780w, 40-45a. Can the stock alternator handle it?

cr445671

Forum Member
Joined
Jul 20, 2018
Messages
66
Location
florida
Car Year
2002
Car Model
forester
Transmission
auto
Just got some new arb intensity knockoff lights, measured at 130w each. Those on the bumper combined with 4 130w hella 500's on the roof basket makes 780w total and from 40-45a. Can the stock '02 forester alternator handle that kind of load?
 
I think that it has an output of 75A. Check.

Also make sure your wiring is taken from the battery, not the car's wiring loom.
 
Where would I find that information? And yah that's how I'm running it. Also I just realised I put down the wrong amperage, it's 60a @ 13v, 40a is just for the hella's. So I guess I should ask what the a/c and other electrical draw is, sounds like I may be maxing it out.
 
Your alternator will have a specs plate on it.

Personally, I wouldn't go over about 75-80% of maximum, but also fit a higher capacity, reputable brand battery. I use Bosch batteries. Older one is now 10 years old, and still works perfectly.
 
My SG alternator is rated at 90 amp; maybe the SF is the same?

It varies with revs according to the specs:
1,500 rpm — 36 A or more
2,500 rpm — 65 A or more
5,000 rpm — 86 A or more
 
Yah it's 75a. Do y'all know of any higher rated ones that can drop in? Excluding the expensive aftermarket ones.
 
With a 75A alternator, you should be fine as long as your battery is good and not a standard 46 AH. As I said before, get a good quality, higher capacity battery.

My light bar is like a small nuclear device, and only draws 9.5 amps ...
 
@Ratbag You sure? I recently got a duralast platinum battery, so I figure it should be ok, I was under the impression though that alternators lost 15-20% of capacity once they get up to temperature. If it did that my lights alone would be maxing it out, adding in radiator fans, the ac, etc, it sounds like I'd be killing my battery pretty quick.
 
cr445671 said:
@Ratbag You sure? I recently got a duralast platinum battery, so I figure it should be ok, I was under the impression though that alternators lost 15-20% of capacity once they get up to temperature. If it did that my lights alone would be maxing it out, adding in radiator fans, the ac, etc, it sounds like I'd be killing my battery pretty quick.
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Sure about what?

You are highly unlikely to be using every high drain device simultaneously for extended periods. However, driving in the bush, you may well have driving lights on for extended periods. Probably not all of them (use of roof mounted lights on public roads is illegal IIRC).

Ditto, your alternator will not be charging at full capacity on a 100% duty cycle.

So a 75A alternator should be absolutely fine.
 
I guess that's the situation I was thinking of, forest service road at night with all the lights on. In the summer it's a guarantee I'd have the a/c running also. I just don't know a whole lot about the subject, it was only last night that I considered I may be overloaded after adding up my wattage.

do sg alternators fit in sf's?
 
Air-conditioning uses little electrical power, just the cabin fan.

Forest service roads at night is a very dangerous activity, unless you are very familiar with the particular track.

When you are young and very inexperienced, it is very easy to die, unfortunately.

[Edit] by the way, any forest track you are legally allowed access to is a "public road". If it's not, your insurance will probably be null and void. [End edit].
 
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Idk I'm not real concerned about insurance, the only thing I'm covered for is liability anyhow, wouldn't make a difference where any damage occurred. And as far as danger from the road goes I can't think of a situation where I've done much more than 20mph off of pavement, and most of these roads are actually fairly decent.
 
Of more concern is what your alternator puts out at low revs.

eg: my gen3 outback is:
100A nominal
49A at 1500rpm
79A at 2500rpm
 
cr445671 said:
do sg alternators fit in sf's?
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Will need double checking but I believe so - especially if your SF has the 2.5 engine (I think it was the standard issue in the USA?) But in your case you may need something with more output.
 
I think I'm going to get a 110a '05+ legacy one off of eBay, it should fit and I'm not sure how I could go higher without spending a lot of money.
 
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