Voltage meter reading in a C2 99 Coupe

[Hroussard]

The handbook states that a charge between 12 an 16 volts is acceptable. What do you all see as a normal reading? I am driving my car almost daily and as such I believe the battery is well charged, immediately after start-up I see just over 14 Volts ( I guess to replace the juice from the starting effort), then I see a downward trend to about 13 to 13.5 Volts where it seems to stabilize but I notice some visible small slow movements of the needle between 13V and 13.5V sometimes even going up close to 14 volts. It this what I can expect.? Do you see movements and if so is that a tell tale sign of something going wrong such as a battery or a voltage overload protector/relay ?? Appreciate some input..

HarryR C2 1999

[P.Viby]

It is all normal. The battery will need a high charge just after engine start, and will then drop down between 13 and 13.7 volt. The visible slow movements can be from when turning on/off heavy electrical loads (light, seat heater or rear window heater).

As long as you don´t se below 12.8 and above I would say 14.5 you don´t need to worry.

Drive safe ;-)

[Costa]

Ideally a lead acid battery (12v nom) should be held at a float voltage of 13.8v. When charging a battery, this can go upto around 14.6v depending on the charge current. The faster the charge ( the higher the charge current) the the higher the terminal voltage will be under charging conditions.

When you place a load on the battery the terminal voltage will fall immediately unless the battery is being charged at the same time. A fully charged standalone battery (ie not connected to car etc) when connected to a heavy load will drop its voltage to around 12.5v and stay around that voltage while still holding charge. As it starts to go flat this voltage will start to slowly start to taper off and when it reaches about 11.0v the battery is considered as flat and requiries a charge.

When a battery is used in your car it is constantly kept charged by the alternator. With a minimal electrical load and a fully charged healthy battery, the alternator should maintain a terminal voltage of 13.8v. If you turn on your lights and as many electrical items you can think of, the voltage should drop slightly when the car is idling (no less than approx 13v though) If you keep the revs up say at 2500rpm or higher, the alternator will supply more current and the voltage will creep back up to 13.8v but no more.

As you have noticed though, the voltmeter sometimes reads slightly higher than 14v. This will tend to only happen when the car is cold. This is because the alternator (when cold) will run more effeciently and due to some minor temperature variations in the voltage regulator circuit (in the alternator) it will supply some excess current which tends to slightly fast charge your battery. As the engine heats up and so the alternator, you should notice the voltage drop to a steady 13.8v when driving indicating it is operating correctly. If you run the car at idle RPM the voltage will drop slightly depending on how much electrical load you have on the system because the alternator tends to operate correctly at RPM's slightly higher than idle eg >1200RPM or so.

This all assumes you have a good , fully charged battery. You will see greater fluctuations in the above readings if the battery is suspect. At no time should the meter drop down to 13v or less or go higher than 14.5v with your engine running at say > 1500RPM

If the voltage falls too much, this will tend to indicate a faulty alternator or a really dead flat battery. If the voltage is too high then it is a possible voltage regulator fault in the alternator.

Edited December 3, 2004 by ZX7R

[P.Viby]

Hey ZX7R.

Very good and precise description. Mine where the short version of all that :P

Drive safe but fast... :drive:

[wwest]

Lead-acid automotive battery charging systems are designed to charge the battery to a higher level, voltage, during predominantly cold weather vs summertime. The old mechanical regulators had a bimetalic strip to sense ambient temperatures and the electronic regulators do it with a thermistor.

Lead-acid batteries can safely hold a heavier charge in lower temperatures that when warm.

[Danyol]

New to the PCar world and my first post.

I've been lurking on the board for a few months, I am continually impressed by the knowledge and passion on the board.

On batteries, I know a little, battery voltage changes with temperature. The charging voltage is temperature compensated from the alternator.

Everything you need to know,

http://www.repairfaq.org/ELE/F_Car_Battery.html

Peace

[Hroussard]

Gang, I am back on this subject. Now here is what I am seeing. I start the engine, voltage meter goes ( generally in the morning or first drive of the day) up to just over 14 Volts as it is supposed to. Now, about 10 minutes later or so, the voltage meter starts to drop , slowly , say over 5-10 minutes BUT when the reading goes just under 14 Volts I "hear" a little click and the click I hear comes from the radio as a metallic click sound and it seems that this is also an indication that a relay or regulator suddenly kicked in or out because I also see the instrument panel lights flicker at the same time. Thereafter I see the voltage gauge read maybe 13 volts but I would suggest more like 12.8 volts or there abouts. The reading stays there and seems not to vary much for the rest of the ride / trip. What do yu'all suggest ?? Voltage regulator gone on the blink?? Or is it the battery?

One more thing, when I switch the ignition on, the voltage meter reads about 11 volts or so, then, during the actual start sequence the reading drops to about 2 volts or so but comes back up as soon as the engine fires. I am not sure that an engine start would draw that much current but I do not know. Maybe it's another indication of something

Appreciate some insight / experiences here.

HarryR C2 1999

[P.Viby]

hmm, The voltmeter should never go down to 11 volt. I would consider changing the battery, or at least try with another battery for a few days to see if everything is the same. Everything else you describe sounds normal.

Good driving :drive:

Edited December 25, 2004 by P.Viby

[1999Porsche911]

You may have a bad cell in the battery. Like suggested above, swap battery with one of your other cars and try it for a few days. Or just go to Sam's Club and get a new $50 battery.

[wwest]

Watching the voltage meter is this manner is a lot like watching the instantaneous MPG indication and trying to determine if the engine is operating propery.

If the car starts reliably each and every time you wish then it is highly likely the system is operating as designed.

If you really want to get an idea of the consistency of the battery charge level then each evening turn the headlights on in high beam mode, engine off, read the meter, and then track it via this method each day.

[Tool Pants]

To change the subject a bit. I have looked at the instrument cluster on a few 997s and did not notice the voltmeter has been replaced by an oil temp gauge. I realized the change when I read the 997 tech info.