Have you seen my 4 volts?

[FL911SC]

Yesterday I finished up replacing the alternator on my 1979 911SC Targa, after the mine suffered catastrophic mechanical failure while driving home from an auto-cross. The voltage regulator is about 18 months old, so only the alternator was replaced.
After everything was buttoned up, I fired up the car and started checking voltages. I had my grandson hold the throttle open between 2,500 and 3,000 RPM. I have a digital voltmeter plugged into the cigarette lighter location, and it showed 13.8 volts, which is an improvement over the old one. However, when I put a multi-meter at the battery, I only showed about 10 volts. I checked this with the engine at various speeds, and it always showed around 10 volts.

If it helps to know, I have a “red top” Optima battery. The battery was 100% charged before starting the car after the replacement of the alternator.

I have checked grounds and I am 100% on the ground strap on the transaxle.

While looking at the wiring diagrams in the Bentley Manual, I did notice I have an extra ground wire (See black wire in below picture). When I pulled the engine a couple of years ago, I noticed this wire was looking rough, so I replaced it with the assumption that it was supposed to be there. After all, the condition of it seemed appropriate to the age of the car.

Lastly, I am assuming that since the volt meter in the car is showing just under 14 v, that the alternator is outputting correctly and the voltage regulator is good.

The three questions are:
1) Does anyone have any thoughts on why the voltage at the battery is so low? Would the starter have anything to do with it? (it is the only thing between the alternator and the battery)
2) Do I need the extra ground wire which goes from the alternator to the crankcase?
3) If “no” to #2, then will it hurt to leave it until the next time I pull the alternator?

[timmy2]

The ground to the case is a good thing and should be there.
Check for a bad through body starter cable from the battery by disconnecting both ends and checking the ohms reading. Should be very low around an ohm or maybe 2.
You may have corrosion in the cable that is causing a voltage drop.
Likely at the battery terminal end.

[bpu699]

Your battery is bad. That's the only way to get 13.8 off the alternator and less at the battery...

Under load, your battery isn't putting out the voltage it should...

Get the battery replaced...

A battery can read "fully charged" at >12v off a charger, and be dead. the moment it sees a load.. A new battery, with NO FUNCTIONAL ALTERNATOR would still read >12v while the car is running...

[turbo owner]

Optima battery's suck, I had 2 fail in18 months just sitting on a shelf.

[Joe Bob]

Try to shock the battery back to life with hi and low alternating voltage. The best to use is an old school analog charger. Do it with a battery OUTSIDE of the car.

If you can't get it back to 13v consider replacement. The procedure may take a day or more.

If in a hurry you can take it to the retailer but their methods may be to just make you buy another....depends on how patient you are.

[FL911SC]

As always, thank you for the replies.
I am picking up on a theme with regards to the battery. Because it is fairly new, (< 36 months) I would like to be sure before replacing it. So here are follow up questions to help all of you, help me.

I do know about a battery showing 12 volts and still being bad, i.e. unable to start a car. However, with the alternator sending 13+ volts to the battery, wouldn't I see that on my meter when I measured the voltage at the battery terminals? Why would there be a voltage drop at the battery?

I currently use a "Battery Tender Jr." to keep the battery charged while the car is parked. It has always been my understanding that keeping the battery charged prolongs the its life.
Have any of you had a poor experience with one of these chargers?

Joe Bob - you mentioned an analog type charge which might 'refresh' the battery. Would you provide an example?

As for corrosion, that is certainly a possibility. I have checked and don't see any, but admittedly, only looked in the easy to see connections. I will say that the battery terminals are clean.

As always, thank you for your help.

[T77911S]

optima is now made in mexico, not as good as they use to be.

I don't know what kind of MM you have plugged into the cig lighter, but it could be wrong.
use ONE MM for checking voltages.

resistance would cause a volt drop to the battery IF the cig ltr MM is correct.

you have to CLEAN the connections. check the ground strap to the body at the battery.
the tranny ground needs to be clean. do just look at them or measure the resistance.
measure the volts at the starter.
what is the batt volt with engine off.
measure volts at batt from the + to - then from + to chassis with engine on.
does the ALT light come on with engine off and key on.

