Is a 13.07V alternator voltage too low?

Question

I'm having some problems with a 2006 Mini One D (1.4 l diesel engine). About a month ago the alternator warning light turned on (it only turned off over about 4000 rpm). The belt looked good, so I brought the car to a mechanic and they said the alternator wasn't working and replaced it. It all seemed to work fine, but about 3 weeks later, after 4 days from the last use, the battery was completely dead, not even enough to open the car or to turn the dashboard on (the voltage was close to 0 V). I started the car with another battery and took it back to the mechanic that replaced the alternator. There they told me that the alternator looked fine and it was probably the battery that was shorted. As I had replaced the battery (a Bosch S4 008: 680A, 74Ah) a year before, I went to the shop where I had bought it to see if it still was on warranty. There they charged the battery and told me it was ok, so I kept it.

Now when the engine is running, I get about 13 V of voltage: last time I checked, the tension was constant at 13,07 V (with no electric loads on), both at idle and at higher rpms. I have no mechanical experience, but looking up on the internet I found that a voltage around 14 V should be expected. Is it possible that the alternator they replaced is faulty? Or is this voltage normal for this car?

Doing some tests, I found that the problem is a parasitic draw: at rest and locked the car draws around 620 mA, while I found that a normal draw should be below 50 mA. I tried to identify the circuit drawing this current testing for a voltage drop across fuses, as shown in this video, but I did not find any significant draw there: the only drop I measured is a 0.4 mV drop in the F4 (5A) fuse (it should correspond to about 34mA of draw and I think it's normal as that fuse seems to be connected to the body control unit).
I could try to remove all fuses and see if there's still a draw, but I didn't have time to do that yet.

Could a faulty alternator be responsible for the draw? Unfortunately I didn't find a way to easily disconnect the alternator as the engine bay is quite cramped and I didn't even find where the positive cable is connected. So I was unable to test this directly.

Should I take the car back to the mechanic and claim warranty on the alternator?

Answer 1

Trying to find the parasitic draw by looking for a voltage drop across the fuses is not likely to be the best method.

If the current is 620mA through a fuse - which is a really low resistance anyway then trying to measure a really really small voltage with a multimeter...

I always found parasitic draws by putting the ammeter in series ie between the batt positive and the pos terminal and then removing fuses until the ammeter dropped to zero - that way I found the circuit, which then left disconnecting components to find the culprit.

13V does seem low, I would expect 13.7 to 14.8V (and on my car it can go up to 15.3...). However, have the voltage checked with a quality or professional meter - I have seen people using those cheap "chinesium" 10 buck meters which can have an error of 1V or more...

Get an Autoelectrician to test the car charging system and battery - they will know how to check the voltage and current output...

Answer 2

Now when the engine is running, I get about 13 V of voltage: last time I checked, the tension was constant at 13,07 V (with no electric loads on), both at idle and at higher rpms. I have no mechanical experience, but looking up on the internet I found that a voltage around 14 V should be expected. Is it possible that the alternator they replaced is faulty? Or is this voltage normal for this car?

It could be that the battery has low state of charge. Batteries are charged in two or three stages:

• Constant current.
• High constant voltage, over 14 volts.
• Reduced constant voltage, around 13.5 - 13.8 volts in room temperature. (optional; sometimes high constant voltage might be enough, especially on cars that are not driven 24/7)

If you are in the constant current stage of the charging, it is very well possible that the voltage just happens to be 13,07 volts to supply this current.

Charge the battery so that it's full. You can do this by driving around in the car for few hours continuously, or by a dedicated battery charger. Then when the battery is full, let it rest for an hour and repeat the test of the alternator charging voltage. Is the voltage still 13 volts? Use a quality multimeter such as Fluke; avoid cheap Chinese brands.

The answer to your question is: no, this voltage is not normal, if it's the final voltage. But if it's just one voltage reading during the middle of a constant current charging regime, it could be normal.

It should also be noted that outside temperature affects the optimal charging voltage. So in a very hot climate, 13 volts could be normal, but then again in such climate, you can't expect the battery to have a long life.