Very worried about amount of draw


My BMW 3-series has a 120amp alternator. I'm looking at putting in 2 amps (4-channel and a mono) that will draw a total of 65amps. I'm worried that this is too much total % of my alternator's rating. Do I need to consider trying to find someone who can do a HO BMW alternator? Do they even exist? Most of the websites I've seen listed here only do American and Japanese labels.

Are there any other problems that a HO alternator might bring to my car? Will it affect any of the stock electrical systems negatively? These german cars seem to be very touchy/tuned in terms of their electronics, not very flexible. Will an HO alternator fix one problem but potentially cause others?

I've never done this before, and my cousin had a "pro" installer screw up his BMW to the tune of over $5k in repairs due to wiring/current issues (toasted something related to the airbag brain or ECU, I don't know details).

Gold Member
Username: Jonathan_f


Post Number: 2687
Registered: May-04
65A on a 120A alternator isn't horrible, it just depends on the reserve they left. Hard to tell, something you'll just have to see about, I think you'll be ok. If you were to replace the stock alt. for an HO one, you wouldn't hurt a thing. The voltage regulator will prevent the alternator from overcharging, and it will maintain a steady voltage.

One of the 2 amps was class D instead of AB as I thought, so I miscalculated. Their calculated draw is 57.6 Amps on a 120 Amp alternator. I guess I trimmed a bit by disconnecting the junky stock amplifier that the stock head unit uses, so I saved a bit there, plus the car is wired for DVD/Nav, which I don't use, so there's another 10 amps saved. I guess it should be okay, and thanks for helping ease my worried mine a bit.

Silver Member
Username: Carguy

Post Number: 707
Registered: Nov-04
There's one way to find out. If your lights flicker, then your stock alternator isn't supplying enough power. If the amp's current draw and your power reserve are pretty close, a capacitor might just help you get by.

Nevermind, I just got the English translation of the fusebox circuits. I found several pre-wired options that aren't installed on this car that well exceed that 57.6 amp total anyway, so I should be fine.

As for the "try and see" approach, that's exactly what I wanted to avoid. Once you try, it's too late to prevent possible damage. BMW's electronics systems are very touchy. That's the reason for the super-caution up front BEFORE any circuit is completed...but I do appreciate the feedback. As for caps, total hogwash. The only thing a cap will help me with, in my personal opinion, is sudden WHUMPS of heavy current draw that outpace the battery's delivery speed. Caps don't help power charging problems, they just store some up for quick access in a heavy spike situation...I believe Glasswolf has expounded on that point in previous posts too...but in any case, thanks to all for your helpful feedback. I'm good to go now after some deeper digging...
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