[Joe Bob]

I have an old Sears charger and a digital one. The analog has a meter with three settings. 2, 6 and 10 amp. The digital would flat refuse to charge on my yellow top when the battery was discharged while the old Sears would jolt it back to life by days end by switching between settings.

It was killed at least 20 times over its 12 year use in my 911.

[bpu699]

Originally Posted by FL911SC

As always, thank you for the replies.
I am picking up on a theme with regards to the battery. Because it is fairly new, (< 36 months) I would like to be sure before replacing it. So here are follow up questions to help all of you, help me.

I do know about a battery showing 12 volts and still being bad, i.e. unable to start a car. However, with the alternator sending 13+ volts to the battery, wouldn't I see that on my meter when I measured the voltage at the battery terminals? Why would there be a voltage drop at the battery?

I currently use a "Battery Tender Jr." to keep the battery charged while the car is parked. It has always been my understanding that keeping the battery charged prolongs the its life.
Have any of you had a poor experience with one of these chargers?

Joe Bob - you mentioned an analog type charge which might 'refresh' the battery. Would you provide an example?

As for corrosion, that is certainly a possibility. I have checked and don't see any, but admittedly, only looked in the easy to see connections. I will say that the battery terminals are clean.

As always, thank you for your help.

If you hook up a 15V battery charger to a dead battery reading 1 volt, what do you think the voltmeter would measure at the battery?

Hint: It would be just a tad above 1 volt. Then, assuming the battery can take a charge, it rises slowly to 13.4V or so.

Now, just because the battery shows 13.4 V, it doesn't mean that it can maintain that (ie. amperage is shot)... The moment it gets a load, it falls back to <10v...

A new battery, will put out >12V for over 300 amp/hours. Meaning, you could drive your 911 with no alternator for a couple hours before it went dead... The battery in a 911 is HUMONGOUS.

[Trackrash]

If the battery starts the car, is it dead?

Use your volt meter to check every connection in the charging circuit.

Put one lead on one side of the connection and the other lead on the other side of the connection with the car running. If you see a voltage reading, there is resistance in that connection. A digital volt meter is best.

Don't try to just measure resistance, it will not be the same as when there is current in the circuit.

Probably in the battery cable connections.

[bpu699]

Originally Posted by Trackrash

If the battery starts the car, is it dead?

Use your volt meter to check every connection in the charging circuit.

Put one lead on one side of the connection and the other lead on the other side of the connection with the car running. If you see a voltage reading, there is resistance in that connection. A digital volt meter is best.

Don't try to just measure resistance, it will not be the same as when there is current in the circuit.

Probably in the battery cable connections.

If the original poster is accurate, and he put the voltmeter on the battery posts and it read 10v, then the battery is bad. There is nothing else that can cause that, assuming you have the voltmeter on the + and - posts of the battery...

Disconnect the battery from all its connections. Measure voltage. Should be >12.8 at rest...

Or, just take it to Sears or Autozone and get it checked under load...

At 10v, it will flip the car over, and start it. Barely.

A short can cause low voltage. But if you have a healthy battery, pushing 850 CCA through a short with enough fury to drop it to 10volts... you probably have sparks and smoke flying .

Now, if the PO is testing the battery voltage differently, that's different...

[gomezoneill]

Battery is "only 36 months old". That's three years, they don't last much longer than that. Especially in a car that's not started every day.

[Trackrash]

Originally Posted by bpu699

If the original poster is accurate, and he put the voltmeter on the battery posts and it read 10v, then the battery is bad. There is nothing else that can cause that, assuming you have the voltmeter on the + and - posts of the battery...

Disconnect the battery from all its connections. Measure voltage. Should be >12.8 at rest...

Or, just take it to Sears or Autozone and get it checked under load...

At 10v, it will flip the car over, and start it. Barely.

A short can cause low voltage. But if you have a healthy battery, pushing 850 CCA through a short with enough fury to drop it to 10volts... you probably have sparks and smoke flying .

Now, if the PO is testing the battery voltage differently, that's different...

