mcapinha

mcapinha

Active Member
2.5 DSE here.

My battery has been getting weaker, so I decided to check my alternator output. Following a troubleshooting guide I found online, there are the readings I got:

- Voltage at battery after main beams on for 2 minutes: 12.37V (guide says that less than 12.40V the battery is shot);
- Voltage at alternator (measured to alternator body or to the engine block) at idle: 14.1V
- Voltage at battery, at idle, 14.0V;
- Voltage at cigarete lighter, at idle, 14.0V;
- Voltage at cigarete lighter, at 2000rpm, 14.0V (shouldn't this be higher ?);
- Voltage at cigarete lighter, at 2000rpm with headlights on: 13.9V;
- Voltage at cigarete lighter, at 2000rpm with main beams: 13.85V.

Also, Nanocom reads voltage as 13.60V on the BECM and 13.55V on the EAS module :confused:

My question is, since these seem to be borderline values, should I get my alternator checked ? I don't want to put on a new battery to find it dying on me just after 3 years, as it's happening to the one I got right now.

Also, what's happening with the Nanocom readings ? I get 14.0V inside the car, reading in the cigarete lighter, but the Nanocom reports a much lower (too low to me) voltage at the BECM and EAS ?

Cheers,
Manuel
 
Should be closer to 14.2 at 2K....

A duff cell in the battery can drag Alternator output down...

If you have access to a known good battery, sub that out and re-test...same readings, Alternator could be getting weak....if the results are more in line with expectations...Original Battery was duff
 
Saint.V8 said:
Should be closer to 14.2 at 2K....

A duff cell in the battery can drag Alternator output down...

If you have access to a known good battery, sub that out and re-test...same readings, Alternator could be getting weak....if the results are more in line with expectations...Original Battery was duff
Click to expand...
+1. Not sure why you included cigarette lighter readings, they are irrelevant. The Nanocom readings are lower due to the volt drop on the very thin wire used to feed the OBD port.
Sounds like you may also have a connection/corroded wire problem, the voltage at the battery should be within about 0.1 of a volt to the voltage across the alternator shell to the output.
 
Thanks for both of your replies.

I'm going to try changing the battery with one from another car, not sure if the terminal positions are compatible though.

I measure the voltage at the cig lighter because I was alone and couldn't check it at alternator or battery while revving the car to 2000rpm. My idea is that the voltage delta, at the cig lighter, between 750 and 2000rpm will be the same as at the battery or alternator. I do have a voltage drop of 0.1V between alt and battery but I'm not sure if that's significant ?
 
Voltage at battery on idle should be around 14.0 and 14.2 with loads on should be around 13.7 to 13.9 depends on loads etc at 2k they should be slightly higher. The reason why you feel your battery is weak due to the lower temps and all loads on ie lights heated seats heated windscreen etc etc and if you only do short journeys they alternator doesnt get chance to replace what has been use!!! Or it could be down to what the others have said also
 
matt1987 said:
Voltage at battery on idle should be around 14.0 and 14.2 with loads on should be around 13.7 to 13.9 depends on loads etc at 2k they should be slightly higher. The reason why you feel your battery is weak due to the lower temps and all loads on ie lights heated seats heated windscreen etc etc and if you only do short journeys they alternator doesnt get chance to replace what has been use!!! Or it could be down to what the others have said also
Click to expand...

You'll be lucky to see more than 13.9v at idle, should be 14.2v at 2K rpm. 0.1volt drop is normal.
 
So, I replaced the battery and all is well now but while I was playing with the Nanocom I noticed the expectable battery voltage drop with the main lights on. So I thought, let's see how low I can get it to.
With everything turned on (everything being dipped beans, main beans, heater running the demist program, front and rear fog lights and even forcing the compressor to run) I got 12.2V measured at the battery on idle. The voltage was dropping steadily so I'm sure the car was by now running of the battery charge. Now, this can't be normal for sure ? I'm sure the alternator must have been designed to provide more than 13V on full load ?
 
mcapinha said:
So, I replaced the battery and all is well now but while I was playing with the Nanocom I noticed the expectable battery voltage drop with the main lights on. So I thought, let's see how low I can get it to.
With everything turned on (everything being dipped beans, main beans, heater running the demist program, front and rear fog lights and even forcing the compressor to run) I got 12.2V measured at the battery on idle. The voltage was dropping steadily so I'm sure the car was by now running of the battery charge. Now, this can't be normal for sure ? I'm sure the alternator must have been designed to provide more than 13V on full load ?
Click to expand...

You missed one the abs pump , your far too paranoid, the only thing you needed to test was your battery , you have a alternator light to tell you if its duff .
Obviously if you think your battery is struggling then a quick check that the alternator is giving out put is enough to know your battery Is duff.
 
Well, you have to be a little paranoid when thinking about preventive maintenance. My previous battery lasted 3 years. On other cars I've owned from new, I've seen batteries last between 7 and 9 years. Doing the math, it seems more beneficial to replace the alternator now than to have to replace the alternator and a new battery in another 3 years. If replacing/repairing the alternator results in doubling the battery's life than that's beneficial from an economic standpoint.


But, the main point of my previous post was that I find it hard to believe that the alternator in not enough to power all these systems. So, either the alternator specification is flawed (by not being enough to handle the systems peak demand) or my alternator is at fault.
 
mcapinha said:
So, I replaced the battery and all is well now but while I was playing with the Nanocom I noticed the expectable battery voltage drop with the main lights on. So I thought, let's see how low I can get it to.
With everything turned on (everything being dipped beans, main beans, heater running the demist program, front and rear fog lights and even forcing the compressor to run) I got 12.2V measured at the battery on idle. The voltage was dropping steadily so I'm sure the car was by now running of the battery charge. Now, this can't be normal for sure ? I'm sure the alternator must have been designed to provide more than 13V on full load ?
Click to expand...
The alternator would only keep up with that load at 2K rpm or more, below that the battery will make up the shortfall of amps. That assumes your alternator is in tip top condition.
 
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