Test
Bad diodes are a common cause of alternator failure. The diodes are part of the rectifier assembly that converts the alternator's AC output to DC. The alternator's charging output flows through six diodes in the rectifier assembly before it goes to the battery and electrical system.* To check for a possible bad alternator diode, switch your voltmeter to a low setting on the AC (alternating current) voltage scale. * With the engine running, touch the meter probes to the battery terminals. Any amount of AC voltage would indicate a bad diode, so you'll need to replace the alternator.
More details
Best Answer: One or more diodes in the alternator have failed. You can measure this yourself if you have a digital voltmeter - place the voltmeter across the battery with the engine running. The AC volts should be well under 0.1 volt, and I bet yours reads more than 0.5 volts AC. An oscilloscope gives and even better picture, but the meter tells the story. When I worked in aviation I saw dozens of those, and I have seen maybe half a dozen in cars. Recharging batteries by running the engine is a common cause.

Bad diodes reduce alternator output 33%, 67%, or 100%, depending on how many go bad, and the ripple can drive electronics crazy. If you have a bad diode and remove the battery lead for an old-fashioned alternator test the ripple can reach most of a hundred volts and ruin thousands of dollars worth of computers, controllers, sound system and security system components.

__________________

2005 X5 4.8IS
The Blue ones are always FASTER....

Current Garage:
2005 X5 4.8is
2002 M5 TiSilver
2003 525iT
1998 528i
Former Garage Stable Highlights
2004 325XiT Sport
1973 De Tomaso Pantera, L Model
1970 Dodge Challenger T/A 4 sp Alpine White
1970 Dodge Challenger T/A 4 sp GoManGo Green
1971 Dart Sport, “Dart Light” package
1969 Road Runner 383
1968 Ply Barracuda 340S FB Sea-foam Green