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Old 03-30-2021, 09:42 AM   #1
1ofeach
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2001 S2000, 2017 FoRS

Default Increase alternator voltage for AGM Battery

I've been using Optima Yellow and other AGM batteries in my '02 WRX for a while now, mostly because regular batteries were eating away at my holder bracket and bolts.

But I've never been happy with how long they last in the car. It seems like my original OEM battery lasted like 5 or 6 years and the AGMs are only lasting about 3. When you factor in the 2x cost of an AGM, it's getting annoying.

Anyway, from the same experience in my Tacoma, I know there is a simple fix where you replace a particular fuse with a diode (that is in a mini fuse package) and this fools the alternator into boosting the voltage by about 0.5V, which is perfect for AGMs, since they need higher charging voltages to extend their lifetimes.

When I measure the voltage at my battery in my Suby when it's warmed up, I'm only getting 14.06V so I know this car has the same problem -- the AGMs are not getting fully charged up with reduces their life dramatically.

But I searched around this forum and one other and I don't see that anybody has found a simple mod on the Subies like for my Taco. Has anybody out there figured this out and can share their fix? Basically I'm looking for the Alternator Sense line (ALT-S) and an easy way to fool the alternator into some more voltage.
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Old 03-30-2021, 10:03 AM   #2
T-37
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This might be of some help

https://forums.nasioc.com/forums/sho...3#post23868833
starting from post 35
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Old 03-30-2021, 10:15 AM   #3
1ofeach
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Originally Posted by T-37 View Post
This might be of some help

https://forums.nasioc.com/forums/sho...3#post23868833
starting from post 35
Thanks Tank! I saw that thread title and looked at the first few posts and it didn't originally catch my eye.

I have an extra mini-diode from my Tacoma, I can probably just plug it into the right slot in the right direction right away and give it a shot.
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Old 03-30-2021, 10:33 AM   #4
cdska
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i have a 1N4004 rectifier diode in line with the power to the alternator to accomplish this
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Old 04-22-2021, 02:22 PM   #5
Clickedspider
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Why not just get a high output alternator
And like a xspower d3400 I think I have mine for 8 years
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Old 04-22-2021, 06:20 PM   #6
1ofeach
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Quote:
Originally Posted by Clickedspider View Post
Why not just get a high output alternator
And like a xspower d3400 I think I have mine for 8 years
Because that is not what an HO alternator does. HO puts out more current, not necessarily more voltage. HOs are not needed unless you have a thousand watt audio system, and they rob more power from the engine than a standard alternator. Also a diode costs $3 and takes 1 minute to swap

Does your HO alternator put out more than 14.1V in a bugeye when warmed up? Each aftermarket alternator has a different voltage regulator, and may have different default settings.

I'll look into the XSPOWER next time I get a battery. Maybe I just had a bad yellow top, but it only lasted me 3 years and like I posted my experience with my other car is that the voltage output of the alternator was not high enough to charge Optimas more than 80%. My bugeye does seem to put out more voltage than my Tacoma, so maybe it's not that, but a $3 try is easy enough.
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Old 04-25-2021, 09:06 PM   #7
Stretch916
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Quote:
Originally Posted by 1ofeach View Post
Because that is not what an HO alternator does. HO puts out more current, not necessarily more voltage. HOs are not needed unless you have a thousand watt audio system, and they rob more power from the engine than a standard alternator. Also a diode costs $3 and takes 1 minute to swap

Does your HO alternator put out more than 14.1V in a bugeye when warmed up? Each aftermarket alternator has a different voltage regulator, and may have different default settings.

I'll look into the XSPOWER next time I get a battery. Maybe I just had a bad yellow top, but it only lasted me 3 years and like I posted my experience with my other car is that the voltage output of the alternator was not high enough to charge Optimas more than 80%. My bugeye does seem to put out more voltage than my Tacoma, so maybe it's not that, but a $3 try is easy enough.

XSpower is no good for under the hood though without thermal wrap. They swell up and lose they’re ampacity. I think you’re on to something with the lower charge voltage. I run a lithium battery in the spare tire compartment and run 3k watt sub and I only ever see 14v. External voltage regulators are the real way to do this which you can run with a HO alt. I got a guy if you are interested in those options. I’m just running my current alt into the ground before I get something like a 240amp alt.

Check out Erni Audio and Electric on FB. Great dude and knows everything about car voltages and alternators
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Old 04-25-2021, 09:49 PM   #8
MoreWagons
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Quote:
Originally Posted by 1ofeach View Post
Thanks Tank! I saw that thread title and looked at the first few posts and it didn't originally catch my eye.

I have an extra mini-diode from my Tacoma, I can probably just plug it into the right slot in the right direction right away and give it a shot.
Did you end up trying this out?
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