Current Draw: Size Your Audio System To Your Electrical System
Similar to how your vehicle has a towing limit or the miles per tank you can drive, your electrical system has limits that directly pertain to car audio. You can make changes to help your truck safely and reliably tow bigger loads, and you can make your car use fuel more efficiently. In either case their are realistic upper limits to expect. It’s extremely common to overlook the alternator as a choke point when designing a sound system. I want to show you how to estimate the maximum watts you can run.
First you’ll want to check what size your alternator is. Your watt output is directly linked to how much current your alternator can supply once you factor in 50-60 amps needed to run the car with all accessories on.
Calculating Amperage With Watts and Voltage
Total Watts / Voltage = Current
In our sample system we have a 100W x 4 amplifier for the speakers and a 1,000W x 1 amplifier for 2 subwoofers. Let’s assume our charging system puts out 14.4V at 2,000 RPM. The current required for maximum output totaling 1,400 watts is calculated as follows:
1,400 / 14.4 = 97 amps
If you looked up the size of your car’s alternator you might be looking at this figure of 97 amps and saying, “No way, this doesn’t work, or else I’d need a bigger alternator right now.” We’re not done yet, because not only does your audio system not use 100% of the watts, but your amplifiers don’t normally draw all of the amperage available.
Why the “Class” of Your Amplifier Matters
We’ll simplify this discussion and say there are Class A/B amplifiers and Class D amplifiers. Class A/B topology is roughly 50% efficient, while Class D topology may be 75% efficient. What this means is your A/B amplifier, which our sample system has for the speakers, is going to use 25% more current than our D amplifier, which is powering our sample system’s subwoofers. However, we’re still not down to the point where this can all make sense in real world use, so don’t give up on me yet.
How Does This Information Change Our Current Draw?
Class A/B on speakers = 400W, which is 50% efficient
400W / 14.4V = 27.8 amps
27.8 amps x 1.5 = 41.7 amps!
Class D on subwoofers = 1,000W, which is 75% efficient
1,000W / 14.4V = 69.4 amps
69.4 amps x 1.25 = 86.75 amps!
Accounting for amplifier efficiency, this puts our maximum current draw at a whopping 128 amps! WOW!
You’re Nuts! I Know This Isn’t How It Actually Works
Why does this math not reflect what actually happens with this common car audio setup with a Class A/B 4-channel amplifier for speakers and a Class D amplifier for bass? To recap, if we figure 60 amps just to run the car with every accessory on(headlights, wipers on full, heater on full, etc.) and then add our sound system that means at full output on test tones we need an alternator that can provide 188 amps at 2,000 RPM.
Did you see that I mentioned test tones? This is one factor that completely changes the actual current draw demanded of the alternator. Test tones are the easiest way to get full power from your amplifiers, in addition to frying your speakers if you aren’t careful. Music has quiet passages with occasional bursts of loud passages. The alternator might be cruising quite comfortably supplying on average only 20 amps to our 2 amplifiers while we listen to music at a moderate level. This is why a typical 90 amp alternator can handle a 1,400 watt sound system without your battery dying.
If we flip this around…
20 amps x 14.4v = 139 watts
That 1,400 watt sound system you bought might be coasting along at 1/10th the RMS rated output.
We want the “excess” power to be there for the LOUD PASSAGES, which can double in loudness(or a gain of 10 dB), which then requires 10 times the power!
Another way to look at this is to take the output, Power as we measure it in Watts, and back into the current required to produce it.
P is Power
I is current
E is volts
How much current would it require to make 1,000W in a car that holds a steady 14.4V with the engine running?
1,000W = I x 14.4V
Solve for I
1,000 / 14.4 = ~69.44
We can’t have a 100% efficient amplifier, and the voltage drops, so it’s accurate to assume it will take more than 70 amps to create a sustained 1,000W.
Practical application and sound system behavior with music makes that swing up and down constantly, which is part of why we can get away with relatively small alternators in our 12V mobile audio sound systems.