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Old 07-15-2015, 11:37 AM   #1
portista
 
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[Electrical] Current consumption

Hello folks,
I am planning to install an amplifier in my 2003 yaris. From what I've seen the alternator is capable of producing 70A. However I don't know what is the current consumption of the car for a standard yaris (no A/C). Do you know which is?

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Old 07-15-2015, 01:45 PM   #2
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How much of an amplifier? The alternator is designed to keep the battery charged under the worst case scenario (High Beams + Fogs + Stereo on max + Rear defogger + All wipers running on max + Aux Heater + etc etc etc). The A/C does not draw any significant amount of current.

If you're just talking a standard amplifier for a reasonable subwoofer, or an amplifier for some regular car speakers, it would not be an issue. Worst case, if you manage to draw more than 70 Amps, you'll start to drain the battery. The alternator won't blow up the moment 71 A is requested from the electrical system, although from heavy usage it will break down sooner than expected.

Someone could measure the current consumption, I suppose, but considering the average car will kill a battery in about 2 hours if the alternator dies (especially with DRLs), and a typical sedan has about 60 Ah of battery, I'd say 30 Ah average draw for a normal car being used in a typical fashion. Just a guess, though, really.
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Old 07-15-2015, 03:04 PM   #3
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Well,
From what I read, a normal car uses on average 40-45A of current. Assuming that the alternator from a P1 Yaris produces 70A, let's say that we have 30A available.

30*14.4V=432W or at 12V=360W... I think that watts are watts, assuming that these RMS (continuous), it may supply a 200W subwoofer and a pair of 80W RMS speakers... Not bad if I am not mistaken, but can the Yaris have a spare of 30A ??? That is my question!

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Old 07-15-2015, 08:49 PM   #4
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I just did some testing:

Off: 0.23 A

Engine not running, ignition on: 9.3 A

Engine not running, low beams on: 23.1 A

Engine not running, high beams on: 24.0 A

Engine not running, Brake lights on: 14.0 A

Worst case - Engine running (idle) , high beams on, AC on, brake lights on, fog lights on, radio on, power steering moving: -7.0 A

Best case - Engine running (idle), nothing else on: -23.2 A

So, really the Yaris only has about 7.0 A to spare in a worst case. The alternator reaches its peak output around 3,000 RPM, so you would likely have about 30 A to spare there, but any time you are sitting at idle you would be drawing from the battery, rather than alternator, when running everything else and drawing more than 7 A or when running nothing else and drawing more than 23 A.
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Old 07-16-2015, 09:07 AM   #5
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CTScott,
First of all let me thank you and congratulate you because you were the only person in many of the internet forums that I access that was capable of giving me a concrete answer to a problem!! Cheers mate!

So let me see if I got this straight. I suppose you used a multimeter between the positive side of the battery and the main cable that goes to there, correct?

The measures that you took state that at idling with everything shut down you have around 23A of current to spare and with the lights and so on you have about 7Amps of current to spare correct? So 7*14.4V (I suppose that the alternator provides this voltage) gives us a total of 100W... This doesn't make it even for a subwoofer , am I correct?

Also I am getting worried since your car is a p2 yaris, whose alternator has a capacity of providing 90A instead of the 70Amps from the p1, am I correct?

Greetings
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Old 07-16-2015, 07:40 PM   #6
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Quote:
Originally Posted by portista View Post
CTScott,
First of all let me thank you and congratulate you because you were the only person in many of the internet forums that I access that was capable of giving me a concrete answer to a problem!! Cheers mate!

So let me see if I got this straight. I suppose you used a multimeter between the positive side of the battery and the main cable that goes to there, correct?

The measures that you took state that at idling with everything shut down you have around 23A of current to spare and with the lights and so on you have about 7Amps of current to spare correct? So 7*14.4V (I suppose that the alternator provides this voltage) gives us a total of 100W... This doesn't make it even for a subwoofer , am I correct?

Also I am getting worried since your car is a p2 yaris, whose alternator has a capacity of providing 90A instead of the 70Amps from the p1, am I correct?

Greetings

Our P2 Yaris has a 70A alternator as well. You are correct on ~100 Watts when everything else is running, but that should not be the case most of the time, so the 23 A (331 W) or more would be your more your typical availability. Remember that I measured these at idle, so there would be more surplus with the car in motion. Also, your 1000 W amplifier is only drawing that much while your ears are bleeding, so the 100 - 300 W supply would probably be more than sufficient for normal use. During the occasional blasting of music you would be drawing from the battery, as the alternator would not be able to keep up.
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Old 07-17-2015, 09:49 AM   #7
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100 watts is also a LOT of power for a speaker. I know amplifiers can be rather inefficient (Class A can be around 60%, though nowadays you'd find subwoofer amps are Class D which would typically be 90%+ efficient).

I'll use my subwoofer speaker as an example, though in the "small sealed car" design to show how much sound 100 watts *should* generate.

http://www.eminence.com/pdf/LAB_15_cab.pdf

If we take a 35 Hz sine wave (typically, for cars, where you get the best sound energy for your input) you'll notice the maximum safe input power is 100 watts. At that you'll experience 110 dB at 1 meter. 110 dB is equivalent to standing beside someone operating a power saw, and only slightly less loud than a rock concert. It's only *somewhat* below the threshold of pain.

Of course, 100 watts is going to draw 110 - 160 watts.

So, I think you can see if you're going for reasonable, you have plenty of power already. If you're going for "blow out the windshield" you'll definitely need to figure out a way to install a secondary alternator.

FWIW, many companies lie about watts and write on the product the output power at which the amplifier fails (Often spectacularly, we always joked that PMPO = CSA fire rating) rather than what it outputs before clipping. I have a 2100 watt "real" output amplifier (plugs into the wall). It weighs about 100 lbs, has a 12 AWG cord (and would need a 20 A outlet for full power output), and has shock hazard warnings on the output (2100 watts @ 8 Ohms = 130 volts).

I have that exact speaker in the "Large Deep Bass" cabinet. Before reaching 1/4 of the amplifier's output the floor above in my house starts to shake in ways that break dishes and worry me that I might damage something. At 20 Hz (typical house resonance) the speaker only outputs 112 dB to get things to that point, consuming just 200 Watts of power. Yes, I have to sit on the speaker or it tries to walk away.

That's not to say that having a large amplifier isn't useful (the extra wattage means small bursts of musical energy are handled cleanly and easily), but it will normally not draw anywhere near that much power constantly, just in short bursts. And your battery can handle those bursts for one reasonably sized sub (or 4 regular speakers) while the alternator charges the rest of the time.
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