R56 Alternator not charging battery, but...
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Joined: Jul 2024
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Alternator not charging battery, but...
I am having charging issues on my 2011 r56. I have dug through the forum and am finding my experience to be a bit... unique.
I discovered my battery unable to start the car one morning, so jumped it, only for it to stall after a few minutes, indicating the alternator must not be charging the car, or the battery is so toasted with a dead cell / 100A parasitic drain.
Charged the battery a bit with an external charger into the low 12s, and fired it up. My ultraguage was showing 11.6V, decreasing with the car running, which leads me to believe the alternator is shot. Probing the alternator output shows the same voltage as right at the battery, low, and the exciter lead going into alternator shows between 9-10V. Disconnecting it does not start charging. Next test, following some recommendations on the forum, I pulled the negative lead off the battery while the car was running, expecting it to stall if the alternator was bad.... it keeps running. The ultraguage voltage jumps up to 14+ volts, alternator is alternating, wtf. So okay, alternator must be good.
I throw the battery on the charger for a couple more hours and come back to it showing a "BAD BATTERY" error. Fire the car up and once again see 11.6ish volts when running again. Welp, must be a bad battery. Buy and Install new battery, fire up the car. Ultraguage shows only 12 volts... decreasing... back down to 11.6. Go for a drive and see the voltage increasing slightly when I am revving the heck out of it, to a max of 11.9, but never up to the 14+ I would expect.
So is the alternator seems to output a little power... sometimes? Enough to run the car without a battery connected. But apparently not enough to bring the system + battery up to alternator voltage. I am hoping to do a little more diagnostics before spending $500 on a rebuilt alternator just to wind up in the same boat.
I also do not seem to have a battery management wart on the negative lead, so I did not attempt to register the new battery. I may try this tomorrow morning anyways, could the BMS be somewhere else if not right on the neg clamp on some cars?
I discovered my battery unable to start the car one morning, so jumped it, only for it to stall after a few minutes, indicating the alternator must not be charging the car, or the battery is so toasted with a dead cell / 100A parasitic drain.
Charged the battery a bit with an external charger into the low 12s, and fired it up. My ultraguage was showing 11.6V, decreasing with the car running, which leads me to believe the alternator is shot. Probing the alternator output shows the same voltage as right at the battery, low, and the exciter lead going into alternator shows between 9-10V. Disconnecting it does not start charging. Next test, following some recommendations on the forum, I pulled the negative lead off the battery while the car was running, expecting it to stall if the alternator was bad.... it keeps running. The ultraguage voltage jumps up to 14+ volts, alternator is alternating, wtf. So okay, alternator must be good.
I throw the battery on the charger for a couple more hours and come back to it showing a "BAD BATTERY" error. Fire the car up and once again see 11.6ish volts when running again. Welp, must be a bad battery. Buy and Install new battery, fire up the car. Ultraguage shows only 12 volts... decreasing... back down to 11.6. Go for a drive and see the voltage increasing slightly when I am revving the heck out of it, to a max of 11.9, but never up to the 14+ I would expect.
So is the alternator seems to output a little power... sometimes? Enough to run the car without a battery connected. But apparently not enough to bring the system + battery up to alternator voltage. I am hoping to do a little more diagnostics before spending $500 on a rebuilt alternator just to wind up in the same boat.
I also do not seem to have a battery management wart on the negative lead, so I did not attempt to register the new battery. I may try this tomorrow morning anyways, could the BMS be somewhere else if not right on the neg clamp on some cars?
3rd Gear
Joined: Mar 2014
Posts: 178
Likes: 2
From: Long Beach, CA
1st Gear
Joined: Apr 2024
Posts: 14
Likes: 2
R56 Alternator Issues
- if you're still having issues shoot me a text. Just spent two weeks with same issue and it about drove me crazy. 321 332 5893
I discovered my battery unable to start the car one morning, so jumped it, only for it to stall after a few minutes, indicating the alternator must not be charging the car, or the battery is so toasted with a dead cell / 100A parasitic drain.
Charged the battery a bit with an external charger into the low 12s, and fired it up. My ultraguage was showing 11.6V, decreasing with the car running, which leads me to believe the alternator is shot. Probing the alternator output shows the same voltage as right at the battery, low, and the exciter lead going into alternator shows between 9-10V. Disconnecting it does not start charging. Next test, following some recommendations on the forum, I pulled the negative lead off the battery while the car was running, expecting it to stall if the alternator was bad.... it keeps running. The ultraguage voltage jumps up to 14+ volts, alternator is alternating, wtf. So okay, alternator must be good.
I throw the battery on the charger for a couple more hours and come back to it showing a "BAD BATTERY" error. Fire the car up and once again see 11.6ish volts when running again. Welp, must be a bad battery. Buy and Install new battery, fire up the car. Ultraguage shows only 12 volts... decreasing... back down to 11.6. Go for a drive and see the voltage increasing slightly when I am revving the heck out of it, to a max of 11.9, but never up to the 14+ I would expect.
So is the alternator seems to output a little power... sometimes? Enough to run the car without a battery connected. But apparently not enough to bring the system + battery up to alternator voltage. I am hoping to do a little more diagnostics before spending $500 on a rebuilt alternator just to wind up in the same boat.
I also do not seem to have a battery management wart on the negative lead, so I did not attempt to register the new battery. I may try this tomorrow morning anyways, could the BMS be somewhere else if not right on the neg clamp on some cars?
Charged the battery a bit with an external charger into the low 12s, and fired it up. My ultraguage was showing 11.6V, decreasing with the car running, which leads me to believe the alternator is shot. Probing the alternator output shows the same voltage as right at the battery, low, and the exciter lead going into alternator shows between 9-10V. Disconnecting it does not start charging. Next test, following some recommendations on the forum, I pulled the negative lead off the battery while the car was running, expecting it to stall if the alternator was bad.... it keeps running. The ultraguage voltage jumps up to 14+ volts, alternator is alternating, wtf. So okay, alternator must be good.
I throw the battery on the charger for a couple more hours and come back to it showing a "BAD BATTERY" error. Fire the car up and once again see 11.6ish volts when running again. Welp, must be a bad battery. Buy and Install new battery, fire up the car. Ultraguage shows only 12 volts... decreasing... back down to 11.6. Go for a drive and see the voltage increasing slightly when I am revving the heck out of it, to a max of 11.9, but never up to the 14+ I would expect.
So is the alternator seems to output a little power... sometimes? Enough to run the car without a battery connected. But apparently not enough to bring the system + battery up to alternator voltage. I am hoping to do a little more diagnostics before spending $500 on a rebuilt alternator just to wind up in the same boat.
I also do not seem to have a battery management wart on the negative lead, so I did not attempt to register the new battery. I may try this tomorrow morning anyways, could the BMS be somewhere else if not right on the neg clamp on some cars?
2nd Gear
Joined: Jan 2025
Posts: 79
Likes: 14
Disconnecting the ground lead off the battery is not a great idea. Doing so might have forced the alternator into action by changing the apparent voltage at the alternator. Yes, if the battery was basically a dead short, you could have gotten a similar result, but not after having replaced the battery. It sounds like a really standard bad alternator to me. I'd suggest monitoring all the voltages at the alternator, using the alternator body as the ground connection.
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