Alternator output drop
05-13-2005, 11:15 PM
#1
Burning Brakes
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Alternator output drop
Over the space of a 2hour drive, my voltmeter dropped from about 13.5 to 12V and sat there for the rest of the drive. I checked the battery fluid and it was fine. With the car off, it was reading 12.6V.
Will alternator output drop if the car gets hot ? What else coudl cause it ?
Hoping it's not serious - I'm down in NC for a track day at Rockingham !
Will alternator output drop if the car gets hot ? What else coudl cause it ?
Hoping it's not serious - I'm down in NC for a track day at Rockingham !
05-13-2005, 11:30 PM
#2
Nordschleife Master
It is heat that is dropping it. External and internal ..... the diodes just hate heat.
But 12.6v is pretty darn low. Make sure you have the heat shield on and the vent hose from the fromt of the air dam.
But 12.6v is pretty darn low. Make sure you have the heat shield on and the vent hose from the fromt of the air dam.
05-14-2005, 06:17 AM
#3
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Originally Posted by IceShark
It is heat that is dropping it. External and internal ..... the diodes just hate heat.
-Mark
05-14-2005, 09:05 AM
#4
Nordschleife Master
Originally Posted by Mark944na86
Can anything be done? Like replacing the diodes?
-Mark
-Mark
You could also try and do it for cheap and attempt to replace the individual diodes if only a couple have gone bad but I don't think that would work out too well as they are pressed into the metal frame. When you look at the retifier you will see what I'm talking about.
05-14-2005, 01:42 PM
#5
Three Wheelin'
Part of the "problem" may be heat related, but I wouldn't go nuts trying to "solve" this one.
First off, when ever you start the engine the starter motor draws a bunch of current. This initial high draw lowers the "charge state" of the battery.
When the engine is running, the voltage regulator in the alternator "sees" a "low" battery voltage and starts to charge the battery, which causes the INDICATED battery voltage to increase. What you are actually seeing it the alternator's output voltage which must be higher than the actual battery voltage in order for charging current to flow.
As the battery is charging, it's voltage starts to increase, and charging current starts to decrease. At some point (And this depends on how "low" the initial voltage drop was and how much capacity the alternator has in amps.), the battery will reach a "fully charged" state. There will be very little if any charging current flowing through the battery. At this point the alternator will be supplying power for what ever electrical loads (head lights, DME, fuel pump, radio, etc.) you may be running at the time, and INDICATED battery voltage will be determined by the amount of resistive (I squared R) loads in the electrical system, which includes resistive loads due to loose/dirty connections.
So, in a nut shell, the voltage "drop" you are seeing is "normal." However, the magnitued of this voltage drop can be reduced by insuring that ALL electrical connections are clean and tight, and there are no "salt tracks" or "acid spills" on the battery or in the battery box. Loose/dirty connections add resistance to the electrical system, and "salt tracks" are conductive and can become "parasitic resistive loads".
Finally, the in-dash volt meter should be used more of a "go/no-go" indicator as opposed to a Digital Volt Meter which is inherently more accurate, or qualitative. However, the in-dash volt meter is highly repeatable.
The idea here is to use a DVM to measure system voltage under various conditions (system loaded, system not loaded, ect.) AT THE BATTERY, and then compare those readings to the in-dash indications. This way you can establish a base line for what is "normal" volt meter behavior under various driving conditions.
If you still have doubts about the "health" of the electrical system, have the battery and alternator tested. Most places will do this for free with the hope of making a sale.
First off, when ever you start the engine the starter motor draws a bunch of current. This initial high draw lowers the "charge state" of the battery.
When the engine is running, the voltage regulator in the alternator "sees" a "low" battery voltage and starts to charge the battery, which causes the INDICATED battery voltage to increase. What you are actually seeing it the alternator's output voltage which must be higher than the actual battery voltage in order for charging current to flow.
As the battery is charging, it's voltage starts to increase, and charging current starts to decrease. At some point (And this depends on how "low" the initial voltage drop was and how much capacity the alternator has in amps.), the battery will reach a "fully charged" state. There will be very little if any charging current flowing through the battery. At this point the alternator will be supplying power for what ever electrical loads (head lights, DME, fuel pump, radio, etc.) you may be running at the time, and INDICATED battery voltage will be determined by the amount of resistive (I squared R) loads in the electrical system, which includes resistive loads due to loose/dirty connections.
So, in a nut shell, the voltage "drop" you are seeing is "normal." However, the magnitued of this voltage drop can be reduced by insuring that ALL electrical connections are clean and tight, and there are no "salt tracks" or "acid spills" on the battery or in the battery box. Loose/dirty connections add resistance to the electrical system, and "salt tracks" are conductive and can become "parasitic resistive loads".
Finally, the in-dash volt meter should be used more of a "go/no-go" indicator as opposed to a Digital Volt Meter which is inherently more accurate, or qualitative. However, the in-dash volt meter is highly repeatable.
The idea here is to use a DVM to measure system voltage under various conditions (system loaded, system not loaded, ect.) AT THE BATTERY, and then compare those readings to the in-dash indications. This way you can establish a base line for what is "normal" volt meter behavior under various driving conditions.
If you still have doubts about the "health" of the electrical system, have the battery and alternator tested. Most places will do this for free with the hope of making a sale.
