Battery and Alternator Basics, Questions

[Ozzie]

Hey all, I have been diagnosing and observing an electrical glitch on another car ('85 Toyota). Am posting here because "the basics" should apply to our cars, and I know there are some electrical gurus and overall very car knowledgeable members here that I value and trust their opinions.

So.. I'd like to refresh on the Battery and Alternator interaction. Here is how I think it works, please correct me:

Car Crank and Starts:
On purely battery power only, at (...whatever the battery has)Volts

Immediately after Start:
Alternator takes over, supplying 13.8V
Alternator now also begins to start re-charging the battery.

After the car is warm, and at an idle condition, like at a stoplight:
The battery becomes the main voltage supplier, not the alternator.

Once you idle up, out of the stop:
The alternator then takes over as the main supply, and continues to re-charge the battery.
A good working alternator, should always be providing 13.8V independent of great than idle speed.


Q1: I am mostly interested in the last two steps above. It seems to me that I recall seeing that effect through the voltage gauge on the dash of the D (and Toyota). Is that correct?

Q2: Where is this switch from battery to alternator happening, through relays?

Q3: On a long couple of hour non-stop trip, once the battery is recharged, say topped off to 12.8V or so, does the alternator stop recharging or does it continue to trickle charge the battery

[dn010]

I always thought of the alternator's purpose as being there to "maintain" battery charge, and it does this through the voltage regulator (q2). There are many alternators that need to reach a certain RPM to get excited (mainly 1 wire) but not all need to, there are many that do in fact "maintain" battery charge/voltage even at idle. I'm sure someone will be along to answer each answer exactly.

[content22207]

Alternator voltage and amperage increase with speed. If your regulator goes bad (not all regulators are internal to the alternator -- Ford's long time practice was to use an external regulator) voltage will keep going up into the upper teens/lower 20's. Amperage draw is of course determined by load bearing devices on the car (it won't go through the roof, even if the regulator fails).

There was some discussion on this forum recently about John Hervey's practice of using smaller alternator pulleys to increase rotation speed, thus voltage/amperage, at lower engine RPM's.

Ask Steve Rice how far his DeLorean can travel on just the battery alone.

Bill Robertson
#5939

[Ozzie]

...and BTW: I recently plugged in a digital voltmeter/accessory charger, into my daily driver's ('08 Mazda) accessory plug. I glance at it every now and the voltage readings are consistent with a distinction of 13.8 = alternator supply, and 12.something = battery supply. ...but

I don't recall if at a stoplight it drops to under 13.8 ...but this is different car, newer, less load, different alternator than our Ds and my '85.

[Bitsyncmaster]

The diodes in the alternator prevent battery voltage from discharging in the alternator. So there is no relay that switches the alternator on, it is always on. A diode drops about 0.7 volts so when the alternator produces 0.7 volts above the battery voltage current starts flowing out of the alternator. Most cars the alternator at idle speed provides more current than the load of the car so you never draw battery power only for starting.

As long as your alternator is producing 12.8 volts (average or RMS) your battery is not being discharged.

To charge your battery fully takes quite a bit more voltage than 12.8 volts. It's temperature dependant but voltages from 14 to 15 volts are need in cold weather.

[Ozzie]

Oh, so you're saying that once the car starts it never runs on battery, it is ALWAYS running on the Alternator.

BTW: on my drive home, the Mazda would never drop below 13.8V so that definitely was the alternator.

So, the drop that I see at a stoplight, is just that the alternator can't turn as fast at idle?

[Bitsyncmaster]

So, the drop that I see at a stoplight, is just that the alternator can't turn as fast at idle?

Yes, you current load exceeds the power of the alternator at the idle RPM. My daily driver idles at 500 RPM but with the AC on it bumps the idle up to 700 RPM. That bump improves the AC and also improves the alternator output.

[Ozzie]

Ahhhhh, I see now (and have observable data to match!).
Thanks!!

[content22207]

Oh, so you're saying that once the car starts it never runs on battery, it is ALWAYS running on the Alternator.

Both the battery and the alternator are on the same electrical system. The car does not switch between them. If the alternator is putting out more voltage than the battery, everything in the car will run at that higher voltage including the battery. If the battery is putting out more voltage than the alternator, then everything in the car will run at that lower voltage including the alternator (which should ground and illuminate the dash indicator).

Bill Robertson
#5939

[Ozzie]

Oh, ok, but maybe I'm being obtuse here, but:

When you say:

Both the battery and the alternator are on the same electrical system. The car does not switch between them. If the alternator is putting out more voltage than the battery, everything in the car will run at that higher voltage including the battery.

So when: Alternator V > Battery V ...car's electrical devices are powered by the Alternator.




If the battery is putting out more voltage than the alternator, then everything in the car will run at that lower voltage including the alternator (which should ground and illuminate the dash indicator).

...but when: Battery V > Alternator V ...car's electrical devices are (still) powered by the Alternator. ?

[content22207]

...but when: Battery V > Alternator V ...car's electrical devices are (still) powered by the Alternator. ?

No. What happens when the alternator belt snaps -- the car's electrical system continues to operate as normally as the battery will allow. That is just the most extreme example of Battery V > Alternator V.

Remember that on a DeLorean, the alternator belt also turns the water pump -- do not try to limp home with a broken alternator belt. Other V belted cars tend to put the alternator on its own belt.

Bill Robertson
#5939

[Ozzie]

Oh, ok, it's semantics then the way you wrote it. The "lower voltage" as you wrote is really "a" lower voltage, but it is still Battery V > Alternator V.

Thanks!

