Alternator Notes

[evm1024]

Just thought I would jot down some background info on alternators

Some basic background - Ohms law

Ohms law describes the relationship between voltage, current and resistance. In any given circuit all 3 of these values are in balance. They are always in balance. The basic formula where current and resistance are known is: E = I x R (voltage = current times resistance)

So if you want to know voltage you use: E = I x R
If you want to know current then you use I = E / R
and if you want to know resistance you use R = E / I

As a side note power (watts) is W = E x I

Alternators and regulators

Automotive alternators typically have an internal voltage regulator. These voltage regulators monitor the output voltage of the regulator at the B+ (output) terminal on the alternator. They are designed to keep the output voltage at 14.4 volts (at 75 degrees) and to decrease the output voltage as the alternator warms up. Plus they may have a high temp shutoff to prevent the alternator from burning up.

The regulator changes the field current to the alternator field coil which in turn changes the output of the alternator. Low field current and you get low alternator output. High field current and you get high alternator output. The regulator changes this field current to meet the demands of the cars loads (head light etc). Notice I said alternator output, not alternator voltage or current.... More in a bit.

Alternators have a designed output voltage (14.4 volts that the regulator is set to) and a maximum current capacity (65 amp, 95 amp etc). It should be noted that no automotive alternator is designed to run at 100% capacity for any length of time. You will not get 95 amps out of a 95 amp alternator day-in day-out without burning it up. It also should be noted that alternator belts have their limits as well. A 3/8" belt can handle about 80 amps without dying a quick death. Beyond that the lifespan of the belt goes down quickly (and leaves dust everywhere).

Voltage, current and resistance

The voltage regulator sets the cars voltage at 14.4 volts. With no loads on the car and the battery fully charged you could expect to measure 14.4 volts at the alternator, on the battery + terminal and here and there around the car. With little load, the resistance that the alternator sees will be high, and the need for current will be low. Remember that I = E / R so the current (I) will be low when the voltage (14.4) is divided by a high resistance. This is the "magic" in understanding how ohms law works.

As you add more loads (say you turn on your head lights) the resistance decreases. (It sounds backwards but is not, a dead short is a 0 ohm path) With a lower resistance the circuit needs more current and because of ohm's law the voltage starts to drop. The regulator detects this and increases the field current causing the alternator to supply more output and the voltage starts to rise. The regulator keeps increaseing the field current until the output voltage reaches 14.4 volts. This happens very fast so we do not see much of a voltage drop then recovery. It appears that the alternator has only increased its current output. But remember that this current is what is needed to make ohms law balance out.

We could go on increasing the load (which means a decreased resistance) by turning on more and more thins (heater fan, big stereo etc) and the voltage would try to drop (ohms law at work), the regulator would sense this low voltage and increase the field current which generates more output (current) that allows the voltage to rise until it again reaches 14.4 volts. This could go on until the alternator reaches its output limit (whether the heat limit built into the regulator or by the alternators physical capacity to produce current (65 amp, 95 amp alt etc). At that time the alternator is overloaded and the voltage starts to decrease.

How is that for a start... I'll tale a look at voltage drop in the next post.

[RRoller123]

Good stuff, thanks for adding to the knowledge base.

[evm1024]

Current flow

We know that current flows in a circuit. It leaves the battery and finds it way to a load then back to the battery. THis applied to the alternator as well. Current flows in a circle. If we break this circle we have an open circuit and the current stops. This can be a good thing or a bad thing. It is a bad thing if it is your ignition circuit - your car just dies. Or a good thing when you flip the headlight switch (to make an open circuit in the headlight circuit) and shut the headlights off.

If we take a 9006 low beam headlight lamp for example we know that they are 55 watts at 12.8 volts. But how many amps do they take. Remember that Watts = Volts/Amps so 55w=12.8v/?a or to rewrite amps = 55/12.8 which is 4.3 amps. We also know that R=E/I so 12.8/4.3 = 2.98 ohms resistance in the headlamp bulb.

N.B. There are lots of roundings, and other factors that I am ignoring. These do not change the ideas so bear with me.

