TheSamba.com :: View topic - The alternator charges 13.8 volts. Is that OK?

Fri Apr 23, 2010 3:33 pm

I measure 13.8 - 13.9 volts output from the alternator. It's a 1976 model with the latest type alternator with built-in charging relay.

Is 13.8 Volts OK, or should it be more?

Fri Apr 23, 2010 3:35 pm

13.8 volts sounds good to me try with the headlights on at idle.....

Fri Apr 23, 2010 3:55 pm

mpl2438 wrote: 13.8 volts sounds good to me try with the headlights on at idle.....

OK, good advice.

Reason I ask is that I'm in the process of identifying the reasons why my battery is dead after the car sits for a couple of days. I have measured that the stereo takes 0,5 amps, so I will fix that.

I believe 13.8 proves that it is actually charging the battery, but I have read somewhere about 14.5 Volts output from the alternator?

Just wondering what you guys read on yours.

Fri Apr 23, 2010 8:05 pm

13.8 volts while running means your charging system is working. So either bad battery or parasitic drain.

When battery is fully charged, disconnect the battery negative terminal while it sits those few days. Then install that back onto the battery, see if it starts. If not, battery issue. If it does, then parasitic drain.

If you mean 0.5 amp for the stereo "memory", that is a lot.

Sat Apr 24, 2010 5:23 am

Cusser wrote: 13.8 volts while running means your charging system is working. So either bad battery or parasitic drain.

When battery is fully charged, disconnect the battery negative terminal while it sits those few days. Then install that back onto the battery, see if it starts. If not, battery issue. If it does, then parasitic drain.

If you mean 0.5 amp for the stereo "memory", that is a lot.

Yes, I believe it is a lot. I disconnected the stereo and the DAB radio antenna amplifier, and it dropped to 0.15. I guess that is OK, or should I consider 0.15 to be a drain too?

Also, I really want to be sure that the alternator is charging allright, so therefore i asked about the 13.8 volts.

[Feel free to think what you want about my basic level questions ;-). I'm really better with metal than with electricity.]

Tue Apr 27, 2010 5:18 am

In the dealer, our generally accepted limit is .040 amps, in a Cadillac, .15 amps is alot. If it doesn't kill the battery at .15, I guess I could live with it. As long as the alternator is putting out 13 volts or better, it is ok. A better way to check the alternator is with an inductive ammeter. Hook it around the large positive lead coming from the alternator, start the engine, turn on all the electrical functions in the car, (lights, stereo, etc.) and rev the engine to about 1500-2000 rpm. At this point the meter should indicate within about 10 percent of the amperage of the alternator; E.G. if it is a 50 amp alternator, it should put out 45 amps with a full electrical load. I have seen alternators put out 13-14 volts WITHOUT the proper amperage, and amperage is what actually charges the battery.

Tue Apr 27, 2010 8:04 am

I would find the source of the drain if it is still .15 amp, that's many times what it should be. Paul is correct with his charging voltage and parasitic draw info, the industry standard is .040a (40MA) parasitic but that is on a computerized car. VW's should have very little to zero parasitic, depending on the type of stereo and clock they have. I would consider .040a high (but acceptable) for a carburated VW. If the stereo (may have 2 power inputs) and clock are disconnected, VW's should be zero parasitic. If you still have .15A parasitic with your clock and stereo disconnected, try disconnecting your alternator and regulator, they are on the top of the hit list for parasitic drains.

Charging voltage is usually 13.5-14.5v at speed, perhaps lower at idle. Paul's amperage output test is mostly correct, but he forgot to mention applying a carbon pile load to the battery to create a demand for amperage. If amperage output is near capacity at 1500-200RPM without a load, either the battery is at a very low state of charge or bad. Sears sells a simple clamp-on DC ammeter for about $60 retail, perhaps in Norway someone sells something similar. Make sure it handles DCA as most are ACA only.

http://www.sears.com/shc/s/p_10153_12605_03482369000P?keyword=ammeter

Good Luck!

Tue Apr 27, 2010 11:27 am

Most modern cars charge @ 14.4 volts, but with very little amperage.

Your 13.8 is fine.

Pull all fuse's and put them back in 1 by 1 to find the drain.

Wed Apr 28, 2010 1:00 am

Like said earlier, a VW should have no drain when the ignition is off, with a good battery it should be able to stay for 6 months and still be able to pull the starter. The VW battery is normally 45 amp hours, that means that if you multiply the drain current by the hours standing idle, you will see if there is any juice left. For instance, it should be able to deliver 1 amp for 45 hours, or 0.1 amp for 450 hours. However, as the battery ages, the amp-hours decline, typically the car cannot stand with the parking lights on for more than 8 hours, and that is maybe 3 amps times 8 hours = 24 ah. If the battery is a few years old it may be only half the amp-hours it used to, and the capacity drops faster if the charging voltage is less.

Wed Apr 28, 2010 2:40 pm

Gentlemen, thanks a million for your comprehensive and instructive answers!

Bruce Amacker wrote: Sears sells a simple clamp-on DC ammeter for about $60 retail, perhaps in Norway someone sells something similar. Make sure it handles DCA as most are ACA only.
http://www.sears.com/shc/s/p_10153_12605_03482369000P?keyword=ammeter
Good Luck!

Anyone tried one of these? From what I hear, they have som difficulties reading values below 1 Ampere.

Thu Apr 29, 2010 4:55 am

They aren't accurate under about 5-10 amps. Under that value, you use a digital multimeter in line. But don't use a multimeter in line with the alternator, you will damage the meter.