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How many watts (RMS), in addition to what it already takes to run the car, can the stock alternator handle ?

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    I know I would have more luck on a car audio forum, but I want to get a thread started in here for the people who actually still come on here and search things..I want to know if anyone here might know some more on this and wants to elaborate a bit. Or if anyone currently has a powerful (over 1000w rms) system on the stock electrical system, and has listened to it at demanding volumes without any problems.

    So I have read that our alternator is 105 amps, and an alternator ideally runs at 14.4 volts.. So theoretically, since watts = amps X volts, the stock electrical system should be able to handle a 1512 watt load. Since 13.5 volts is a more realistic number for the alternator to produce, that would still mean the electrical system could handle a 1417.5 watt load.. Even at 12 volts, we're still producing 1260 watts. I'm not sure what kind of efficiency all this has, or how many watts it takes just to run other electrical equipment on the car.. But that's just what I have gathered so far.

    For Example: currently, I have an alpine pdx f4 amp that tested at 464 watts rms for the component speakers, the OEM bazooka sub that is 100 watts rms, hid low beams and fog lights (35watts each) so that's another 70 watts. Added up, that's 634 watts.. How many more watts could I safely run on my audio system without putting an overload on the alternator? This isn't talked about much because there are just so many factors (ie, Efficiency, and other things like how many other electrical components are you using at the same time, like opening windows, sun roof, using the brake lights, interior lights, whatever else uses electricity.. Are you listening to the music at full volume, etc)

    I'm just throwing this out there because I'm about to get rid of my bazooka sub and get new subs and amp (500w rms amp for the subs + 464w rms for component speakers, so with my headlights on and music turned up, that's 1034 watts right there) and I want to learn more about this subject!
    Wow that's a really long title lol
    I'm running two Sundown Audio SA-10s on an Audiopipe amp which is at 1100 watts RMS. I'm on the stock alternator. However I did the Big 3 upgrade and I have a deep-cycle battery. I can blast it all day and the car is fine. Shoot I even bump it with the car on accessory power, no problems. The Big 3 helps a lot. But if you go for something crazy like 5000 watts, then you might want a bigger alternator lol.
    What kind of battery do you run? I have heard mixed reactions on optima red/yellow tops, that's the only thing from keeping me from getting a deep cycle battery. And I looked a little into the big 3, but it didn't know how much it would really help. How long did it take you to do?
    joshtc- thanks for the informative post! I was looking through the stuff I have from when the tc came out, it was saying the spec is 1200 watts for the stock alternator. yes your calculations are correct! I think Toyota did a great job on a stock alternator! I always wondered if they figured out we would upgrade since that was the whole marketing of scion to "make it your own".. Including audio upgrades. And that's why they put a bigger alternator in.. well it's happy thinking either way..
    Thanks! That's what I was trying to do with this post, make it informative for anyone who is wondering when they might be running too big of a load for the car. And good to know that my calculations were right! If anyone has any more. Information or input feel free to post it up here for future refrence
    Joshstc wrote:
    What kind of battery do you run? I have heard mixed reactions on optima red/yellow tops, that's the only thing from keeping me from getting a deep cycle battery. And I looked a little into the big 3, but it didn't know how much it would really help. How long did it take you to do?


    I believe I just have a Super Start deep cycle battery. In my previous vehicle (Chevy Silverado) I ran an Optima Yellowtop with the Big 3 and same subs/amp and had no problem. I've also heard mixed things about Optima batteries. In fact, people always told me I was one of the lucky few who never had a problem.

