ananthu0000
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battery in mAh needed for running rpi3 for an hour..?

Tue Dec 06, 2016 7:11 pm

how can i calculate how much mah of battery is necessary for running a rpi3 for 1 hour assuming rpi3 uses 5V 2A

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mikronauts
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Re: battery in mah needed for running rpi3 for an hour..?

Tue Dec 06, 2016 7:16 pm

5v * 2a = 10W, so for one hour that is 10Whr

USB power bank, 2A 5V, 10,000mAh / 0.85 (efficiency of in-built converter) = 11,7640mAh

11,764 * 5.0/3.7 = 15897mAh

So you need a power bank with a minimum of 16,000mAh rated capacity
ananthu0000 wrote:how can i calculate how much mah of battery is necessary for running a rpi3 for 1 hour assuming rpi3 uses 5V 2A
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dasmanul
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Re: battery in mah needed for running rpi3 for an hour..?

Tue Dec 06, 2016 7:23 pm

Out of curiosity: Where does the 3.7 come from?

wildfire
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Re: battery in mah needed for running rpi3 for an hour..?

Tue Dec 06, 2016 7:24 pm

FWIW I bought a 1800MaH power bank for £10 from the corner store, primarily planned to power a zero (a lot less power requirements) but it should be capable of powering your Pi3 for an hour or so.
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gregeric
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Re: battery in mah needed for running rpi3 for an hour..?

Tue Dec 06, 2016 7:38 pm

dasmanul wrote:Out of curiosity: Where does the 3.7 come from?
Powerbank vendors quote their capacity in mAh based not on the output voltage, but on that of the internal cell. Bigger number = better to the uninformed purchaser. The value 3.7 is the voltage of an assumed internal lithium cell.

Heater
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Re: battery in mah needed for running rpi3 for an hour..?

Tue Dec 06, 2016 7:40 pm

@ananthu0000,

How many milliamps are there in an amp? Yes, that's right 1000.

So if you want to supply 2 amps for an hour, that is 2000mA for an hour or 2000mAh.

Let's say you used a big old 12 lead acid battery and a 7805 linear regulator to make 5v for the Pi. The current demand will still be 2 amps. So you need a 2000mAh battery.

Problem is you are wasting half your energy as heat in the regulator.

More realistically you would use a modern switch mode buck converter to make 5v from 12v because it's far more efficient. Then you may only need half the current so a 1000mAh battery would do.

Conversely you maybe using a 3.7 volt Li-ion battery and a boost converter, in which case it will have to deliver twice the current (very approximately) to make the necessary power. You will need a 4000mAh battery.

Remember, power equals volts times amps.

You requirement is 5 volts times 2 amps = 10 watts.

If you use the 12 volt battery and buck converter we have 12 volts times 0.8 amps = 10 watts.
That is 800mAh.

If you use that li-Ion battery and boost converter we have 3.7 volts times 2.7 amps = 10 watts.
That is 2700mAh.

You will need to take the efficiency of the converter into account as mikronauts points out.

@mikronauts,

Woah, 16 amps to run a Pi! I think not :)

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Re: battery in mah needed for running rpi3 for an hour..?

Tue Dec 06, 2016 7:59 pm

Hi Heater,

OP wants 10W for 1hr, that is 10,000mAhr

Divide by .85 to compensate for a modern buck converter, compensate for the "informative" 3.7V mAhr rating of USB power banks, then run the math... you get 16,000mAhr that you need from the USB 5V bank quoted at 3.7V

Mind you, the Pi3 is unlikely to draw the 2A the OP was assuming constantly, I gave worst case calculation.

Don't get me started on the "interesting" choice of rating power banks by the 3.7V capacity... which degrades... and not counting 5V conversion efficiency.

With RoboPi, a Raspberry Pi Model B, and a WiFi stick, I can get ~3h-4h run time from a 5200mAh Patriot Fuel Cell+ drawing average of <400mA from the Fuel+ (I run the motors from a separate NimH battery pack) ... I'll have to do some run time tests with Berry (Pi3 based, w/RoboPi, wifi, etc)
Heater wrote: @mikronauts,

Woah, 16 amps to run a Pi! I think not :)
LOL... it is not... it needs 16AHr capacity at the 3.7V quoted power banks at the OP's figure of 2A draw @ 5V :)
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Re: battery in mah needed for running rpi3 for an hour..?

