Voltage / charging issues

[Sinister]

Ever since I purchased my 2001 996 TT, whenever one measures voltage at the battery, engine started, I get 13.2V with everyting off but the engine running, 12.94V with AC on, and 12.88V with headlights, radio, AC, etc. on.

With many short trips, my battery eventually disharges, requiring a full recharge about every 8-10 months, with a battery charger.

This weekend I fully charged the battery and when I measured the voltgage at the battery, with the engine running, it measured 14.1V. When I went to drive the car, it automatically droped to 13.4, then 13.2, and now it is back to 12.8V. When measuring at the alternator end, voltage is 13.5-13.6.

I removed the alternator and took it to an alternator shop. On the test bank, it puts out 14.2V, so alternator and regulator are OK.

So as is now, the car tends to read a voltage of 12.88 at the battery end, and 13.5 at the alternator end.

With the car off, the voltage on both ends (battery and alternator end) is 12.46V.

Am I losing current? All my grounds are tight, and the battery tests fine.

[fastboydave]

When you took it to the alternator place, did they 'load' it up?? That is try and draw current from it?? Its no good just checking the voltage output when there's no load, those alternators should supply over a hundred amps at the correct voltage. Suppose they know what they are doing? You will get a Vdrop between alt and battery as you have resistance in the copper which the more current you draw, the more Vdrop you have.

However, something may not be right. Think i've heard on here you can have a bad headlight switch or ignition switch that drains you battery?? Check if you have too high parasitic drain. Take of positive lead and check 'standby' current with a DMM. Connect it right (in series) else you'll get a nice spark! This should tell you if you have a short/drain somewhere. It maybe that you arent doing enough long trips and it has been discussed on here alot. I plug mine into a battery maintainer every weekend if not using it to keep it fully toppped up.

[plima]

The battery won't charge when its terminal voltage drops much below 13 volts. The voltage at the alternator seems about right. When you see a drop in voltage between the alternator and the battery like you are observing there is an excessive drop in voltage in the wiring (unlikely) or a connection (much more likely). The drop in voltage at each connection depends on the amount of current flowing; thus it is much smaller when there are no loads (like lights, air con, etc.). I'd check for voltage drops at each connection point (like at the alternator itself, or at the battery. My guess is you will find the missing half volt or so at one of these connectors. If so, remove the cable, clean mating surfaces, and reconnect tightly. Good luck.

[Sinister]

When you took it to the alternator place, did they 'load' it up?? That is try and draw current from it?? Its no good just checking the voltage output when there's no load, those alternators should supply over a hundred amps at the correct voltage. Suppose they know what they are doing? You will get a Vdrop between alt and battery as you have resistance in the copper which the more current you draw, the more Vdrop you have.

However, something may not be right. Think i've heard on here you can have a bad headlight switch or ignition switch that drains you battery?? Check if you have too high parasitic drain. Take of positive lead and check 'standby' current with a DMM. Connect it right (in series) else you'll get a nice spark! This should tell you if you have a short/drain somewhere. It maybe that you arent doing enough long trips and it has been discussed on here alot. I plug mine into a battery maintainer every weekend if not using it to keep it fully toppped up.

Hi! They loaded it up on a machine.

As for parasitic drain, even if the car is not driven for a week or sometimes more, it will still start just fine. Not sure of if that means anything. How do you connect it in series in this case, to check?

[Sinister]

The battery won't charge when its terminal voltage drops much below 13 volts. The voltage at the alternator seems about right. When you see a drop in voltage between the alternator and the battery like you are observing there is an excessive drop in voltage in the wiring (unlikely) or a connection (much more likely). The drop in voltage at each connection depends on the amount of current flowing; thus it is much smaller when there are no loads (like lights, air con, etc.). I'd check for voltage drops at each connection point (like at the alternator itself, or at the battery. My guess is you will find the missing half volt or so at one of these connectors. If so, remove the cable, clean mating surfaces, and reconnect tightly. Good luck.

Should the voltage at the alternator still not be 14.1 or so?

I did remove and sand the negative terminal at the battery and to the chassis at the battery end, as well as the positive cable at the battery (but not to the firewall, as it seems to have a security nut holding it.

