battery voltage or alternator issue

[ace123]

Hi Guys,
Here is the issue I have been having. I voltage on my car is fluctuating and I'm not sure why. When I take the car for a drive for 10 minutes or so the voltage is fine around 13.75 volts while running. Then I turn off the car and start it up again the runs at 12.00 volts.

What's up with that?

I have a brand new battery.

What do you think is the issue?

I'm using one of the devices that you plug into the lighter to get the readings. I got that tip from the previous research here

Thanks!
Ace

[mreid]

When you restart it, rev it once to kick the alternator then measure the voltage at idle. The alternator only charges the battery when it needs it and in most cars will not start to charge until the rpms exceed around 1500.

[ace123]

After the restart, I start driving it and I get up to 4000k rpm or whatever and it is still only at 12 volts. Is that normal?

I can test the what you said and report back if that's different from what I said above.

[The cap'n]

Where and how are you reading the voltage?

The Cap'n

[wwest]

Originally Posted by ace123

Hi Guys,
Here is the issue I have been having. I voltage on my car is fluctuating and I'm not sure why. When I take the car for a drive for 10 minutes or so the voltage is fine around 13.75 volts while running. Then I turn off the car and start it up again the runs at 12.00 volts.

What's up with that?

I have a brand new battery.

What do you think is the issue?

I'm using one of the devices that you plug into the lighter to get the readings. I got that tip from the previous research here

Thanks!
Ace

Cranking the engine for starting will always "remove" the "top off, float charge", anything above 6X~2.15, and some of the "core" charge. The core charge must be restored before you will begin seeing the "top off" voltage level.

12.00 volts seems a bit low but if it always recovers to ~13.75 or above, then you have no worries.

[mysocal911]

Originally Posted by ace123

After the restart, I start driving it and I get up to 4000k rpm or whatever and it is still only at 12 volts. Is that normal?

I can test the what you said and report back if that's different from what I said above.

That's not normal. You most likely have an intermittent connection problem from the
alternator to the battery (alt - starter - battery), an intermittent alternator, or an intermittent regulator. Also, the alternator light must always come 'on' with the key in
the 'run' position and go 'out' once the engine starts.

[wwest]

Originally Posted by mysocal911

That's not normal. You most likely have an intermittent connection problem from the
alternator to the battery (alt - starter - battery), an intermittent alternator, or an intermittent regulator. Also, the alternator light must always come 'on' with the key in
the 'run' position and go 'out' once the engine starts.

What's not normal?

Battery charging voltage is rarely a function of RPM....

That, battery terminal voltage rising with engine RPM, only happens with a more fully discharged battery and/or a heavy electrical load, headlights, heater blower, etc, etc.

[wwest]

How long, how far, must you drive before the battery voltage begins to climb, reaches 13.75 volts or more?

[Determined]

A good battery should put out just over 14V (13.8-14.1) range, from what I've seen on all my cars. If not then you may have an alternator going out, however please check all your grounds including the transmission ground. I found on my Carrera that my transmission ground was disconnected and my voltage was jumping around everywhere even with a new alternator.

When the car is turned off the batter should have about 12.5-12.6 Volts. If you battery functions as I mentioned above but when turned off it reads less than 12 or slight above but below 12.5 Volts then you may have a dead cell in the batter.

I hope it helps, good luck.

Best Regards,
Determined

[ischmitz]

Assuming you have a new battery (like you said in your OP) you might have an issue with your alternator.

Most likely the brushes are at the end of their life and sometimes it puts out power and sometimes it doesn't. As a result sometimes you see the voltage going to the 13V - 14V area (battery is charging) while at other times you are running off battery power only and you only read 12V.

And as others suggested check the GND strap and the connections at the starter, the alternator and battery. Especially, if someone was in there recently.

Ingo

[wwest]

Originally Posted by Determined

A good battery should put out just over 14V (13.8-14.1) range,

Only during charging might you see (lead/acid) battery post voltages in the higher ranges, above 6X2.15 volts, ~12.9-13.5 volts.

from what I've seen on all my cars. If not then you may have an alternator going out, however please check all your grounds including the transmission ground. I found on my Carrera that my transmission ground was disconnected and my voltage was jumping around everywhere even with a new alternator.

When the car is turned off the batter should have about 12.5-12.6 Volts.

Yes, IMMEDIATELY post engine shutdown after a reasonable period of charging.

After cranking the engine...NOT!

