# Battery Pack Longevity (and Math)

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I've been reading several discussions on battery packs.  Did a little bit of math, and I think I came up with a way to get a decent estimate on how long a particular battery pack should last running Tetra.

Basic electrical theory:  Watts = Volts x Amps.

Tetra requires 24 watts "for normal stable operations" at 5vDC.  So the math works out to be 4.8 amps (A), or 4,800 milliamps (mA) -- (5 x 4.8 = 24)

Milliamp hours (mAh) is a measure of power over time.  So a 10,000 mAh battery pack provides 10,000 milliamps for 1 hour (at it's given voltage).

To figure out the runtime, you just divide the mAh rating by the milliamps the device is drawing:  10,000 mAh/ 4,800 mA = 2.1 hours.

A 20,000 mAh pack would provide about 4.2 hours, and a 28,600 mAh pack should give you about 5.6 hours.

Am I correct on this, or is my theory wrong?

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Oh, and of course the battery pack itself has to have at least 4.8 amps total output in either one or two USB ports.

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The biggest unknown is actual load of the tetra.  24 watts is most likely MAX with it running all transceivers TX at full power.  So the real question becomes what are the characteristics of the module you plan to run and its effect on power load? Lets say you are only scanning (RX) so your load will be lower meaning more battery life.

That is just one example.  If you are concerned about being super accurate for a project, I would recommend doing a load test for an hour with it in operational mode and see what the average is.

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Yeah, that's a very valid point.  This was more just my thinking trying to answer the question, "How big of a battery pack should I get?"

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