I did remove and visually check all of the other grounds, but I guess I'll be jacking the car up and sanding the groud straps that I find.

Any connections you suggest for me to check specifically?

Since the voltage at the wire that comes out of the alternator (the red one) I am assuing the loss must be somewhere between the back of the car and the trunk in front, where the battery is?

Also, what is the best way to check for voltage drops?

Edited April 22, 2008 by Sinister

[Sinister]

Updates:

Voltage-drop tested between alternator houring and chassis. 0.12 loss. Between battery post and wire: 0 loss. Bewteen battery ground wire and chassis: 0 loss. I htink this menas that a ground is not my issue?

This rules out having to test the large ground from the engine to the chassis in the rear.

Drop tested between positive post and cable, at battery: 0 loss. Between cable and firewall: 0 loss.

Another thing that puzzles me is that the warmer the car, the more I lose. In the morning it is about 13.5 in the rear and 13.2 up front. By the end of the day, it was the same in the rear, but 12.60 up front (basically battery voltage). If I turen off the AC, it goes up to 13.2 up front. This happens everyday and has happened since the car was purchased originally.

What would cause tmperature to matter?

Why do I never (except last Sunday, when I had just charged the battery, with a a battery charger) do I get 14.1 volts (in the rear, but 13.8 up front). As the car was started to days later, I no longer saw those numbers. That is the only time I have seen anything close to 14 on this car.

If it is relevant, ai 3k rpms I have what sounds like a bad ground, but it does not come from the stereo. But it could be the turbos or something...

[fastboydave]

When you took it to the alternator place, did they 'load' it up?? That is try and draw current from it?? Its no good just checking the voltage output when there's no load, those alternators should supply over a hundred amps at the correct voltage. Suppose they know what they are doing? You will get a Vdrop between alt and battery as you have resistance in the copper which the more current you draw, the more Vdrop you have.

However, something may not be right. Think i've heard on here you can have a bad headlight switch or ignition switch that drains you battery?? Check if you have too high parasitic drain. Take of positive lead and check 'standby' current with a DMM. Connect it right (in series) else you'll get a nice spark! This should tell you if you have a short/drain somewhere. It maybe that you arent doing enough long trips and it has been discussed on here alot. I plug mine into a battery maintainer every weekend if not using it to keep it fully toppped up.

Hi! They loaded it up on a machine.

As for parasitic drain, even if the car is not driven for a week or sometimes more, it will still start just fine. Not sure of if that means anything. How do you connect it in series in this case, to check?

You first need a current meter or DMM (digital multimeter). Plug the positive lead into the "current" input (this is importnat!). Select the current (DC) setting. You then need to remove the +ve lead to you battery. Connect the DMM +ve input to the +ve battery terminal and the negative DMM lead to the cable you have just removed from the battery. This then puts the DMM "in series" with your current path.

YOU MUST insulate exposed terminals else you will have great fun and sparks. Please take care. You ofcourse need to run the DMM leads outside the vehilce, shut all doors and windoes to get the indication of 'standby' drain. The car should shut down i believe to about 40mA according to Loren (i believe it was) when it goes to standby. Not sure if this was with alarm on or off though. It takes a few hours to achieve this, so be patient.

Going back to the 'charging issue'. If your alternator is fine and been tested, your battery is fine and been tested, it is clear that the battery is not being charged sufficiently. Now, this could do with poor connections causing you a Vdrop hence not charging, or you arent running you car long enough to charge it up. It is recommended you charge your battery a few times a year to keep it in top form anyway, especially if the car is not being used.

What is the voltage on you instrument cluster reading? Mine starts off at 14V (ish) then drops to 13.2-13.5V after a while. This is normal.

[Sinister]

Mine starts out at 13.5 and eventually goes lower. It has nothing to do with the car not running long enough in this case, as the voltage with ac on eventually drops to 12.8 at the battery and 13.5 at the back. That menas the battery os not charging at all.

Not sure why I never see the 14.

Where am I loosing current? I've run out of ideas.

