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2019 Forester Sport
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Discussion Starter · #1 ·
I'm looking to add some emergency vehicle lights and siren to my 19 sport. Peak draw should come in around 400 watts or ~30A. My question is would I need to upgrade the alternator to support the extra wattage? The usual draw should be approximately 250 watts since most of the 3 watt bulbs will be alternating.
 

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Super Moderator
2001 Forester S, 4EAT
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3,358 Posts
I would since alternator output is tied to engine RPM. And that's an additional 20AMP nominal.
 

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2019 Forester Sport
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4 Posts
Discussion Starter · #3 ·
In that case, are there any good recommendations for an OE fit upgrade?
 

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2019 Forester Limited
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1,191 Posts
I've not heard of anyone upgrading the alternator in a Forester. It's a "smart" alternator, so I'd assume that factors in.
 

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2007 Forester Sports XT 4EAT
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I've seen members install upgraded, some pretty hefty alternators for serious audio upgrades!

Here's one link I had bookmarked. I'm not sure they make one for your specific model year?
Bobby...

['07 FSXT Member Journal] ['03 X Member Journal]
 

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2021 Forester Limited
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321 Posts
Unless you are constantly using them, you might be just fine with the stock alternator. As long as you don't have every electrical load maxed out while using the sirens and lights, it will probably handle it, and enough drive time afterwards if the battery was being tapped into.

Not sure on the 2019, but the owner's manual for my '21 states it is rated at 150 amps.

I don't see too many vehicles with sirens and lights on traveling the entire time at an idle, except for maybe a parade.

You might look into the Ascent alternator specs, and if it is the same as far as mounting and pulley placement. If it is, and has a higher rated output, it should be fully compatible as far as the "smart" features.

I found one post in an Ascent forum that has it at 190 amps. Now whether or not it will physically fit is another matter. You would want to possibly upgrade the cable from the alternator to the battery. Or cables depending on how it is routed. that's an additional 40 potential amps. Or more if the 2019 is less than 150...
 

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2020 Subaru Forester
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9 Posts
I have a Whelen Airhorn, several LED's and flashers, Radio's and a Scanner on a 2020 Forester Premium. No alternator upgrade and it's all working very well. Let me know if you have any questions on install.

 

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2019 Forester Sport
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Discussion Starter · #8 ·
Thanks for your help folks. I'll give it a try with the stock alternator and see if I have any voltage issues. If the vehicle is left idling, it'll just have lights on and no siren. And I won't have my stereo cranked up with the system on either.
 

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2021 Forester Limited
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321 Posts
Are the lights something you are supplying yourself, or equipment supplied by the emergency response team?

Is there any way LED's could be used vs incandescent?
 

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2015 Forester CVT
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697 Posts
I'll be using LEDs. Each bulb in each light uses 3w max. Rear light bar is 36 bulbs. Several other 6 bulb lights around front and sides.
Odd that battery capacity was not mentioned. I would install the highest capacity battery that would fit. That extra reserve of power means the alternator is a bit less of an issue.

GD
 

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2001 Forester S, 4EAT
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@Green Darter I have to believe no one brought up extra capacity because only a Group 24 will barely fit and depending on the vehicle modifications would have to be made slightly. Anything bigger than a Group 24 will not fit the existing battery tray. You also add 4-5 lbs more weight to the vehicle.
 

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2021 Forester Limited
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321 Posts
Those are some pretty bright lights...

I thought of mentioning more battery capacity, but even if the alternator wasn't keeping up 100% of the time, it would still need to recover the battery's state of charge. Not sure what engine rpm the alternator puts out the closest to the rated output, but I would guess somewhere around 2000 rpm.

If I were to replace the battery, I would go with at least AGM since they can recover quicker than a flooded cell battery. This would help take some stress off the alternator.

One issue with extended idling under a heavy load is will the alternator have adequate cooling?

But I'm sure there are plenty of other emergency response vehicles that are under the same conditions. Whether or not they have upgraded charging systems is something you might ask the others in your group.

If nothing else, you could get/borrow an inductive DC ammeter, and check the current flowing from the alternator wire with everything on in the vehicle, then with everything off and with the lights on. At least you'll have a baseline as to what the vehicle's accessories max amp draw is.

