I'm a freak about reliable power.. I don't want my stero to leave me stranded so I bought another battery and I want to add it to the charging circuit without running crazy wire straight to the main battery..

I bought an Isolator, but I don't know for sure which wires are which coming from the alternator, so I wanted to ask if anyone else has done this before I reinvent the wheel?

Also if anyone has any info on what the stock 04 WRX alternator puts out AMPs wise, that would be great to know too!

Thanks!

just wire up your second battery in parallel to your existing battery.

Um, no-
The isolator is a must.
There is really just the one big fat wire from the alternator to the fuse box.
The stock WRX alternator is a 75A unit. It's really just barely adequate to keep the stock battery charged with a moderate electrical load, and I do not believe it could handle the added demand of a second battery. To do this properly the alternator should be upgraded to a much higher capacity.

would the stock alternator really be working that hard though? unless both batteries are fully discharged. since current drops in parallel, whatever charging power the alternator has (75A), that would just get split into each of the batteries...right?

so under regular use the alternator would be just fine. and if both batteries are almost fully discharged then the alternator would have to work not harder, but longer to charge them both. so the longer charging time might make the alternator conk out.

of course, this is speculation. feel free to correct me.

It's not that simple, you can't just parallel the batteries. The charging load on the alternator will be doubled and the battery with a higher voltage will try to discharge into the other one (what happens when you do a jump start).
The isolator is absolutely necessary to run two batteries, both for electrical/safety reasons and practical ones- the whole purpose is to have one battery for starting and running the car, and the second one solely for running the high-current accessories.
To support this system an alternator of much higher charging capacity is needed, at least 150A.

Originally posted by 420WRX
I'm a freak about reliable power.. I don't want my stero to leave me stranded so I bought another battery and I want to add it to the charging circuit without running crazy wire straight to the main battery..

I bought an Isolator, but I don't know for sure which wires are which coming from the alternator, so I wanted to ask if anyone else has done this before I reinvent the wheel?

Also if anyone has any info on what the stock 04 WRX alternator puts out AMPs wise, that would be great to know too!

Thanks!


heheheeh 420

The RS alternator is 90A;)

Well the isolators I had to choose from were med, large, and way too big. The med was too low (something like 65 amps?) the large is rated to 140 amps, and the largest one I didn't even look at cause it was huge and wouldn't fit anywhere. Even the 140 amp unit is going to be tricky to fit under the hood, which is where it is supposed to be mounted.

I don't want to go to a HO alternator for more than just the cost issues. I have done it in the past and I always had problems with the fan-belt tension and wear on the belts. I would get all sorts of squeals and screaming when I drove by playing my loud tunes, but it was coming from under the hood. :(

So what about getting a smart switch that only throws a charge to the back battery when there is excess amps to spare? Is this a product I can look for?

I don't know if you can find a higher capacity alternator. I have a 95 impreza, and I went Through the same thing with auto zone, I was running a small 500 watt amp, and
I couldn’t keep my battery charged. Btw my stock alternator only puts out 65 amps. They checked all the books and told me that you can’t buy an after Market alternator for a Subaru. If anyone knows where I can get one let me know airdevil85@yahoo.com

I'm not sure about Subaru but most OE alternators can be re-wound for higher output?

Originally posted by 420WRX
I'm not sure about Subaru but most OE alternators can be re-wound for higher output?

Yes. It shouldn't be too hard to find a shop that can re-wind the OE alternator.

Ruiner was re-winding them, not sure if he still is.