    As for the Big 3, it took me a couple hours, mainly because the chassis ground required me to remove the battery and battery tray to get to it. But it does help a lot, seeing how small of a gauge the stock battery wires are. It definitely helps to have giant wires to keep the battery charged. The lights in my tC have never dimmed on the hardest bass notes with the Big 3. I run 1/0 gauge wire with Rockford Fosgate battery terminals. You should keep in mind though that you may have to cut off the stock battery terminals to do the Big 3 depending on how you do it. I went ahead and chopped them off and have the stock wires hooked up to the RF terminals.
    I never really got around to updating this thread but for anyone wondering, so far I'm running 2 hid kits (35w each), A 500w alpine mono amp for my subs, and a 470w alpine 4 channel amp for my speakers (total 1030w). I've been running this setup for a little over a year and play the crap out of my system without any problems. My battery has yet to die on me even though I occasionally play my music with the car off. I'm not using any kind of dry cel or deep cycle battery either. So it forsure can handle 1000w+
    I appreciate all the info you have posted bud! Are you running a capacitor?
    No problem Spence306! I'm actually not running a capacitor, I thought about it but heard a lot of mixed opinions on them. My lights never dim or anything so I didn't feel the need for one.
    Okay. Yeah I'm leaning toward not using one. I'm planning on running two 12's in a sealed box with a 1200 peak watt amp. Shouldn't be too much of a draw. The only other electrical change I have done is a low beam HID 35w kit.
    Yeah I read that in general to use a 1 farad capacitor for every 1000w you run but all a capacitor really does is store energy for short term bursts your speakers draw on your amp, like if you have your volume all the way up on a bass heavy song and your subs are drawing a lot of power. But I've played my subs to their limit and didn't notice any dimming of my lights so I didn't want to spend money on something I didn't have to have. I actually read that capacitors actually draw more current from your alternator because they need to recharge often. So with all the mixed info about capacitors I opted out. Also, incase you didn't know make sure you look at the amps RMS output not the peak output. I've seen some 5000w peak amps that only run at 1000w rms lol
    Joshstc wrote:
    Yeah I read that in general to use a 1 farad capacitor for every 1000w you run but all a capacitor really does is store energy for short term bursts your speakers draw on your amp, like if you have your volume all the way up on a bass heavy song and your subs are drawing a lot of power. But I've played my subs to their limit and didn't notice any dimming of my lights so I didn't want to spend money on something I didn't have to have. I actually read that capacitors actually draw more current from your alternator because they need to recharge often. So with all the mixed info about capacitors I opted out. Also, incase you didn't know make sure you look at the amps RMS output not the peak output. I've seen some 5000w peak amps that only run at 1000w rms lol


    yeah I can see your point, to keep the system more simplified. And if the voltages in the system aren't dropping then the alternator is keeping up. That's true a lot of amplifiers are over-stated their specs! I think that's why they've come up with more standardized testing with the CEA compliant tests. Finally, a real test that shows the frequency response plus the THD and the RMS, now that is better. Instead of someone saying 1000watts rms, and it turns out it's only at 1khz! lol.. that's pointless.

    The amp o just bought is 450rms at 4 ohms. I'll be running two 12's on it so I don't believe I'll have any issues. I just want some more bass than the stock sub can offer. Lol
    Update to this thread: my alternator finally went out lol. It took 3 years of abuse so i can’t be too upset. Debating on going the expensive route and getting a HOA and upgrading my big 3 and replacing my JL w0’s with w6’s and another 500w rms amp, or just going the cheap way and getting an alternator from autozone. My water pump has been leaking lately, when i change my oil i notice lots of crystals forming around the water pump and dripping down to the oil pan. Although the coolant reservoir remains relatively full, i am going to change my water pump while I’m in the area changing my alternator. I know not many people come on these forms anymore, but i just thought I’d update this anyways lol
    Joshstc wrote:
    Update to this thread: my alternator finally went out lol. It took 3 years of abuse so i can’t be too upset. Debating on going the expensive route and getting a HOA and upgrading my big 3 and replacing my JL w0’s with w6’s and another 500w rms amp, or just going the cheap way and getting an alternator from autozone. My water pump has been leaking lately, when i change my oil i notice lots of crystals forming around the water pump and dripping down to the oil pan. Although the coolant reservoir remains relatively full, i am going to change my water pump while I’m in the area changing my alternator. I know not many people come on these forms anymore, but i just thought I’d update this anyways lol


    some of us are still around lurking on these scion forums thanks for the update!

