Alternator Voltage Reading Level

Hi All! I'm creating a club level wiring harness for my 2013 BMW 135i, but i've seen to run into a bit of a roadblock. Most of the harness documentation is completed - besides this one issue, where I don't know how I can get the OEM alternator to read the voltage level of the battery. This is used to determine how much voltage the alternator sends out in real time, but the OEM alternator seems to be based on running with a BSD line (Bit Serial Data) - I believe BMW specific.

I don't believe I can access this line, since I believe it's encrypted, so my current plan is I would leave the alternator voltage level reading sensor unplugged, and just let me output the maximum voltage. I'm not sure that this is the best idea though, as I don't want to possibly risk overloading actuators and some sensors. Does anyone have any suggestions for how I can go about this? The aftermarket selection of alternators that fit this vehicle are pretty selective, and I'm not sure I would be able to fit a universal one without some fabrication.

Thanks Guys,

Michael

Where does the BSD line come from? Is it possible to leave whatever module (B895?, Dash?, ECU?) that sends that in the car to continue to function.

Assuming that's not possible, here are some options I would consider, in the order I would probably try them:

- Try the alternator with the BSD line disconnected -- what is the maximum voltage? If it's less than 15v, I would not worry, and just run it.

- Consider using a stand-alone voltage regulator, like vehicles used in the '70s and '80s. Something like this might work: https://www.summitracing.com/search/part-type/voltage-regulator-and-rectifiers?N=voltage-output%3A13-5-constant-volts&SortBy=Default&SortOrder=Ascending

- Fabricate a bracket and use a new-style single-wire alternator.

Interesting, I didn't know that voltage regulators existed. From the looks of it, I'm not going to be able to salvage the existing BSD as it can only be read with the OEM modules, which have all been removed. I don't believe the alternator exceeds 15v, as after a bit of research, it seems like it averages an output of 14.4v. But from what you're saying, is that if it stays below 15v - it shouldn't matter? Doing so won't overload the 12v supply? It seems like to me, it would increase the battery voltage level to 16v+ no?

Thank you,

Michael

A battery will charge just fine with any alternator that provides no more than 14.8 - 15v. If the battery is discharged (like from starting the car), the voltage will be lower as the alternator is current limited (especially at low RPMs), but as the battery charges, the voltage will rise to the maximum the alternator (which has a built-in voltage regulator) voltage it provides if there is no load.

I like to see between 13.5 and 14.5volts in my race car on track. If your alternator setup with no BSD achieves that, you should be good.

Thanks for the help David, I'll make sure to measure to voltage output just to ensure stable voltage levels once I get the car back up and running. If too high for whatever reason, I'll look into a voltage regulator, and worst case a new alternator.

Thank you,

Michael