So we can say that with 12.8 volts at the headlight the headlight resistance of 2.98 ohms limits the current flow to 4.3 amps.

Now let's suppose that we have one of the headlight wires on the supply side chafe through and contact the metal of the car. The current instead of flowing round to the headlight then then back to the battery goes directly back to the battery taking a shortcut if you will. This is a short circuit. Remember that the resistance of the head light (2.98 ohms) limits the current flow. The resistance of the wire that has been shorted is much less than the resistance of the headlamp. Let's just say that the resistance of the short circuit is 0.1 ohms. This resistance will try to limit the current flowing in the (short) circuit and using ohms lay we see that the current is I=E/R or 128 = 12.8 / 0.1.... SO this short will have 128 amps flowing through it. Look at it another way, watts= E*I sow 12.8 volts * 128 amps = 1638.4 watts. You now have your headlight wiring turned into a space heater and watch it go up in flames! Poof!

As a side note: Current is measured in amps. Amps are defined as a coulomb of electrons moving in a wire per second. That is to say that 1 amp means that 6.241×10^18 electrons per second travel round the circuit.

Voltage drop

We know that for any given voltage the resistance of a circuit limits the current flowing in that circuit. The less resistance the greater the current. In the case of the headlamp example we have 12.8 volts and 4.3 amps. If we measure the voltage on one side of the headlamp we get 12.8 volts and on the other side we get 0 volts. The voltage drop in the headlamp is 12.8 volts (12.8-0=12.. All of the power is used in the headlamp (55 watts worth).

But let's just say that you have a bad ground on the headlamp. This bad ground is in fact a resistance. It is in series with the headlamp so the resistance of the ground and the resistance of the headlamp add together. This total resistance now it the limiter of the current flowing in the headlamp circuit. For this example let's just say that the resistance of the bad ground it 0.9937 ohms. We add this into the headlamps resistance and end up with a total resistance of 3.973 ohms.

With ohms law we see that 12.8 volts / 3.973 ohms gives a current flow of 3.22 amps. But wait, resistances add together (when in series) but current is the same everywhere in a (simple) circuit like this. 3.22 amps flows through the headlamp and through the bad ground. If we now measure the voltage going to the headlight we get 12.8 volts. When we measure the other headlamp prong instead of 0 volts we get 3.2 volts. Measuring across both headlamp prongs we get 9.6 volts. Our headlamp "sees" only 9.6 volts and is pretty dim.... The bad ground has a voltage drop of 3.2 volts (12.8 - 9.6 = 3.2). Also, that bad ground is getting hot. This is because there is 3.2 volts at 3.973 amps being dissipated there. This is 12.7 watts. I have a 12 watt soldering iron that has a 700 degree tip temperature. Expect the ground to burn up.

The brown wire

All wire has a rated voltage, current and temp to name a few specs. Voltage limits is mostly due to the ability to prevent that voltage from going through the insulation. THink about the ignition wire verses the spark plug wires. If you used regular wire for your spark plugs you are going to get shocked and your car would really misfire badly. Current carrying capacity of the wire is limited mostly by the actual size of the wire. More electrons (more current) needs more wire to go through. And the temp limits of the wire tell us when the wires insulation will melt....

All wire has resistance (ok lets ignore super conductors) and the more current that you run through it the hotter it gets due to the voltage drop over the wires resistance. For any given wire as you increase the current that passes through it the hotter it gets.

In the case of something like the brown wire let's just say that that wire has a resistance of 0.005 ohms, Not much. If we have almost nothing turned on and the brown wire only passes 10 amps current we can calculate that the brown wire has a voltage drop of (E=10*0.005) 0.05 volts. So the brown wire dissapates only (w=0.05*10) 0.5 watts. Not bad. If we add on the lights and what not we might have a current of 20 amps going through the brown wire. The voltage drop would be (E=20*0.005) 0.10 volts. Power dissipation would be (W=0.10*20) 2 watts. The wire is warm but still OK. Lastly we put in some new high power headlights and the wiper motor is needing more power due to gunk instead of grease inside. The voltage drop on the brown wire would be (E=40*0.005) 0.2 volts with a power dissipation of (W=0.2*40) 8 watts. The brown wire has gotten hot and really made a mess of our key switch.

