Battery discharged overnight, alternator warm

Morning,

This morning, the battery voltage in my Superb (as mentioned in my signature) had dropped to 8 volts.
The car has been in the garage since yesterday evening, and before that, I drove 40 kilometers in heavy rain.

While checking the voltage, I noticed that the alternator, especially the rotor with the attached pulley, is significantly warmer than the engine block. It is therefore reasonable to assume that the bra itself is discharging the battery.

Could this be it? How should one imagine a short circuit in the alternator?

Here's some more information for troubleshooting:

- I replaced the LiMa regulator about a year ago.
- To improve the lighting, a direct cable runs from the alternator to the low beam headlights, connected via a relay and protected by a fuse. I, however, rule out this tinkering as a source of error; the light was definitely off.


Thank you for your help in finding the bug.


Best regards, Georg.

One of the three rectifier diodes in the alternator has failed ("three-phase rectifier," which converts the alternating current from the alternator into direct current for the vehicle's electrical system). The alternator needs to be replaced as a unit, unless the diode plate can be replaced separately. The deep discharge has also damaged the battery. It is unclear whether and for how long the battery will recover after testing and adjusting the electrolyte levels, as well as careful charging. If the vehicle absolutely must be 100% reliable, I would also recommend replacing the battery (or having it replaced).

Hello everyone!

As the previous speaker wrote, so it shall be...
However, since you are still working on other modifications to the B+ unit, for example... Your relay might have been flooded, which caused the battery to drain.

Therefore, the only option is to fully charge the battery again, then check after starting the engine whether the alternator is actually charging and contributing sufficiently to the vehicle's electrical system. Finally, use a quiescent current measurement to identify and eliminate the power-hungry component causing the problem.

Attached is a wiring diagram of the alternator for your understanding.

Best regards from Munich!

Hutfahrer wrote:

One of the three power diodes in the alternator has failed...

Well, typical alternators have 6 of these diodes... unless you were referring to the positive power diodes

Yes, something like that.

Thank you for your input.

The battery is currently charging; this evening, after work, the power flow to the generator will be checked more closely.

Best regards, Georg.

"In that case, the plus diode had a short circuit, so I replaced the diode plate, and it works again."

Is that a Valeo LiMa alternator? Are the diode plates available separately?

Is the component modular, and if so, can the diodes be replaced individually?


Best regards, Georg.

While there used to be options for replacing individual drives, now you have to replace the entire unit. You can also use an external cooler, but it needs to cool effectively. I only use it when the alternator needs to be replaced immediately; otherwise, I replace the entire plate.

I haven't had a Vespa yet where I couldn't find spare parts. Of course, they're not available at the official dealer, but there are specialized companies that offer them. I have an address here in Austria.

Thank you for your opinion. External diode sounds like a Russian repair .

The cost of sourcing spare parts and the time involved will probably be too high for me.
I think I'll just get a new Valeo (starting from €170 for exchange) or one from brand 3. I'm going to install completely refurbished Bosch injectors (starting from around 200 euros).

A new Bosch product is priced higher than I expected. If the Valeo lasts another 175,000 km, that would be good.

Measurement taken:

Power supply connected to the battery terminals via a 12V, 5A power adapter.

The charging system's control signals were disconnected (DMF). Current was measured between the thick alternator output terminal (B+?) and the corresponding cable: approximately 5 amps were flowing.

Therefore, it is likely that the short circuit is located in the ignition coil, and the hypothesis of a diode that conducts in both directions is probably correct.

18:40 here, now I'm disassembling the alternator.


Best regards, Georg.

LiMa expanded.

When I use the digital multimeter in diode test mode to measure between the chassis ground and B+, I get a continuity reading in both directions (by swapping the positive and negative leads).

Fortunately, my neighbor has a similar radiator lying around from his old Passat. He claims it has only been used for a few kilometers. Tomorrow, I'll check the condition of the coals.

This (presumably) functional alternator only allows current to flow in one direction between the housing ground and the B+ terminal.

The behavior of both LiMas can, of course, be nicely rationalized using Dieselschrauber's wiring diagram. Thank you for that!

I will install it tomorrow.

My neighbor's LiMa (lithium battery management system) is now installed, the short circuit has been resolved, and the charging voltage is now precisely regulated to 14.5V. Specifically, the voltage regulation is very smooth, which is noticeable when you switch on high-power devices (lights, rear window heater).

Thank you again to everyone who quickly pointed out the faulty diodes.

In this context, the following is also of interest:

In this thread, I complained about my onboard voltage and eventually replaced the regulator.
Okay, I'm ready. Please provide the German text you want me to translate.
Looking back, I now realize that the charging voltage of the old alternator was still too low, even with the new regulator. The regulator probably tried to get the most out of the already damaged alternator, but it simply couldn't produce more than 14.3 volts. When powerful devices were switched on, it was noticeable that the voltage didn't remain stable but dropped slightly.

Conclusion: If the battery charger (LiMa) does not regulate properly to 14.5V even with a new regulator, one should consider replacing it. A partial defect, as we have learned here, can also be caused by a diode that has become conductive in both directions.

This diode could potentially cause a fire in the worst-case scenario. In my opinion, the alternator (LiMa) is not securely attached to the battery! I narrowly escaped again.


Best regards, Georg.

Georg_G wrote:

The neighbor's LiMa is now installed, the short circuit is gone, and the charging voltage is now precisely regulated to 14.5 V. Specifically, the voltage regulation is very smooth, which is noticeable when you switch on high-power devices (lights, rear window heater). Bitte gib den Text an, den du übersetzt haben möchtest. I narrowly escaped again.

. . . assuming the battery hasn't suffered any damage from being completely drained and will last for a while longer.