Tue Dec 06, 2016 8:16 pm

mikronauts,
OP wants 10W for 1hr, that is 10,000mAh @5V
No.

He wants 10 watts from a 5 volt supply. That is 2 amps or 2000mA.

He wants to do that for an hour. So our mythical 5v battery will need a capacity of 2000mAh.

You have mixed up units there, watts times time cannot result in mAh.

An actual battery is going to put out more or less volts so it will have to deliver less or more current to meet the power requirement when converted to 5 volts.

Then as you say there is the efficiency of the converter to think about.

@dasmanul,
Out of curiosity: Where does the 3.7 come from?
3.7 volts is what it says on the side of my Tenergy LI-ION batteries. LiPo is the same isn't it?

If you are using a power bank that is a bit of a lottery. As noted above the mAh rating they claim may well be the internal battery rating, not what you actually get a the 5v USB output. Even then I bet there is a lot of exaggeration going on.

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dasmanul
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Re: battery in mah needed for running rpi3 for an hour..?

Tue Dec 06, 2016 8:23 pm

Thanks for the explanation, both of you. I'm a bit surprised at this though. If someone sold me a 5V 10.000mAh power bank with the capacity based on 3.7 volts I'd consider that close to actual fraud.

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mikronauts
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Re: battery in mah needed for running rpi3 for an hour..?

Tue Dec 06, 2016 8:25 pm

Ooopsie!

I need more coffee...
Heater wrote:mikronauts,
OP wants 10W for 1hr, that is 10,000mAh @5V
No.

He wants 10 watts from a 5 volt supply. That is 2 amps or 2000mA.

He wants to do that for an hour. So our mythical 5v battery will need a capacity of 2000mAh.

You have mixed up units there, watts times time cannot result in mAh.

An actual battery is going to put out more or less volts so it will have to deliver less or more current to meet the power requirement when converted to 5 volts.

Then as you say there is the efficiency of the converter to think about.

@dasmanul,
Out of curiosity: Where does the 3.7 come from?
3.7 volts is what it says on the side of my Tenergy LI-ION batteries. LiPo is the same isn't it?

If you are using a power bank that is a bit of a lottery. As noted above the mAh rating they claim may well be the internal battery rating, not what you actually get a the 5v USB output. Even then I bet there is a lot of exaggeration going on.
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MarkTF
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Re: battery in mah needed for running rpi3 for an hour..?

Tue Dec 06, 2016 8:30 pm

Depending upon the operating environment there may be additional battery derating for temperature and for high discharge rate.

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mikronauts
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Re: battery in mah needed for running rpi3 for an hour..?

Tue Dec 06, 2016 8:33 pm

Updated calculation, with mythical 10,000mAr battery bank based on 3.7V internal voltage:

10Ah * 3.7V = 37whr

37whr/5V = 7.4Ahr @ 5V

7.4Ahr * 0.85 efficiency = 6.29Ahr @ 5V

So such a power bank should run a Pi3 drawing 2A constantly for ~3.1 hours

Now I'll go make some coffee.
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Heater
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Re: battery in mah needed for running rpi3 for an hour..?

Tue Dec 06, 2016 8:51 pm

@mikronauts,
...I need more coffee...
Make it an Irish Coffee.

It's Independence Day holiday here and I'm on the Baileys :)

@dasmanul,
...I'd consider that close to actual fraud.
Me too. Still, that is one of thousands of frauds consumers have to put up with now a days.

Mind you, my one and only power bank I have here actually states: 12000mAh @ 3.7V in the small print on the back. As far as recall that was not in the big print on the packaging.

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Re: battery in mah needed for running rpi3 for an hour..?

