What size alternator should I get?

Printed From: the12volt.com
Forum Name: Car Audio
Forum Discription: Car Stereos, Amplifiers, Crossovers, Processors, Speakers, Subwoofers, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=32565
Printed Date: May 08, 2025 at 7:24 PM

Topic: What size alternator should I get?

Posted By: lavamadness
Subject: What size alternator should I get?
Date Posted: May 22, 2004 at 4:32 PM

I need to get a new alternator, as the bearings are going on my current one. The current one isn't stock. It's a large case one, and it's 70amps (at least that's what it says on the case).

Anyway, I have a stereo, it's a 200W deck, and a sub that's wired for 600W RMS. I'm also thinking of getting driving lights and possibly a neon undercar kit (hold the groans posted_image ).

So I'm wondering what alternator I need to get. I know stock is 105, and it seems 140amp and 200amp alts are easy enough to get. Since a 200amp is about twice that of a 140, I'd like to know for sure if a 140 isn't gonna do the job before going for the 200.

I know people say buy the most you can afford, but that's kinda like throwing parts at a problem instead of finding the one part that was the problem.

I've heard it mentioned that a HO alt will produce noise in a sound system. Is this the case all the time? And can this be corrected?

Are the large case alternators better with overheating than the small case ones (like what came stock)? I the 200amps all seem to be the large case ones, but if I should go with a 140, would a large case be better or does it matter?

Any comments/suggestions/explainations are greatly appreciated.

Replies:

Posted By: lavamadness
Date Posted: May 23, 2004 at 12:53 PM

bumping to the top

Posted By: haemphyst
Date Posted: May 23, 2004 at 10:03 PM

1) Your deck DOESN'T make 200 watts, so we'll just call it a 5 amp load
2) Your 600 watt amplifier (depending on the honesty of the mfr) could be as much as 75 amps
3) Your engine will use 12 amps JUST TO RUN
4) Your AC will use about 30 amps
4) Your high beams (2 lamps at 55 watts) will use 7.7 amps
5) Driving lights will duplicate the high beams - 7.7 amps
6) Clearance lights total will be around 10 amps
7) Neon could add as much as 10 more amps (depending on how many you are going to add

5+75+12+30+7.7+7.7+10+10=157.4 amps total load, with everything running foll out... You are not going to be using at all, all the time, but I would recommend the 140A at least.

The manufacturer of your alternator will HOPEFULLY have figured in the thermal mass of the alternator, and built the unit appropriately for the output of the device, so case size should not be a determining factor in your purchase decision, except "will it fit?"

If your alternator is a decently built unit, there should be no problem with noise. The battery should filter any ripple that might be present in the output of you alternator.

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It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."

Posted By: lavamadness
Date Posted: May 24, 2004 at 2:54 AM

Thanks for the reply! I think I'm going to go with the 200amp alt, since the 140 seems to be "just enough".