081 and I would take this info for my car compared to some ad-hoc battery site / blog on the internet.

I would always go to the workshop manual / manufacturer information for any specifications such as this, probably because it. was how I was trained - apprentice trained vehicle electrician...

I stated in my comment that just over 15V was given for my car and this is actually 15.3V with the generator (alternator, 120A) at 25 Deg C and producing 5 A.

Jaguar alternator output voltage

This should be measured with the probes on the battery terminals with a quality meter that has been calibrated or at least checked against a known good meter for readings (10V, 12V and 15V, or use the battery and system and see if the meters read the same).

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