Tue Dec 06, 2016 9:15 pm

mikronauts wrote: OP wants 10W for 1hr, that is 10,000mAhr
Thou shalt not mix units! What you have given is equating 10Wh with 10Ah. Not the same thing at all.
OP wants 10W for 1 hour, so he needs a capacity of 10Wh, or 10K mWh, if you want to be silly about it.

Assuming for the moment that the battery will provide 2A at 5v (most *small* "powerbanks" are only rated for 1A), and taking note that the Ah rating are actually at 3.7v for the reasons people have already given, to get 10Wh you need 10Wh/3.7v = 2.7Ah. Adding a bit for the converter efficiency, one could call it 3Ah, or 3000mAh.

Again, taking ignoring that most small "powerbanks" are only rated for 1A output, one can observe that the two typical small units are 2400mAh and 4000mAh. I would go with the 4000mAh unit...if you can find one that will provide 2A.

Grump...doesn't *anyone* study dimensional analysis any more?

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Re: battery in mah needed for running rpi3 for an hour..?

Tue Dec 06, 2016 9:25 pm

All in all, I suppose we should be grateful that this is all DC. If it were AC we'd have the problem of the marketing department doing all the labeling in PP voltage, while the equipment usage would be spec'd in RMS voltage and Those Who Know would be having to explain where the 1/2 sqrt 2 came from.

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Re: battery in mah needed for running rpi3 for an hour..?

Tue Dec 06, 2016 9:42 pm

Now I guess we have totally confused our OP :)

I have here a USB power bank sold as 12000mAh. Dual output, maximum of 3400mA (For both I presume).

The small print mentions the 3.7 volt thing but never mind that, what does it actually do?

As I recall it ran my Pi 3 for about 3 hours, which seems to be in line with what someone calculated above. That is with the Pi just idling doing nothing mind.

I am now going to try it again and will report back...

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Re: battery in mah needed for running rpi3 for an hour..?

Tue Dec 06, 2016 10:02 pm

I use an 11Ah powerbank with 1A and 2A outputs to power a Pi2B and RPF 7" display. Not doing anything in particular, it will run for about 11 hours. That's probably on the high end of expectations. I also use a 4Ah powerbank to run a Pi Zero with a 3.5" display (connected to the GPIO block). With a static, graphic image, it will run in excess of 12 hours. Monitoring power, it typically draws from 180mA to 190mA with occasional higher spikes (probably writing to the SD card), so call it 1W. That is in line with expectations, given the usual assumptions about Lithium batteries.

(To show the math on the latter system... 180mA-190mA, rounding up to 200mA times 5v = 1W. 4Ah times 3.7v = 14.8Wh. Expected run time: 14.8h, dropping it a bit for converter efficiency and discharge limits--do NOT fully drain a Lithium battery--call it 13h. The math is simple if you put both source and drain in the same units: Watts for power, Watt-hours for energy. For DC, but not for AC, W=vA.)

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Re: battery in mah needed for running rpi3 for an hour..?

Tue Dec 06, 2016 10:42 pm

I know, it was sad.

Heater already pointed out my caffeine-free mistake :)

My bad for posting quickly without sufficient caffeine.

Off-the-cuff quick&dirty analysis without coffee = mistake can easily happen.
W. H. Heydt wrote:
mikronauts wrote: OP wants 10W for 1hr, that is 10,000mAhr
Thou shalt not mix units! What you have given is equating 10Wh with 10Ah. Not the same thing at all.
OP wants 10W for 1 hour, so he needs a capacity of 10Wh, or 10K mWh, if you want to be silly about it.

Assuming for the moment that the battery will provide 2A at 5v (most *small* "powerbanks" are only rated for 1A), and taking note that the Ah rating are actually at 3.7v for the reasons people have already given, to get 10Wh you need 10Wh/3.7v = 2.7Ah. Adding a bit for the converter efficiency, one could call it 3Ah, or 3000mAh.

Again, taking ignoring that most small "powerbanks" are only rated for 1A output, one can observe that the two typical small units are 2400mAh and 4000mAh. I would go with the 4000mAh unit...if you can find one that will provide 2A.

Grump...doesn't *anyone* study dimensional analysis any more?
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ananthu0000
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Re: battery in mah needed for running rpi3 for an hour..?

