Slow alternator voltage rise time

[JHZR2]

I put a digital voltmeter into the cigarette lighter plug on two of my cars. My S10 seems to have some sort of a small short that causes my typical load to be 100 mA that I haven't had time to fully diagnose yet. My 91 BMW just seems to have a low charging voltage from the alternator.

The thing I've noticed on both of these vehicles is that when I start the car it can take 10, 15 or even more seconds before the alternator actually gets up to normal voltage.

It seems that the other cars especially the newer cars are much much faster to get up to full voltage, over 14.

The BMW output is a little bit low; with headlights, AC, radio and operating on the highway the voltage is only about 13.4. On the S 10 the voltage under load and operating normally is 14.4 to 14.5 which seems to be just fine.

They just seem to be really slow to actually hop off of 12 V and up into a normal operating range. On the BMW at least it appears that revving the engine makes the voltage come up much quicker.

Should I be concerned? Thanks!

 

[HerrStig]

How do you measure a 100 mil "load" with a voltmeter. To do that you have to put an amp meter in series with either battery lead.

 

[JHZR2]

Originally Posted By: HerrStig
How do you measure a 100 mil "load" with a voltmeter. To do that you have to put an amp meter in series with either battery lead.


Where did I say that I measured that load with the voltmeter??? I'm measuring DC voltage with the voltmeter, via the cigarette lighter port which is constantly connected to the battery even when the car is fully off.

I know from previous diagnosis what the standby load is on the truck and I've had to pull some fuses to get it from 200 to 100.

 

[river_rat]

Sounds like a voltage regulator is not putting out field current and the residual magetism of the field winding is the only thing letting the alternator put out any charging current at all.
It could also be a high resistance in one of the slip rings feeding the field.
Just my guesses.

 

[Chris142]

Check the actual voltage at the battery and compare it to the lighter plug.

 

[Dakota1820]

i agree with chris.

 

[wrcsixeight]

Is your Alternator internally regulated, or regulated by the engine computer?

If the Latter, perhaps BMW decided to delay activating the alternator until the engine has spun a couple thousand times first.

I can ask my engine electrical system to either provide amps to the starting batteryonly, or the starting battery and 2 other depleted deep cycle batteries by the turn of a switch.

A Depleted pair of group 27 deep cycles ask for everything the alternator can make . That is about 58 amps at 800 cold engine Rpm and about 110 amps at 2500 rpm according to my digital battery monitor, and tapers quickly as the alternator seemingly heats up faster then the engine to provide this juice.

Since it takes ~1 engine HP for every 25 amps the Alternator produces, the delay might be a way for the engine to run smoother when cold, and perhaps your observations are intended by Teutonic engineers.

My Digital battery monitor has taught me that in terms of alternator contribution, and battery longevity, Ignorance was truly bliss.

Until I had to replace another set of prematurely failed batteries due to chronic undercharging.

When I start my Vehicle on my fully charged starting battery only, that battery asks for only 25 amps right after start up and quickly tapers ( 20 seconds) to about 12 amps as the voltage rises to 13.7

Extra RPM does not significantly increase amperage at this point, and my monitor does not read engine battery voltage as it is wired to monitor the voltage Deep Cycles. But as all alternator current passes through the shunt, it monitors everything the amps to the battery(s) I have the switch set to.

If I bring the depleted deep cycles into the charging circuit, the amps will jump from ~ 12 to 62, and the engine note changes significantly with the extra load, and the Voltage will rise to the Mid 14's with enough time, and or RPM. Without the RPM the alternator has a hard time bringing them to 14.5, even when delivering 50 amps.

It is very obvious to me when I do the first acceleration of the day whether my alternator is feeding one relatively fully charged starting battery(~12 amps), or that battery and two large depleted deep cycle batteries(62+ amps).

I've seen you post plenty of knowledge on battery threads before. As long as you know your healthy battery is not being chronically undercharged, consider the bliss of ignorance, as BMW probably intended.


I am well aware you are not trying to charge a bank of batteries as I am. I am only trying to relate my observations of alternator amperage at differing States of charge by batteries involved in the circuit. Few people have an accurate real time digital read out of the amperage an alternator is producing after start up, at differing RPM's engine , alternator and battery temperatures.

 

[JHZR2]

Well, on the BMW, being a 1991 with a vltage regulator that is easily replaced on the alternator, I am going to say that it is not controlled. I am not positive about that, but my impression is that it is not a smart alternator, and just gets its excitement via the 12V source that is fed through the battery/alternator light in the instrument cluster, as is common.

On a 98 Chevy truck, the situation may be different. It has a standard Delphi alternator and to be honest I am not sure where the regulator is...

 

[meep]

delayed start.

or

remember running ammeters? Batteries pull a HUGE burst recharge current for the first 10-15 seconds, decreasing over the next 45 seconds, settling at a minute or so.

So the alt maybe be at full output but the battery is absorbing it all. you don't have enough info.

M

 

[TFB1]

Originally Posted By: meep
delayed start.

or

remember running ammeters? Batteries pull a HUGE burst recharge current for the first 10-15 seconds, decreasing over the next 45 seconds, settling at a minute or so.

So the alt maybe be at full output but the battery is absorbing it all. you don't have enough info.