[Ron]

The diodes in the alternator prevent battery voltage from discharging in the alternator. So there is no relay that switches the alternator on, it is always on.Generators and old style alternators (most externally regulated) used relays to turn them on/off by controlling the field voltage. Todays internal regulators do the same thing but, instead if magnetic coils working against springs, they sense the battery voltage and adjust the field current on the fly. Most try to keep the battery >14.2V and all will virtually shut the alternator off by dropping the field voltage if the battery voltage gets too high.

[stevedmc]

Ask Steve Rice how far his DeLorean can travel on just the battery alone.

About a year ago I managed to drive about 90 miles on battery power alone. For whatever reason my vendor purchased alternator decided to fail the same day I was in Shriever converting my car to run a carburetor.

We simply put my battery on a charger and when it was time to go home placed it back in the car. I drove behind Farrar for about an hour at night with my headlights turned off. After that we went separate routes, I turned on my headlights, and drove solo while praying for the next 45 minutes.

I drove without my radio or ac turned on and surprisingly I made it home. Needles to say, I went to Autozone the next day and bought a lifetime warranty alternator. The vendor repaired the delorean alternator for free under warranty and it has been sitting on my shelf ever since I got it back.

[WelmoedJ]

Another proof of the generator/alternator doing the supply work:
In the mid-sixties I had a Chevvy 56 (dynamo of course, not alternator).
To prevent theft I had a battery cut-off installed (a "hook" style key).

At a certain moment after a long drive entering a city I noticed that at idle my low beams seem to flicker.
After cutting the engine, the car's engine would not turn over (all electrical systems dead).
Turned out a passenger kicked the key into the off position, making the car use the dynamo power for all the lectrical functions.
IMO this proofs what others previously mentioned: the "power house" takes over once the engine is running.

[Bitsyncmaster]

Generators and old style alternators (most externally regulated) used relays to turn them on/off by controlling the field voltage. Todays internal regulators do the same thing but, instead if magnetic coils working against springs, they sense the battery voltage and adjust the field current on the fly. Most try to keep the battery >14.2V and all will virtually shut the alternator off by dropping the field voltage if the battery voltage gets too high.

Turning the field current on and off is how the output is regulated to prevent to much output. The field current does not disconnect the stator windings which provide the output. The diodes are your switch which only allow output when the alternator voltage is > battery voltage. If you have a shorted diode you battery will drain when the engine is off.

The old generators did have a relay to disconnect them in the regulator. The regulator had both a high voltage and high current regulation. Alternators do not have high current limits.

[content22207]

Some vintage tractor alternators will drain the battery through the alternator field when not in use (that's how their alternators are designed), so they have a simple SPST switch to disconnect the alternator from the rest of the electrical system. Not surprisingly old IH Loadstar trucks also have a switch like this on their dashboards.

Bill Robertson
#5939

[David T]

At idle most alternators do not put out a lot of power. This becomes important especially if you have a lot of load on at the time like the A/C and lights and you are stopped at a traffic light. During that period the battery will supply most of the power to keep the car running and once you start moving again above idle speed the alternator will carry the load and replenish (recharge) the battery. The regulator which is built into the alternator senses the battery voltage and temperature and decides when and how much electric the alternator should make. The electrical system is supposed to be sized so that the battery is really only necessary for starting the car and during short periods of idling. The alternator is supposed to carry the majority of the load and still have enough reserve to charge up the battery. If you don't use the car often the battery is always under-charged. The alternator will always be running at high power and never get a chance to rest (cool off) . Eventually it will overheat and die. Batteries are not like sponges. The do not like being emptied much more than 25%. Deep-cycling shortens their lives also. Bottom line is the harder the electrical system works the shorter the life of the components. If you do not use the car regularly you should have a Battery Tender and keep the battery fully charged so the alternator doesn't have to work so hard and the battery doesn't get discharged so deeply.
David Teitelbaum

[Ron]

Generators and old style alternators (most externally regulated) used relays to turn them on/off by controlling the field voltage. Todays internal regulators do the same thing but, instead if magnetic coils working against springs, they sense the battery voltage and adjust the field current on the fly. Most try to keep the battery >14.2V and all will virtually shut the alternator off by dropping the field voltage if the battery voltage gets too high.


Turning the field current on and off is how the output is regulated to prevent to much output.
I said that, last sentence.


The field current does not disconnect the stator windings which provide the output.
I don't see how I implied that??? But actually, if you cut the field current, you loose the magnetic lines of force it produces to induce current in the stator...no output.


The diodes are your switch which only allow output when the alternator voltage is > battery voltage.
The diodes are used to rectify the alternating current into direct current, which I'm sure you know -- But they have nothing to do with controlling output levels, as such. An excellent explanation, See pg 30. (http://www.slideshare.net/ram_ari/alternator-winding)


If you have a shorted diode you battery will drain when the engine is off.
This is true for the Positive diodes. But, if one of the Negative diodes shorts, it in turn, shorts one of 3 stator windings, causing the alternator to lose (some) output.


The old generators did have a relay to disconnect them in the regulator. The regulator had both a high voltage and high current regulation.
Agreed.
And they did this, IIRC, by cutting the field. You can't have one without the other ;)


Alternators do not have high current limits.
Bet you can't get 110A out of my stock D alternator!
J/K!!! I know what you meant...
The amperage is not regulated, no need too. The regulator would see the voltage climb too high and cut the field first.

=====

+1, Bill
IIRC, those are the ones that, if you pulled the belt, the generator would turn!

+1, David T

An EXCELLENT explanation here (http://www.slideshare.net/ram_ari/alternator-winding)