I picked up a relatively cheap (clearance) meter that has an inductive amp clamp that is good for 100 amps. It is a handy thing to have. sliding it over the feed wire to the underhood fuse box of my '99 neon, with all the lights, rear defrost, HVAC blower on high, etc. turned on, there is about 63 amps flowing into there.

That's just a basic econo box. But I do have a 136 amp GT Cruiser alternator in place of the 83 amp neon alternator. That probably helps push a bit more power.

If I have a chance sometime within a week or so, I will see what the draw is in my '21 Forester. I'll post it up here. I have a bit going on, so hopefully I will remember.
 

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2018 Forester Touring CVT
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97 Posts
My town requires a police car at all major road construction. Because of a tragic accident years earlier their cars were fitted with a spectacul array of strobes and flashing lights, something any airport would be proud of. Because of the load, they had to keep their engines running all day. A quick study by one of the light providers determined that an expensive LED retrofit could quickly pay for itself in saved gas, no more idling required....but the lights are still spectacular.
 

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2011 Subaru Forester X, Auto
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I'll be using LEDs. Each bulb in each light uses 3w max. Rear light bar is 36 bulbs. Several other 6 bulb lights around front and sides.
My experience with current LED lights is that they grossly over advertise the wattage output. NOTHING that I have measured has even come close to the advertised capacity in either wattage or lumens. I agree with Black21Limited and measure the draw (individually or collectively) before I ran the alternator rabbit.
 

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2014 Forester 2.5 Limited CVT
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17 Posts
So I’m a little late to the party but I have a similar load on my car currently with a 600watt amp (I get that’s not constant load) and I upgraded my alternator (270amp Singer Alt), battery (X2 Power p/n# SLI35AGMDP) and power/ground cables (1/0ga OFC). To be honest I didn’t notice any voltage issues before but had planned for a much higher wattage system and wanted to have the ceiling to run anything.
Crankset Wheel Motor vehicle Automotive tire Gear Motor vehicle Tool Vehicle door Automotive tire Automotive exterior
 

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2015 Forester CVT
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@Green Darter I have to believe no one brought up extra capacity because only a Group 24 will barely fit and depending on the vehicle modifications would have to be made slightly. Anything bigger than a Group 24 will not fit the existing battery tray. You also add 4-5 lbs more weight to the vehicle.
You are partially correct. Within the group 24 there are several batteries with higher capacity than OEM.
 

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2021 Forester Limited
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Interesting. Do the 2014's use the "smart alternator" technology?

Thinking about it, I'm not sure the alternator itself matters, (I may be wrong) as long as the winding current draw will not damage the vehicle computer controlling it. OEM alternators have not had an integral voltage regulator for quite some time. The output is controlled by either the powertrain control module or another means. The "Smart" part is likely in the controlling module, and not the alternator.

The alternator could be capable of making 500 amps, but that doesn't mean the PCM will allow it to. If the target voltage cannot be maintained, then it will ramp up the voltage to the alternator winding, so it can make more amperage to maintain the target voltage.

The common method is Pulse Width Modulation on the ground side of the winding circuit. Basically the PCM switches the ground path on and off very rapidly, to control the volts from zero to full system voltage available as needed. The pulse width is the amount of time the ground path is connected vs not. Longer pulse time equals higher voltage.

It's the same way that the fuel injectors are controlled. It is a proven reliable technology, so I doubt they have changed it too much.

The added capacity would only be reached if the load on the system kept that target voltage from being achieved.

The target voltage is whatever the PCM deems is needed depending on temperature, overall electrical load, engine load, and probably a few other things.
 

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2019 Forester Limited
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Thanks. That's a really good explanation.

I guess where I was going with that is the ECU (I think that's the module a tech bulliten said the Forester uses to control the alternator) has an algorithm using the sensors you mention to get a desired output. Often, if you change a part in a control system, in this case the alternator, and it responds quicker/differently than the replaced part, it can throw off the "tune" of the control system. I see your point and how that may not be the case here.

If the 2014s use the smart alternator control scheme, and dropping a new one in didn't rock the boat, then that tells me everything you said above is true (i.e the alternator itself doesn't matter that much). That is why I asked the question.
 
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