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Author Topic: How to calculate runtime?  (Read 4143 times)

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Offline willndmb

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How to calculate runtime?
« on: February 23, 2014, 10:56:54 AM »
I have been thinking of switching my batteries and/or gear up a bit but can figure out what battery to keep/get.
If a device uses 5.4 watts and has a 5 volt usb in, how do I see the runtime on a 3000mah, 3600, 10000 (whatever different battery I have/might get)
Does the amp output current effect anything? For example some batteries say 5v/2a while others says 5v/1a or even 3a
Thanks
« Last Edit: February 23, 2014, 10:59:43 AM by willndmb »
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Offline willndmb

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Re: How to calculate runtime?
« Reply #1 on: February 23, 2014, 01:58:40 PM »
Sweet thanks
So if I use a 10,000mah battery I would get approx 9.26 hrs (yes the 5.4 was max)

And if the original formula came out to be say 10w / 5v = 2A or 2000mA then I would want to make sure the battery was 5v and 2a

Always learn something from you
Thanks again
« Last Edit: February 24, 2014, 09:46:08 AM by willndmb »
Mics - AKG ck61/ck63 (c480b & Naiant actives), SP-BMC-2
XLR Cables - Silver Path w/Darktrain stubbies
Interconnect Cables - Dogstar (XLR), Darktrain (RCA > 1/8) (1/8 > 1/8), and Kind Kables (1/8f > 1/4)
Preamps - Naiant Littlebox & Tinybox
Recorders - PCM-M10 & DR-60D

stevetoney

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Re: How to calculate runtime?
« Reply #2 on: February 24, 2014, 09:12:40 AM »
Remember that when you calculate run-time in this manner that it will be an approximation.  Two things that will throw off your calculation are 1) if the device specs say 5.4W, that may not be nominal but peak power draw (which typically means you'll get longer run-times than your calculation), and 2) batteries aren't 100% efficient so you don't get out of them what you put into them (which would offset some of the gain you get from 1) above). 

For most accurate run times, I typically hook everything up the same as I would in the field and drain the batteries in real time.


stevetoney

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Re: How to calculate runtime?
« Reply #3 on: February 24, 2014, 01:11:49 PM »
^ OK, that makes sense. 

So, for example, I think you're saying that when a manufacturer specs out a device at 5.4W, the 5.4W number includes whatever losses have occurred between the battery and the device, such as through heat rejection or just that you don't get as many mah out of the battery as you put into it during a charge cycle.  So in doing a run-time calculation, you wouldn't need to put in any compensation for these inefficiencies.  Right?

« Last Edit: February 24, 2014, 01:14:01 PM by tonedeaf »

Online jerryfreak

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Re: How to calculate runtime?
« Reply #4 on: February 26, 2014, 12:19:20 AM »
also worth noting that many devices step down voltage internally.

AD2K uses 800 mA at 11V or 18V, obviously 11V is more efficient
same with a V2 or V3. they will run on 6V and if you feed them 12 they just burn off the extra as heat.
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stevetoney

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Re: How to calculate runtime?
« Reply #5 on: February 27, 2014, 07:31:42 AM »
 :coolguy:  Thanks Jon.

kirk97132

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Re: How to calculate runtime?
« Reply #6 on: March 01, 2014, 11:02:36 AM »
I = P / V, so 5.4 / 5 = 1.08A, or 1080mA.  Divide mAh by that to get runtime.  Note that 5.4W is probably a maximum rating, so your runtime may be longer.

You need a battery rated for at least 1A discharge.  Higher ratings don't affect anything per se, but a higher capacity battery of the same type probably has lower internal resistance, which means that the converter circuits inside a device may run more efficiently, giving a slightly longer runtime when comparing devices than predicted by the rating.
Jon, Kind of a theory question not sure if it is possible to answer with the limited info I can provide but here goes ...In regard to converter circuits inside an external battery.  Do they have the ability to handle a variety of amperage's as long as voltages are the same?  OR maybe rephrased, are the charging and/or output circuits just voltage dependent and able to respond with a variety of amperage's?  TIA

 

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