Wed Dec 07, 2016 7:44 am

https://drive.google.com/open?id=0B4JqQ ... lgtTTlTelE
https://drive.google.com/open?id=0B4JqQ ... WY4QnNOWFU

check those images i have fully charged the above power bank of 10400mah and its been power the rpi3 for last 2 hours continously and power indicators of power bank still shows 4/4(above 75% left)

i will be waiting till the power bank completly discharged

tpylkko
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Re: battery in mah needed for running rpi3 for an hour..?

Wed Dec 07, 2016 7:51 am

ananthu0000 wrote:https://drive.google.com/open?id=0B4JqQ ... lgtTTlTelE
https://drive.google.com/open?id=0B4JqQ ... WY4QnNOWFU

check those images i have fully charged the above power bank of 10400mah and its been power the rpi3 for last 2 hours continously and power indicators of power bank still shows 4/4(above 75% left)

i will be waiting till the power bank completly discharged
This information might be nice but do you think that it will answer the original post where the poster specifically asks how much capacity is needed to run a pi3 at 2 amps for an hour? Is so, how?

EDIT: I just realized you are OP. So, are you running it on 5V 2A?

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Re: battery in mah needed for running rpi3 for an hour..?

Wed Dec 07, 2016 8:31 am

An idle Pi 3 uses a little over 1 watt. Not many Pis out there using 10 watts (5 volts x 2 amps). It's likely just the reading off the AC adapter.

Regards,
-Moses
Power problems? MoPower UPS for the Pi
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Re: battery in mah needed for running rpi3 for an hour..?

Wed Dec 07, 2016 5:36 pm

mosespi wrote:An idle Pi 3 uses a little over 1 watt. Not many Pis out there using 10 watts (5 volts x 2 amps). It's likely just the reading off the AC adapter.

Regards,
-Moses
Yes, and this was already stated in like the second post, but it still does not answer the question, nor frankly does it have much to do with it.

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Re: battery in mah needed for running rpi3 for an hour..?

Wed Dec 07, 2016 6:31 pm

http://mypi.tech/index.php?helpbookname ... helppage=2

That will run a Pi3 for 24 hours, will run more than one pi, the maths is in page 1 the home page

I designed, built and tested it, it has run a pi2 and pi3 through many test cycles. you can click the drawing to down load it and use the cct program on page 1 to print and edit it.
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Re: battery in mah needed for running rpi3 for an hour..?

Wed Dec 07, 2016 7:10 pm

tpylkko wrote:
mosespi wrote:An idle Pi 3 uses a little over 1 watt. Not many Pis out there using 10 watts (5 volts x 2 amps). It's likely just the reading off the AC adapter.

Regards,
-Moses
Yes, and this was already stated in like the second post, but it still does not answer the question, nor frankly does it have much to do with it.
I should have been a bit more clear. I'll rephrase. I believe OP is mistaken on his true power consumption. A lot of people read off the label and assume that's that. The runtime tests should clear things up if that's the case.

Regards,
-Moses
Power problems? MoPower UPS for the Pi
http://www.allspectrum.com/mopower/

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Re: battery in mah needed for running rpi3 for an hour..?

Wed Dec 07, 2016 9:42 pm

tpylkko wrote: Yes, and this was already stated in like the second post, but it still does not answer the question, nor frankly does it have much to do with it.
Well, that is a bit blinkered.

The original question was answered (as it turns out, inaccurately, due to caffeine deficiency -- but this was corrected).
However, to experienced Pi users it appears that the OP may be asking the wrong question. It is by no means clear that ananthu0000 is running a Pi with a total consumption of 2A. Therefore it is relevant and helpful to explore what the actual scenario is, and then to explain the misunderstandings that led to a wrong question. It is the nature of this forum (which supports an educational project) to explore wider issues that are of educational value or general interest -- often posters are taught how to answer their own question, rather than just given an answer without imparting any understanding of why it is the answer.

If you think an answer is off-topic, you should report it to the mods, not make comments which can inflame things. And you could always give your own concise and correct answer to the OP.
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