If you battery functions as I mentioned above but when turned off it reads less than 12 or slight above but below 12.5 Volts then you may have a dead cell in the batter.

I hope it helps, good luck.

Best Regards,
Determined

Those might well be perfectly normal voltage readings.

[The cap'n]

I just reread the thread. Testing your voltage through the lighter has got to be one of the WORST possible ways to assess the condition of the charging system. You MUST read the voltage directly at the battery posts, even if it means running wires into the cabin from the battery. The lighter is at the end of a lot of connections and wire of indeterminate condition, and will NOT accurately reflect charging or at rest voltage. It's kinda like the volt meter in a 914, commonly known as "eye candy". Do it right, do it once.

The Cap'n

[wwest]

Originally Posted by The cap'n

I just reread the thread. Testing your voltage through the lighter has got to be one of the WORST possible ways to assess the condition of the charging system. You MUST read the voltage directly at the battery posts, even if it means running wires into the cabin from the battery. The lighter is at the end of a lot of connections and wire of indeterminate condition, and will NOT accurately reflect charging or at rest voltage. It's kinda like the volt meter in a 914, commonly known as "eye candy". Do it right, do it once.

The Cap'n

I disagree...system voltage measurement via the cigarette lighter socket will be perfectly adequate as a monitor.

[timmy2]

Originally Posted by wwest

I disagree...system voltage measurement via the cigarette lighter socket will be perfectly adequate as a monitor.

I agree, especially on my car where I ran a fused always hot directly to my lighter socket. Works great with the lighter socket adapter for maintaining the battery too!

[ace123]

OK - Here's what I did this afternoon. I tested the voltage at the battery while the car was off and then compared it with the voltage on the cig lighter meter. They were off by about .1 volts. I also checked when the car was running with low revs and parked . Again, it was off by about .1 volts or so. So I think it is a good enough reading. Here are the readings for the test:

Car off: 12.00volts
Started car, headlights on/off for about 2 seconds, revved to 2000rpm: 13.75 V
Had the car idle for about 5 minutes, backed out of the garage then stopped: 12.00 V
Then drove for 10 minutes and the voltage went to 13.75 v then dropped to 12V

Then 2 ten minute trips with start and stop. The Voltage was mostly 11.5v then at one point it dropped to 8 to 9 volts and the battery light came on and then went off on the dash board. Then back up to about 10 or 11V.

Then I parked it and scratched my head. What do you guys think?

Thanks!

Ace

[ischmitz]

Bad alternator (brushes EOL)

[yanik]

I had the same issue and it was definitely my brushes... I have an extra voltage regulator with good brushes if you need one pm me... If you are really worried I would take your alternator to an auto electrical shop and have them test it.. If you pull your voltage regulator out and the brushes are worn down you may need to get the bearings turned.. My unfortunate alternator was in bad shaped and needed to be replaced... Here is my thread with my situation.

Condition of this Alternator?

[ricks911]

Had a similar issue; voltage would be fine while driving for a bit, 14.4V, then drop to 12V or so. It turned out to be a bad ground cable from the tranny to the chassis. The cable looked good and metered fine, but the new cable solved the problem. However this was after a new battery, alternator rebuild and testing the tons of other electrical testing $$$. I used the little volt meter plugged into the lighter too. Excellent test tool.

If you can reproduce the situation, use a set of jumper cables to simulate a new ground cable. Connect battery ground to the chassis and engine at different spots to see if it fixes the voltage.

[Driver_X]

^ start there

Clean the grounds, one at the trans, another one actually on the alternator (this means you have to remove the alt) and check the one at the a battery. These are the big ones, many more but I would start there.

That being said, it sure sounds like the alternator is tired.....

[jwakil]

Originally Posted by The cap'n

I just reread the thread. Testing your voltage through the lighter has got to be one of the WORST possible ways to assess the condition of the charging system. You MUST read the voltage directly at the battery posts, even if it means running wires into the cabin from the battery. The lighter is at the end of a lot of connections and wire of indeterminate condition, and will NOT accurately reflect charging or at rest voltage. It's kinda like the volt meter in a 914, commonly known as "eye candy". Do it right, do it once.

The Cap'n

I always hoped the lighter voltage was incorrect whenever I had problems on all my previous cars, unfortunately it never once was the case. The wiring would have to be really screwed up if the lighter is reading different than the battery.

I had some similar problems as the poster, and I ended up replacing the voltage regulator which solved the problem.