I believe a car alternator only generates power (and places a load on the car's engine) when the battery voltage drops below some pre-set threshold value. Presumably it also stops generating when battery voltage climbs above some slightly higher value. Assuming I am correct, how can I determine what those threshold values are?
Car alternator on/off limits
- Thread starter Just4Fun
- Start date
Help Support UKworkshop.co.uk:
No, the alternator doesn't care about battery voltage. The battery can't discharge backwards through the alternator and you need a minimum engine speed for the alternator to make enough voltage, but the alternator output depends only on engine speed. If the battery is at a low level of charge, it draws a lot from the alternator and charges quickly, if it is full and has a higher voltage, close to the alternator output voltage, only a small current flows into the battery.
Just put a multimeter on the battery to see the voltage when the engine is running.
Get someone to work the throttle and see the voltage rise a little.
Be careful. 12V is not high but a car battery can provide the amps to melt your test leads if you put them in the wrong place.
Car battery charging is usually done at 13.8V (float) to 14.7V (maximum, fast charging). If you have a bench power supply these two numbers are handy to know when you want to use it to charge your car battery.
Just put a multimeter on the battery to see the voltage when the engine is running.
Get someone to work the throttle and see the voltage rise a little.
Be careful. 12V is not high but a car battery can provide the amps to melt your test leads if you put them in the wrong place.
Car battery charging is usually done at 13.8V (float) to 14.7V (maximum, fast charging). If you have a bench power supply these two numbers are handy to know when you want to use it to charge your car battery.
Last edited:
guineafowl21
Established Member
I once tested the regulator in an old Denso A127 alternator, similar to the Lucas ACR.
12V bulb across the brushes, bench power supply between earth and field. As I increased the voltage, the bulb glowed brighter until around 14.5V, when it went out.
Modern alternators use a more sophisticated field control, such as PWM, but the overall threshold of 14.5V or so won’t be much different.
12V bulb across the brushes, bench power supply between earth and field. As I increased the voltage, the bulb glowed brighter until around 14.5V, when it went out.
Modern alternators use a more sophisticated field control, such as PWM, but the overall threshold of 14.5V or so won’t be much different.
Only thing I would add if if testing at the battery terminals then if you get low readings, repeat connecting directly to the alternator. If you get the same then there is a problem with it. If you get different figures then check the wiring between the two, particularly the earth.
The vehicle battery is there for starting the engine and once it has reached full charge the vehicle electrics are run by the alternator and the battery just sits idle unless you exceed the alternators maximum output at which point the battery will discharge. The difference in a vehicle battery and a forklift battery is that a cranking battery has a very low internal resistance so that it can deliver the amperage to the starter motor, a forklift battery has a higher internal resistance and can deliver an output over a longer duration. The best way to test a cranking battery is under load, the tester is just a low resistance that draws a current from the battery and shows the voltage which it should hold, a duff cell or sulphated battery will not hold a steady voltage which will drop off.
In modern vehicles the alternator often has no regulator as such, this function being managed by the engine control module along with a warning light and the argument is again emisions / performance or maybe down to control freaks who just do it because they can !
In modern vehicles the alternator often has no regulator as such, this function being managed by the engine control module along with a warning light and the argument is again emisions / performance or maybe down to control freaks who just do it because they can !
Let's take those "usual" voltages (13.8V - 14.7V) as given. Now consider a thought experiment in which we make the battery voltage greater than the alternator charging voltage. 15V For example. In those circumstances, would the alternator generate power or would all electrical loads be supplied from the battery?
Any loads would be supplied from the battery.
The alternator would be spinning but as no current would be drawn from it, it would present a minimum mechanical load on the engine.
Once the battery had discharged down to the output voltage of the alternator, current would begin to flow out from the alternator into the loads and the current drawn from the battery would fall to zero.
This is an analogue process with small differences in voltage affecting the size and direction of currents that can be 10's of Amps.
Beware of higher voltages. Filament bulbs have very short lives at voltages only a few percent above nominal and you don't know the design tolerance of car electronics which are all awkward and expensive to replace.
A concept. Any battery or generator can be represented as an idealised source of voltage in combination with some dumb components. Most important is a series resistor. Every real voltage source has some internal resistance, and the wires connecting to it add some more. The more current you draw, the more the voltage at the place you're interested in will sag, because of resistive losses inside the battery and in the wires.
For this reason, there will be temporary situations where both the battery and the alternator are sharing the load, and the share will shift from all of one to all of the other as you continue to run.
The alternator would be spinning but as no current would be drawn from it, it would present a minimum mechanical load on the engine.
Once the battery had discharged down to the output voltage of the alternator, current would begin to flow out from the alternator into the loads and the current drawn from the battery would fall to zero.
This is an analogue process with small differences in voltage affecting the size and direction of currents that can be 10's of Amps.
Beware of higher voltages. Filament bulbs have very short lives at voltages only a few percent above nominal and you don't know the design tolerance of car electronics which are all awkward and expensive to replace.
A concept. Any battery or generator can be represented as an idealised source of voltage in combination with some dumb components. Most important is a series resistor. Every real voltage source has some internal resistance, and the wires connecting to it add some more. The more current you draw, the more the voltage at the place you're interested in will sag, because of resistive losses inside the battery and in the wires.
For this reason, there will be temporary situations where both the battery and the alternator are sharing the load, and the share will shift from all of one to all of the other as you continue to run.
Last edited:
It was also an issue as more electronics were used as they did not like any noise / spikes on there supply and also wanted a very smooth Dc supply. This is why the alternator became a three phase twelve pole unit so that it minimises any ripple from rectification and the battery also helps this process. It was always a difference between commercial /agricultural systems and passenger vehicles that there was more protection on the commercial systems for O/C or S/C .
When you look at the LED arrays used in many vehicles now you often see dead segments, cannot easily replace these so a complete assembly is needed for the sake of a few pence worth of LED's.
Thanks for that. To rephrase my question in these terms, how can I determine the output voltage of the alternator? Access to the alternator is not easy but the battery is easy to get at.
Step 1. Put a volt meter on the battery with the car switched off. Note the reading.
When you start the car, you will pull several percent of the charge out of the battery, so if anything the battery voltage will drop.
Now measure voltage again at the battery, ideally with the engine at a fast idle so it helps to have an assistant.
Odds are that the voltage will be above 14V and as long as it is greater than the battery voltage you noted in Step 1, you can take it to be the alternator output voltage.
Superduner
Established Member
Search Amazon for "car battery monitor Bluetooth". You'll get a readout on your phone and a record of any parasitic loss
Richard_C
Established Member
If a car has stop/start things can get lots more complicated. In "stop" mode like at a junction the battery does far more work that in conventional engines as the alternator isn't running but the lights, cooling water circulation, aircon etc keep going. Some manufacturers (PSA for instance) use a combined alternator and starter on their 3 pot petrol engines. Careful where you stick your probes 