You are probably right. However I had a situation once where there was a poor connection in the battery cable (a kill switch to be exact). It ohmed out ok. But when you put a load on it there was a problem.

I agree the battery should be checked first. But if there is almost 14v at the lighter and 10v at the battery, it makes me think bad connection.

[Quicksilver]

If you are seeing 10V at the battery with it running then you have other issues.
I would add some other tests to the list.
Hook the positive meter lead to the alternator and check the voltage at various grounds.
- I would want to see what voltage it was showing at the body side of the transmission grounding strap.
- I would want to see the voltage at the ground stud where the negative battery cable mounts plus at the actual negative post on the battery.

If you have a huge voltage loss somewhere down that chain you have an issue to work on.

Same basic test on the positive side. Hook the meter to the ground right at the alternator.
- Check the positive voltage at the starter.
- Then check the positive voltage at the positive cable at the battery and then at the actual post.

If you see a huge drop along a cable the size of a starter cable then the cable path has issues.

In all my years working on cars I have seen 10 volts on a running car at the battery but never with it showing 13.8 at the same time at the alternator. There is WAY too much conductor between the two to have that much drop without a lot of resistance in the path.
Someone with an electrical engineering degree could probably calculate how much load would be necessary to get a 4V drop at the end of a 10' battery cable but it would be more like an arc welder then a battery.
I found a voltage drop calculator and put a 10 amp load through an 8 gauge wire 10 feet long and the drop was 0.13V. Now I'm assuming the positive lead is about 8 gauge but it is nowhere near 10' long.

Something else is wrong...
Or the 10V is when the car isn't running.

[Porboynz]

+1 Quicksilver, good advice there. You can also pull the battery and charge it on the bench, leave its a few hours and see what it settles back to. Using a load tester is good advice as well. Once you have a known good battery check voltages again, but with a known good meter. Good luck fella.

[Joe Bob]

15 responses,14 hours and you STILL haven't pulled the battery and put it on a charger?

[DanielDudley]

Just a little corrosion on the battery terminals can play havoc with conduction. Crusty deposits inside the connection on either the battery or the connector can play havoc with the circuit, and still pass electricity.

But I don't think that is your problem. Check anyway.

https://www.optimabatteries.com/en-us/battery-charger

[FL911SC]

Once again, a very sincere thank you for all the replies.
Joe Bob, this one is for you : o).
Last night I pulled the battery out of the car and checked it again. The meter showed just under 10 volts. I then put it on a 'better' charger than the Battery Tender Jr, set the charger to charge at 10 amps and let it go. At this point the charger showed it was outputting about 14-15 volts, so I let it run.
I did, however, check the voltage with the charger connected and it still showed 10 volts. This validates what bpu699 already knew and had stated above.
After about 15 minutes the charger shut off saying the battery was charged. I checked with the meter and got 9.6 volts. I set the charger to 2 amps to put it on a trickle charge and let it run over night. This morning, 9.6 volts.
At this point, I am going to agree that the battery is bad. If it will only charge to ~10 volts then there is a bad cell and it is done.

On a separate note, I have to bring up the most common alternator test, which is to start the car and check the voltage at the battery. Based on what I have learned from all of you and from this experience, there is a BIG prerequisite which is often left out of this test. It appears that you need to know the voltage of the battery before you start the car. If the voltage at the battery is 11 volts, then with the car running, you will only get ~11 volts even if the alternator is putting out 13- 14.

Quicksilver, thank you for the recommendations. If there is still an issue after I remove the battery as a possible problem, I will update the post with the results and any fixes.

Thanks again to all.

[Joe Bob]

"I" would swap in another battery or buy another.

[911pcars]

Charge voltage is different from battery voltage.

If the charging system is operating correctly, the charge voltage will vary depending on the electrical load and the battery state of charge, 12V up to about 14.5 V.

If a charger is connected to a battery for a few days and the battery voltage remains below 12V or so, the battery is borderline. Take it to a parts store for a load test. There are modern bat. testers that can even test a discharged battery for load capacity.

Best wishes.
Sherwood