[fastboydave]

Mine starts out at 13.5 and eventually goes lower. It has nothing to do with the car not running long enough in this case, as the voltage with ac on eventually drops to 12.8 at the battery and 13.5 at the back. That menas the battery os not charging at all.

Not sure why I never see the 14.

Where am I loosing current? I've run out of ideas.

Hmmmm, yes. Lets go through it: Battery tested ok (check) and alternator tested ok (check). So, the main componets here are fine. Therefore you are loosing it through either volt drops or a short of somekind, but you say the car sits there OK for weeks so possibly a 'resistive short' is out unless its only when the alternator is running?? Therefore, as someone said it is most likely to be 'connections' rather than wiring (although it can be wiring, especially if the wire has been worn down (cross sectional area) due to rubbing etc). If you know the routing of the main wire from the alternator to battery, then you take each 'section' in turn (by that i mean from alternator to first connector, from first connector to next and so on) and check the voltage at each point. Use a common ground and probe the positive to see how the voltage differs from the last. If you find a large difference compared to the last 'section' it is that section that is the issue. It is then time to replace/repair the issue. That is what i have done with electrical/electronic test equipment that use high currents when they arent achieving the spec due to lareg Vdrops.

Please be CAREFUL though when the engine is running, we dont want any 'issues'. Failing all that, its time for a Porsche electrical guy........... Good luck my friend.

[Sinister]

I've tested the main ground at the alternator, the alternator to chassis and the battery. I have tested the alternator positive cable and the battery positive and the positive form the firewall. Would you then recommend taking apart the dash to thest the main power fuses and the connection to the firewall from within? Where else should i check?

How come the current drop appears to happen when the car is warm, and especialy when the 2nd fan speed for the front fans kick in? Could it simply be that the fans are draining too much current?

What does everyone elses voltage drop to with the high speed fans on? What is the normal current drop between alternator and the battery? There is no drop when measuing with the car off and measuing batetry voltage at the battery and at the alternator.

[fastboydave]

I would NOT recommend taking the dash apart no, it will never go back right and will probably rattle there after! Plus you may break something?

You will get a larger Vdrop when connections/wires get warm, the resistance of them go up with temperature usually, unless it has a negative temperature coefficient, which copper etc doesnt. You will also see more Vdrop when the fans run as you are then pulling more current and as its directly proportional, thats what happens.

I believe you DO have a bad wire or connection between alternator and battery. When the engine isnt running, you have hardly any current transfer (consumption) and therfore minimal Vdrop, so you will see same voltage at both ends when engine off. Is there any way you can find out what and where the connectors are for the main power lead from alternator to battery??

My volt meter in car will read 14V at startup and drop to 13-13.5V when warm or ran for a long time. It will still maintain this with alternator fully loaded. Think this is the alternator output NOT the voltage at the battery, so dont actually know what the voltage at the terms would be. It may be the same as yours? If i get chance i will check the weekend.

[fastboydave]

Have a look at these:

http://www.renntech.org/forums/index.php?s...p;hl=alternator

http://www.renntech.org/forums/index.php?s...p;hl=alternator

Failing that, go on the Carrera forum and do a search using "alternator" as the topic. Have a good look through, you may see somthing that gives you a clue...........

[Sinister]

How does one test for example at the starter, with the engine on? It is too hot and hard to get in there with the car on.

As per the diagrams, to check the distribution box and main fuse, one would have to pull the dash...there would be no choice.

[fastboydave]

How does one test for example at the starter, with the engine on? It is too hot and hard to get in there with the car on.

As per the diagrams, to check the distribution box and main fuse, one would have to pull the dash...there would be no choice.

I totally appreciate that and this is your problem. Maybe you can find a very accurate with high resolution resistance meter. You can then have your vehicles engine off (DISCONNECT the BATTERY though!!!) and check the resistance from the alternator to the next connection point etc etc. If you see a high(er) resistance at that 'section' there lies the problem. This is a very dificult problem to find.

Have you double checked all GROUND connections (including alternator to chassis) and main positive connections that you can get too??? If you can follow the main wire back from the alterntor as much as poss and 'wiggle' it and unplug/plug any connections you come across, you may find it??

