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I'm looking to build a portable amplified speaker and want to know how long a 1200W car audio amplifier will run on a 12v 50ah battery supply?

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  • \$\begingroup\$ Will you be playing it as loud as you can all the time? \$\endgroup\$ – user65586 Mar 15 '16 at 2:24
  • \$\begingroup\$ At times probably but not all the time \$\endgroup\$ – Antonio Moreno Mar 15 '16 at 2:25
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    \$\begingroup\$ I was being a bit cute. It's hard to answer this question because "1200W" isn't a lot of information, actually. Even assuming this is a stereo (or multi-channel) amplifier and the stated wattage is over two or more channels, it also depends on the design of the amp and what you are driving. \$\endgroup\$ – user65586 Mar 15 '16 at 2:29
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    \$\begingroup\$ I'm not sure if a question about the run time of a retail amp is suitable for this SE, but given you already have a model with ratings and possibly published specs, you could make a guess. Though, honestly, you should just measure how much current it draws when idle, and how that changes as you increase the output, and you will get some idea of run times. More important is paying attention to how the battery is drained and recharged. Some batteries really respond poorly to being run flat, so absolute run times aren't so helpful. \$\endgroup\$ – user65586 Mar 15 '16 at 2:38
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    \$\begingroup\$ Thank you for that it helps a lot ill look at it im hoping for a good result \$\endgroup\$ – Antonio Moreno Mar 15 '16 at 2:44
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As the comment thread states, you can make a guess, but it might be wildly off.

If you measure the current that is being pulled in a 'typical' use case, then your wattage becomes much less of a variable.

Watts = Volts * Amps
Watt*hours = Volts * Amp * hours

Watt-hours in battery = 12V * 50A-hours = 600Wh

This assumes that the battery actually stays at 12V (it doesn't). You can measure the voltage drop as you run the battery down to further remove this variable. Most 12V batteries are nearly out of energy when they are at 12V. Consult your manufacturers data sheet for details and adjust accordingly.

So, we have 600Wh of energy in our battery. If we run it at 1200W, then we have 1/2 hour of energy in our battery. If we run it at 600W, then we have 1h of energy in our battery. If we run it at 300W, then 2h ... and so on ...

W-h / W = hours

The precision with which you measure voltage and current - along with the useful life of the battery - will determine how close you get to the real-world value.

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    \$\begingroup\$ Your '1200W' amplifier is most probably not, ie car audio tends to inflate the specs, or the 1200W is a 'burst' figure for a few milliseconds. You really need to make some proper measurements in order to get any sort of idea of what is really going on. \$\endgroup\$ – F. Bloggs Mar 15 '16 at 10:53
  • \$\begingroup\$ Agreed. That is why the 2nd line says 'measure the current'. \$\endgroup\$ – slightlynybbled Mar 15 '16 at 13:26

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