Howdy, I have a 2004 honda civic coupe, and I recently need a boost to start, so I checked the battery, and it needed to be replaced, after I replaced it, I always do a simple voltmeter test to see what's up, if it was just the battery or if my alternator is going too. I checked it before and it was acceptable (12.6) and after I started it, it went up around 2 volts to 14.4, great, it was just the battery. Alternator is humming along as it should, right? Well, during those test I preformed, I cranked up the volume on the stereo, played with the power locks and windows, and it's all good, still the battery is at 14.4. But when I decided to crank up the fan, the voltage dropped back down to 12.6 from 14.4 I don't know what's up with it as I am far from a mechanic or electrician. Then I looked it up on the forum here, and couldn't find anything similar, only some stuff like the battery is draining, and it said to unhook the battery after the car was started, I thought that was a little weird, but tried it, and everything works, full bright headlights, stereo, power locks and windows, but when I turn the fan on, the car dies. Has anyone had a similar problem? Is there a fan relay or something that I can replace? Or am I stuck without being able to use fans and have the battery charge? Please give any possible solutions, although cheaper and easier ones are better, I'm open to almost anything. Thanks in advance.
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You shouldn't disconnect the battery when the car is on. You can damage the alternator. But you said the alt is humming? It should be silent. If it hums it might be a bearing going bad or the rectifier going out.

my guess alt is bad. Have it tested at an auto parts store while it's in the car.

the fan should pop a fuse of its drawing to much current. Check to see if the fuse wasn't replaced with one larger than its supposed to be.

 

Humming as in, running well, not audibly humming. And I'll go into my local shop, then, thanks.