Incidentally, my contact at Porsche said the voltage from the alternator (meter in vehicle) when up to temp/voltage should be around 13-13.5V. At the battery very similar apparently, depending on state of charge.

[Sinister]

There is no loss with the car off, so I don't think measuring resistance will not work to detect the issue, which is why I am doing the voltage drop tests.

There are only two main grounds, the one at the batter and the one from the engine. Since the voltage drop from the alternator chassis to the vehicle chassis is only 0.1, I believe that is not the problem.

The main issue is that when the car is cold, it is fine. As the car gets warmer, the voltage starts dropping.

My alternator never gets below 13.4-13.5, but at that time the battery voltage is 12.8 or so, so that is the issue.

It can't be normal to have the voltage at the battery drop that low to the point it is not charging...

Edited April 28, 2008 by Sinister

[fastboydave]

"It can't be normal to have the voltage at the battery drop that low to the point it is not charging... "

Ageed.

"There is no loss with the car off, so I don't think measuring resistance will not work to detect the issue, which is why I am doing the voltage drop tests."

No, because if there is no current flow (minimal anyway), you have NO voltage drop. If you put a resistor in line with your positive on your battery and measure the voltage after it, it will measure the battery voltage. If you then try and draw current through it, there will be a drop. Ohms law, it is all propertional.

As i said previously, the voltage at the gauge does come dowm after a while after startup, but should not fall below 13V. If you are seeing considerably less at the battery and it is falling alot more agressively, this is because potentially a bad 'connection' is heating up and its resistance is increasing.

There is another thought that you may have a bad 'device' that is only 'running' when the engine/alternator is. This too could cause you issue??? Check one of those link out i posted before. Basically, check you fuses out, and pull each one in turn to isolate potential issue. If voltage then OK, then it is to do with that.

[fastboydave]

Question?? When you check the voltage at battery, i know it is without engine running, but is it with ignition on??? If not try it then, it will drop as things 'power up' but not too much. As i said before also measure current at "standby" (car sat and instruments off etc etc), then with car 'alive' (instruments lit etc) then with ignition on. There must be spec's (limits) for these stages of 'power up' If you have a large current draw with ignition ON, then it is a 'device' or switch that has a 'short' and not wiring from alternator to battery. Think there has even been problems with ignition key/switch in regard to this before???

Keep an open mind buddy, at the moment you dont have an answer and if you convince yourself it is in one direction, you may miss or never find the problem. Entertain all posibilities, tackle them each in turn to eliminate and i'm sure you'll get there. Good look.

[Sinister]

When I key on voltage does not drop much.

What you say about the connection heating up makes sense, but the only place I have yet to look would be the main fuse and junction box, behind the dash. AM I looking necessarily for a LOOSE connection or could there be some other sort of issue?

As for fuses, what am I looking for? Regardless, seeing as how the voltage drops as the car heats up, must be the fisrt option above.

I did not understand part of your first post above, which said to dry to draw current thoguh it. I thought that is what a voltage drop test is?

What does you voltage read at the battery when the car gets warm, measured with a DMM, and what does it read at the alternator?

[fastboydave]

When I key on voltage does not drop much.

What you say about the connection heating up makes sense, but the only place I have yet to look would be the main fuse and junction box, behind the dash. AM I looking necessarily for a LOOSE connection or could there be some other sort of issue?

As for fuses, what am I looking for? Regardless, seeing as how the voltage drops as the car heats up, must be the fisrt option above.

I did not understand part of your first post above, which said to dry to draw current thoguh it. I thought that is what a voltage drop test is?

What does you voltage read at the battery when the car gets warm, measured with a DMM, and what does it read at the alternator?

Before you remove the dash, try everything else first! Have you, or can you, inspect the total routing of the 'charging' wire from alternator to battery?? You are looking for, loose, dirty, corroded, broken, worn connections/wires. Also, remove, clean and refit grounds too.

When i referred to the fuse box, it was to pull each fuse in turn for 'devices' that 'run' when the engine is running. The theory is that if you have an abnormally high loading 'device' (drawing too much current) then you will see the voltage drop (at battery) due to the fact the alternator cannot keep up with the demand at tick over. If you locate the 'device' that is causing the issue, the voltage should return to normal. What happens when you rev the engine?? Does the voltage recover?? This could be the key to establish if it is a device or not?

