Indestructible battery

[Pumaracing]

Hmmmm. I went out again today and checked the voltage with all the ancilliaries on. Headlights, front and rear screen, fan blower. Only 11.7V even when revving it. It went back to over 14v with all those switched off. I'm now thinking the alternator is not doing the job.

[Noxious89123]

Max_Torque said:

An alternator is a separately excited claw pole device. It uses a small electromagnetic coil on the spinning rotor, to create a revolving magnetic field, that cuts the high current stator windings to produce a sinusoidal voltage output, which is then passively rectified to DC. The higher the voltage applied to the rotor winding, the higher the magnetic flux in the air gap, and hence the higher the output voltage from the stator. Internal to the device is a closed loop voltage control system that controls the field current until the stator output voltage is correct (nominally 14.7, and usually between 14.3 and 14.9Vdc).

Even if you do not connect any load to the alternator this control loop will result in the nominal output voltage being generated at the output terminals. However, as no load is present, there will be zero stator current flow (and only a tiny field current will be required)


Hence, the output current of the alternator, like all electrical devices, depends upon the input impedance (resistance) of the load being driven. In a car, this load is highly variable, and depends on what electrical loads are switched on, and of course the SoC of the battery (and hence it's charging current). For a modern car, the battery charging current is far far smaller than the total current consumption of the electrical consumers, with a "flat" battery charging at maybe 15A maximum, whereas things like heater screens and seats can consume 50A easily.


So, if you check the electrical system voltage with the engine running, if it is above, say 14.3Vdc, then the alternator is generating sufficient current to support the consumers. Of course, if you want to check the alternator, then you need to switch on sufficient consumers to increase system current draw to a point that makes the alternator work hard.

I agree with most of what you're saying, although I'd say the voltage output was controlled by rapidly switching the power to the rotor on and off. I agree with your second point, no current will be produced with no load. The third point I would agree with, but also point out that whilst output increases with load, this is only upto the alternators maximum capacity; if the alternator is faulty it may not be able to supply the required current. Going back to the second point highlighted; Alternator current output must be tested with a battery that is atleast partially discharged and preferably with as much load placed on the system as possible ie. lots of consumers switched on. I still stand by the fact that a voltage test alone cannot tell the whole story, and sometimes a fault will only be revealed when you test the current output. A voltage test is still a very useful diagnosis tool though, and will reveal most common faults by itself.

Pumaracing said:

Hmmmm. I went out again today and checked the voltage with all the ancilliaries on. Headlights, front and rear screen, fan blower. Only 11.7V even when revving it. It went back to over 14v with all those switched off. I'm now thinking the alternator is not doing the job.

11.7v is quite low, sounds like it might be time for a new alternator.

[anonymous-user]

Noxious89123 said:

I agree with most of what you're saying, although I'd say the voltage output was controlled by rapidly switching the power to the rotor on and off.

The magnetic flux in the air gap and hence the stator voltage, is proportional to the rotor field current (and the rotational speed of the rotor of course). That field current is induced by applying the system voltage to the rotor windings, using as you say, a pulse width modulation technique to realise the required average voltage necessary to drive the applicable rotor current. The inductance of the rotor windings acts as a low pass filter, and results in an almost DC rotor current, and hence a stable air gap magnetic flux.

Noxious89123 said:

I agree with your second point, no current will be produced with no load. The third point I would agree with, but also point out that whilst output increases with load, this is only upto the alternators maximum capacity; if the alternator is faulty it may not be able to supply the required current.

If the alternator is faulty, and cannot supply the necessary output current, then the system voltage HAS to fall (V=IR). Hence to test an alternator, you can just measure system voltage, but you must make that measurement under high load conditions (ie with as many consumers as possible switched on) as you say here:

Noxious89123 said:

Alternator current output must be tested with a battery that is atleast partially discharged and preferably with as much load placed on the system as possible ie. lots of consumers switched on.

Noxious89123 said:

I still stand by the fact that a voltage test alone cannot tell the whole story, and sometimes a fault will only be revealed when you test the current output. A voltage test is still a very useful diagnosis tool though, and will reveal most common faults by itself.

In effect, measuring the system voltage under the following conditions tells you everything you need to know:


1) Engine Off, Ignition Off.
2) Engine Off, Ignition On & all consumers on (heated screens, lights etc).
3) After waiting 5 to 10mins at state 2), remeasure
4) Engine running, consumers off (except those needed to run engine obviously, ie fuel pumps etc).
5) Engine running, all consumers on.

Test 1), tells you the float voltage of your battery, and will spot any dead cells or very low SoC
Test 2) Tells you the Ri of your battery
Test 3) Tells you the energy storage capability of your battery (its SoC)
Test 4) Tells you your alternators basic voltage output control capability
Test 5) Tells you your alternators generation capacity and voltage stability


Measurement of alternator current only tells you what the load is, and not what the alternator is doing.

You also cannot measure battery health with alternator current unless you know what the battery SoC is (modern cars monitor battery SoC in real time as they use smart charging (only attempt to charge the battery / supply consumer current) when the car is decellerating etc)

[anonymous-user]

Pumaracing said:

Hmmmm. I went out again today and checked the voltage with all the ancilliaries on. Headlights, front and rear screen, fan blower. Only 11.7V even when revving it. It went back to over 14v with all those switched off. I'm now thinking the alternator is not doing the job.

Repeat test, and measure both across the alternator output terminal to the case of the alternator itself
Then Measure directly across battery, and finally, check the engine earth path by meauring alternator case to battery negative terminal.


It's not unusual (/tomjones) to find corroded and hence high resistance wiring on older cars, resulting in a large voltage drop at high currents

[Pumaracing]

Max_Torque said:

It's not unusual (/tomjones) to find corroded and hence high resistance wiring on older cars, resulting in a large voltage drop at high currents

My mate and I had a play around today with some WD40 and vaseline. Then we looked at the battery and alternator connections. Ooh er missus. Anyway we got the voltage with every possible ancilliary switched on from somewhere in the 11 volts range to over 13v. With just the headlights, radio and fan blower on full it was still over 14v. 14.4v I think. Only the front and rear heated screens dropped it back. I'll call that good enough for now. I rarely use either of the heated screens.