[evm1024]

Back to alternators

Do I need a 95 amp alternator? Well, yes, no, may-be.

If you current needs are low then you really don't need a big alternator. If you car typically uses 20 amp from the alternator while running then a 55 amp alternator would be fine. You would be running at 36% of the alternators capacity. You should be able to do that all day and night.

If on the other hand your car needed 40 amps to run you would be loading a 55 amp alternator at 72% which is likely pushing it very hard. You can expect the alternator to run hot and have a reduced lifespan.

If you run a car that needs 55 amps at peak loads then you can expect your 55 amp alternator to get very hot and not be able to produce enough current to meet the demand of the car. And thus the voltage that the alternator is producing will decrease until everything comes back in balance. Expect dim lights, backfires and a fire at some point.

What does a 95 amp alternator get me

The short answer is that it gets you ample power. It gets you enough reserve power so that you can run your alternator at 50% load or less thus the alternator runs cooler. It gets you a reserve of power so then when you need more electrical power the voltage does not drop as a lesser alternator struggles to supply the current demanded by your car.

Running a 55 amp alternator at 20 amps and running a 95 amp alternator at 20 amps takes about the same power. Actually the 95 alternator may take less power to supply 20 amps because of increases in efficiency of a newer alternator design. 20 amps at 14.4 volts is 288 watts and there are 745.7 watts per horsepower. When you factor in the losses involved to make electrical power this 20 (about 0.38 HP) may take up to a horsepower or so to make.

Considerations for running a 95 amp alternator

A big alternator has the ability to supply major current. This could mean major problem if you have a marginal electrical system. Clean all the connections and grounds. Try DeOxit d5 when cleaning contacts and grounds. Put in relays for headlights, wipers, heater fans to limit the current that has to go through the brown wire (and your ignition switch). Fuse those relays.

When you start the car the battery supplies the needed current. Once started the alternator takes over that job. At first the alternator has to supply the cars needs as well as to recharge the battery. All of the current that the car uses comes from the alternator and thus the alternator B+ wire needs to be sized correctly.

There are many factors that go into deciding the current limits for a wire. Using marine specs for wire rated for 105 degrees C it shows us that a 95 amp alternator should use a #6 wire (rated at 120 amps). Of course you could get by with smaller wire because you are not likely to be taking lots and lots of amps out of your alternator for any length of time. #8 wire is rated at 80 amps for example. Smaller wire has a greater voltage drop and runs hotter at higher currents. Remember that we are talking about circuits - this means that both the + wire needs to be sized for the expected current but that the grounds also need to be sized for the same current.

[evm1024]

Well there you have it. A bunch of ramblings with a large number of details left out. But I hope that it will give the those of us with limited electrical experience a leg up. And give a chuckle to those of us who have been around the "circuit" a few time.

[spider2081]

Thanks you really put a lot of time into your thoughts and sharing them. Just curious have you measured the current drawn from your alternator with everything electrical turned on??? The reason I ask is I have and would like to know what a similar car measured. With everything on in my car including the radio and halogen headlights and driving lights added below the bumper), I measured less than 50 amps. My car has a 65 amp alternator. When I purchased the car in 2003 the alternator was defective and the wire connecting the alternator to the starter was fried. I installed a new #8 wire, a repaired alternator and new battery. I have had no issues with the charging system since. The battery lasted 8 years and I replaced it. So the second battery now has been in the car the last 5 years. I have driven the car about 45,000 miles since 2003. The car shows 173,000 miles on the odometer. An old rule of thumb ( meaning not real technical) was the constant load on an alternator should not exceed 80% of it rated output. So unless I'm driving on a cold rainy day with defroster and wipers on I feel the original alternator is adequate. Also I think it is a plus to have an alternator on a car that was designed to be on that car.
I do have headlight relays, a starter relay, wiper relay and coolant fan relay installed. I installed this primarily to save the switches that control them.

[DRUMMOND]

evn1024

Thank you! for giving us electrical info for dummies Keep it coming!