When i was on about the resistor and drawing current through it, it was only to explain to you that if you had resistance in the wires, you will see the same voltage at both ends until the engine is running an the alternator is trying to charge. You have to pass current to obtain a Vdrop. Hence, my suggestion of checking wires with a resistance meter.

According to my Porsche contact, he says if all is well, you should see similar voltage at battery to what is at alternator.

[Sinister]

I have found no loose, dirty, corroded, broken, worn connections/wires. Not sure where esle to look.

Which fuses for what devices should I look for?

When I rev, nothing changes. What does that point to?

[fastboydave]

Get you manual out, and look in the fuse description section. Just use your common sense from what you know comes 'alive' when the engine is running. Pull each fuse in turn (TURN IGNITION OFF AND KEY OUT EACH TIME YOU PULL A FUSE, AND RESTART ENGINE). You need to run your engine a little while after each one to see if the voltage starts to recover.

Follow me here: If you had a device that was pulling too much current when the engine was on, the battery voltage would gradually fall as the alternator would not be putting out enough current (at tick over) to 'replace' the energy being used. If you REV the engine, the potential output (current) of the alternator goes up, so should then combat the extra current being drawn from the battery by a faulty 'device'.

It needs to be held there though. Is it? Can you get someone to hold it at say 3000rpm for 5-10mins and see what happens???

When you had your alternator tested, was it just put on the tester, thumbs up then off?? Or was it on for a while (soak test) the same kind of time it takes for you to see an issue?? If not it could be that the alternator is 'breaking down' when it starts to get warm etc?? I just hope for you in the end its not a bad engine harness?

[Sinister]

Well, when the voltage drops to 12.8 at the battery, the alternator STILL sees 13.5, so that rules out the alternator breaking down when it gets warm. What happens is that the difference between the alternator and battery becomes larger as the car gets warmer, eventually stopping at 12.6-12.8.

If I find a device that is drawing to much power, what next? Have you tested your voltage? To see how different mine is from yours?

I dount it would be a device, seeing as how increase in voltage drop is directly proportional to warming up...

When I rev it, I do hold it there for a bit, but not 10 minutes, maybe 15-30 seconds.

Let me know.

[klueless7571]

Are you able to run a different cable from the alternator to the battery? There should be no way to drop so much voltage. As a matter of fact, you should not be able to detect any voltage drop in this section of the circuit. If you say your connections are all clean and good, then it has to be a bad cable. I doubt there are any other problems with the rest of your devices. From the sounds of it, you've had a defective cable from day one.

Derek

[Sinister]

The problem is determining what section of the cable. Rewiring would also be a PITA.

Would a bad cable do that when it warms then?

I will try to run a direct cable, and maybe attach it to the VMM to see if there is any Vrop. Likely thre would be, but back to what section of the cable is the culprit.

[fastboydave]

Well, when the voltage drops to 12.8 at the battery, the alternator STILL sees 13.5, so that rules out the alternator breaking down when it gets warm. What happens is that the difference between the alternator and battery becomes larger as the car gets warmer, eventually stopping at 12.6-12.8.

If I find a device that is drawing to much power, what next? Have you tested your voltage? To see how different mine is from yours?

I dount it would be a device, seeing as how increase in voltage drop is directly proportional to warming up...

When I rev it, I do hold it there for a bit, but not 10 minutes, maybe 15-30 seconds.

Let me know.

Good point with the alternator, forgot about that!

I will try and check my voltage over the long weekend we have here this week (bank holiday monday) and feed back into you next week.

It could be a device, the car 'warms up' yes, but the longer you run a high load, the more the voltage in the battery will fall. So it will have the apprearance of the 'warming up' causing the problem. But however, i do suspect it is mor a bad 'connection'

To answer your other question above, YES, poor/broken cable, will cause this issue, i mentioned this before. Good suggestion there above, run a new cable from alternator output to battery. No need to be flash, outside of car is fine, just make sure the cable is lareg enough (cross sectional area) and you arent shorting anything. That should prove your +ve connection or not??