    How many miles were on your alternator? Wasn't that the original stock alternator?

    Don't wait on that water pump, seriously. Toyota updated the water pump around 08-09, so yours is definitely one of the earlier designs (just like mine that failed). On our engine (2az-fe) the cylinder head to block bolts aren't as deep as they should be. When the water pump fails or is failing, it allows the engine temps to creep up. That happens and then the head begins to pull away from the engine, leading to coolant leaks and hydrolocked engines! Not a good scenario. You won't see the leak because the intake manifold usually hides it, until it's too late.

    You won't notice much of any loss in the reservoir. That's the part that's deceiving.

    The good news is, get the water pump from Toyota (mcgeorgeparts.com or sparksparts.com, comes to mind) and you'll get the updated part and you'll have escaped the engine failure/head bolts failure problem. Don't wait..

    About the alternator.. I would go with a HOA (high output alternator), don't screw around with that junk from autozone. Over on another forum people were having one return after another. And they didn't even have a high-wattage audio system! Just normal stuff!

    Thanks for the reply man, i ended up replacing both last week. Took all day but i got the job done. If i had to do it again it should only take 5hours max, I’m just OCD and had to clean my whole engine bay while i had it all apart lol! I ended up going with an alternator from autozone because i didn’t really have time to wait for shipping and HOA’s are like $500.. i did get my waterpump from Toyota though, my friend used to work at Toyota and i had him pick me one up with his discount when i noticed mine leaking last year.. yeah i waited almost a year to change it, i was lazy and didn’t feel like taking it to a shop and i thought since the reservoir and radiator were full it would be fine. But on my last oil change i noticed quite a bit more dried coolant than usual so i finally changed it with my alternator lol.
    • Edited 1 times, last by Joshstc on .
    And yeah it was the original alternator. Atleast that’s my guess, i picked the car up with 80,000 miles on it 4 years ago (it’s an 07) so I’m not sure what the previous owner did, but it was the OEM alternator so unless the previous owner changed it out with the OEM one, I’d say it was the original. It had 115,000 miles on it when it went, and i listen to my music pretty much full blast every day haha that 1000w rms load probably took it out earlier than it should have but it still took 3 years of abuse from it!
    Joshstc wrote:
    And yeah it was the original alternator. Atleast that’s my guess, i picked the car up with 80,000 miles on it 4 years ago (it’s an 07) so I’m not sure what the previous owner did, but it was the OEM alternator so unless the previous owner changed it out with the OEM one, I’d say it was the original. It had 115,000 miles on it when it went, and i listen to my music pretty much full blast every day haha that 1000w rms load probably took it out earlier than it should have but it still took 3 years of abuse from it!


    hey josh! thanks for the update. So it was around 115k when it died. Reason I ask is I wanted to compare it to when everyone's alternator died.. and you're right in the same mileage. It's between 100k-120k. Yeah the denso made alternators are not bad quality they took that abuse and lasted 3 years. I know their a/c compressors are really good. When it was time for the civic to get a new compressor I didn't buy the junk sanden (original supplier), I went with Denso. Sure I take a fuel economy hit, but NOW I have ice cold a/c! Just like the tC (Denso compressor)..

    glad to hear you finally replaced the water pump, hope everything is good for that in the future! And washing/cleaning the engine bay! Hey you're talking to the Scion community here! Of course you'd clean all that! What did you use to clean it? I have to clean mine soon.

    and autozone alternator? Well let's see how long that one will last. It's hit/miss with those china made alternators. So you get to be the test subject!
    Yeah right at 115k it went! And i just took the coolant reservoir and windshield wiper fluid reservoir out while the alternator and waterpump was out and got all the dirt and dust off them and wiped down all the hoses and stuff in my engine bay that you usually can’t reach when all that stuff is in there. I wipe down my engine bay every time i wash it so it stays relatively clean but i just went in deeper this time. And yeah i was kinda in a rush because i needed my car to get to work but i had 2 days off so i just went to autozone and got the duralast gold alternator because i didn’t know how long shipping would take.
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