# Running time in a battery [closed]

How long will 150w speaker run on a 12v battery . I will be using this in intervals each session last around 20 minutes with a microphone ?

## closed as unclear what you're asking by Leon Heller, JRE, Daniel Grillo, Olin Lathrop, RicardoOct 20 '16 at 11:01

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• How many Ah does the battery have? Its possible to calculate for a worst case (150W draw), but its very unlikely that it'll be accurate. – Wesley Lee Oct 20 '16 at 4:49
• Here is a 12V appliance battery, cdn3.volusion.com/9jon3.pys6q/v/vspfiles/photos/… This one will not run your speaker at all. Did you mean a battery like this, or something else? – Ale..chenski Oct 20 '16 at 5:42
• A 150 W speaker will not be drawing 150 W continuously. Also you do not provide the type or capacity of the battery so you question is impossible to answer except with: It will last until the batteries run out. – Bimpelrekkie Oct 20 '16 at 6:06
• Oh, about that long. – Olin Lathrop Oct 20 '16 at 10:57

The problem with your question is that the 150 watt speaker is rated for either 150 watts max, or 150 watts RMS... but either way, when you apply a signal to the speaker (music, a sine wave or whatever) the voltage, and therefore power, to the speaker will fluctuate and will not be just 150 watts all the time. If you've ever seen a waveform, like if you record your voice in a music production program like FL or something, then the biggest parts of the wave will draw the most power, while the small parts of the wave will not require as much power from your amp and therefore battery.

I think I see what you're getting at though. Maybe you have an amp in your car with a 150 watt speaker hooked up to it, and you are going to be hooking up a mic to the amp to talk about how Hillary is the devil and Trump is really no better, so you'd might as well vote for Gary Johnson (or something like that)... and you want to know how long this will take to kill your battery. To me, this sounds like your situation, so if this is close to what you are talking about then this might help.

The amp will draw some amount of current (and power) from your battery, just to stay on--this is the minimum current draw of the amp. You can calculate the power consumption in wattage by using Watt's Law, which is P = I V, which means Power = Current * Voltage (or Watts = Amps * Volts.) Your car is at 12 volts, so we already know that. If your amp requires 1 amp of current just to be powered on, then it will consume 12 volts * 1 amp = 12 watts just to stay on, not even amplifying anything. Once you give it a signal, it will amplify it and draw ANOTHER amount of power on top of the operational power, and it sounds like this will be up to 150 watts on top of the 12 in this example (this is assuming 100% efficiency, or 150 watts input power, but let's ignore that.) So really it all depends on what you are amplifying.

Another huge factor is the capacity of your battery (as Wesley Lee mentioned) but based on my experience I would say anywhere between 15 minutes to like two hours. There is huge variance based on the current drawn by the amp, and especially by the quality / capacity of your battery.

One way to know for sure is just go ahead and test it out, and bring some jumper cables just in case :)

Too many factors to say or sure.

However, 2 of them will cancel out to some extent, so let's give it a try.

On the one hand, your amplifier may take up to 300W or more to deliver 150W to a speaker; on the other, speech and music are usually quieter than their peak volume (or the sound will be very distorted) and that means lower power, so let's take 150W as the actual power you need.

For battery life calculations, it's better to know the current, I, = power/voltage. So,, 150/12 tells us we need 12.5 Amps.

If your battery is 55 Amp hours (as in a small car battery) AND it's fully charged, you'll get 55/12.5 hours = 4.4 hours out of it, then you'll need a ride home.

But it's a bad idea to run a car battery flat, and you don't know exactly how fully charged it is, so practically, 2 hours, and I'd run the engine between 20 minute sessions to top up the charge (and you'll notice if starting gets sluggish).

If that's not enough, get a "leisure battery" designed for deep discharge, as in campers, motorhames, boats etc - most motor parts stores will help you.