M


Agreed... I've never concerned myself with time it takes for alt to recover to full voltage, with a 70-80 amp, it will be longer than on a vehicle with a 130A...

 

[mechtech2]

I am almost sure what is wrong with the BMW.
They use thin wires. Clean all hot and ground cables, and where they seat shiny clean. Yes, BOTH ends. Consider a size upgrade.
And an additional ground wire to the alternator , engine, and body always helps.

 

[JHZR2]

Perhaps it is a ground connection. This past spring I had to pull the alternator to replace some bushings. The main wire from the alternator was in bad shape so I replaced it - I know that it's perfectly clean. The battery connections are also perfect - it happens that the battery is in the trunk.

The batteries connected to the rest of the car by a very heavy 4/0 cable. It is clean and in good shape on both ends. I was curious about voltage drop along it though considering the comment about checking the difference between the battery voltage and the voltage seen at the lighter socket.

With the car off in 100° weather I found that the battery voltage was 12.60 and the voltage at the socket was 12.58. So at least one not underload the voltage drop is not particularly high. I put my battery tester onto the battery and found that the impedance was 2.71 millions. It is lower than I expected, but can be anticipated because of the ambient temperature. So under hot weather conditions it could be that the impedance of the battery is so low that really does allowing excess of charged for rushing in which drops the alternator voltage. I could see this happening on the old BMW but it's a bit hard to grasp on the Chevy truck which is from the late 1990s. I would've thought that the alternator current capability would be far higher, and thus the voltage drop far lower. But I haven't tested the Chevy truck yet to see what the differences are.

 

[brianl703]

Originally Posted By: JHZR2
So at least one not underload the voltage drop is not particularly high.


The load here is a 10megohm (if not higher) multimeter. You can have just about any kind of poor connection and still read full voltage.

Try this, connect one multimeter lead to the battery, pick up the other lead in one hand and touch the other battery post with the other hand.

I bet you read full voltage.

 

[JHZR2]

I believe that the comments that led to this exercise were just to determine the difference in voltage readings from one point to another. In other words to see if the reading from the cigarette lighter socket was accurate.

I guess you are implying that even a very poor connection should still show accurate voltage when there is no load present?

 

[brianl703]

That is exactly what I am saying.

 

[The_Jeff]

Originally Posted By: JHZR2

On a 98 Chevy truck, the situation may be different. It has a standard Delphi alternator and to be honest I am not sure where the regulator is...


If you have an incandescent alternator bulb in your dash the odds are good it's a "dumb" unit. If you have a LED, or a voltmeter with "check gages" LED, you probably have a body control module or PCM running the alt.

"Probably".

 

[JHZR2]

Originally Posted By: brianl703
Originally Posted By: JHZR2
So at least one not underload the voltage drop is not particularly high.


The load here is a 10megohm (if not higher) multimeter. You can have just about any kind of poor connection and still read full voltage.

Try this, connect one multimeter lead to the battery, pick up the other lead in one hand and touch the other battery post with the other hand.

I bet you read full voltage.



OK, so I wanted to get more accurate readings.

First, with the car idling, I checked the accuracy of the cigarette lighter voltmeter against my Fluke 179. The cigarette lighter meter was within 0.02V of the Fluke.

I then turned the AC on with the fan on 3 of 4 for speed, high beams on, radio (which has an amplifier) on. I got roughly 12.4 at the lighter socket and 12.5 at the battery. This was too low to me, as I wanted the alternator to be the sole source, and I figure if loaded voltage was that low, the battery was compensating too.

So I went to low beams, and one is currently burnt out (my job for tomorrow). I got 13.23V at the cigarette lighter, 13.32 at the battery (which is in the trunk, connected with a 4/0 cable), and 13.33 at the underhood connection point where the alternator feeds the battery cable and the car's loads.

So under load, there is roughly a 0.1V drop between the cigarette lighter and battery, and a 0.01V drop on the battery cable that runs the length of the car. Good to know, as when I turned all the loads off, the voltage was about 13.6V (it was 13.9x V when I first started the car, but it had idled to full hot).

13.7 under no active electrical loads, when idling hot in 70F weather is OK I guess... I have a 14.1V regulator in the alternator, FWIW.

What else have I learned? Well, H4 bulbs are 60W on low and 55W on high beam in the low unit, and the high beam unit takes an H1 bulb that is also 55W. So, when high beams were on, all four fillaments were burning, dissipating 220W or 17.6A at 12.5V. With the low beam position on, I was only dissipating 60W, or 4.5A. So, 13.1A additional load caused the alternator to drop 0.8V. Is this indicative of a bad or weak alternator? What should the V/A drop look like on a modern alternator?

Thanks!

 

[Donald]

The alternator sounds fine to me, but if you really want to check it put it on a scope and look at the waveform. Its possible one of the 6 diodes in the bridge is bad.

 

[MrHorspwer]

Originally Posted By: eljefino
Originally Posted By: JHZR2

On a 98 Chevy truck, the situation may be different. It has a standard Delphi alternator and to be honest I am not sure where the regulator is...


If you have an incandescent alternator bulb in your dash the odds are good it's a "dumb" unit. If you have a LED, or a voltmeter with "check gages" LED, you probably have a body control module or PCM running the alt.

"Probably".

For the S10, the alternator is controlled by the PCM.