linkstatic
Posts: 2
Joined: Wed Nov 06, 2013 5:31 am

Running RPi for 20 mins on battery

Wed Nov 06, 2013 5:34 am

Hello everyone, really excited to be here ^_^. So I have this project in which my RPi is suppose to run for max 20 mins and minimum of 12 mins. I wanted to know what could be the lightest battery to power the RPI in this case. Answers related to Button batteries will be much appreciated. I have the understanding that I can use a UBEC to control my voltage. So any ideas what batteries should I use? Weight is a major issue here.

Dutch_Master
Posts: 362
Joined: Sat Jul 27, 2013 11:36 am

Re: Running RPi for 20 mins on battery

Wed Nov 06, 2013 11:18 am

Try batteries from the world of R/C models (planes, boats, cars, etc). You'll need an additional voltage regulator between it and your RPi, with a minimum of 1Amp capacity.

fixitsan
Posts: 58
Joined: Sun Sep 29, 2013 4:28 pm

Re: Running RPi for 20 mins on battery

Wed Nov 06, 2013 9:38 pm

Lightest is going to be lithium polymer.

If your Pi isn't driving anything too power hungry allow for 600mA average current. (you had better measure the current being drawn first)

To power the Pi for 1 hour will therefore require 600mAH of capacity. For 12 minutes yoiu need 600/5= 120mAh capacity and for 20 minutes you need 600/3=200mAh capacity.

Allow a UBEC efficiency of 75%

Now, the trick is that you use a battery which can deliver the high current, whilst being a low capacity cell.

If you need 200mAh of capacity, at 600mA current, then a 200mAh cell needs to be rated for 3C (3 x capacity) of current.

What you tend to find is that as the current goes beyond the 1C value for most batteries (1C for a 200mAh cell = 200mA of current.....1C for a 600mAh cell = 600mA ), then yoiu see a noticeable reduction in overall capacity

This means that to get 200mAh of capacity from a cell, while drawing 3C current (600mA) then you need a very high performance cell. When looking for a suitable cell always check what current load the capacity was measured at. For example a 1000mAh cell, measured at 0.2C current (200mA), might only have a capacity of 700mAh when the current rises to 0.6C (600mA)

I'm sure this is getting confusing, but the basic trick is to measure your current demand first, then look for cells which are strong enough to deliver that current continuously. It isn't enough these days to assume that a 1000mAh cell can deliver 1000mA for 1 hour, because battery capacity isn't always measured at a 1C discharge rate.

Chris

NewWorld-UN-order
Posts: 2
Joined: Sun Nov 10, 2013 4:14 pm

Re: Running RPi for 20 mins on battery

Sat Dec 14, 2013 8:40 pm

Hey, there is some interesting info here thankx ...

But can anyone help im looking for a 2nd opinion, comments & recommendations if any

Would this voltage regulator iv found on amazon work with the Pi set to 5v obviously, but connected directly to the Raspberry Pi's power input port, without making any mods to the Pi its self???
Only i hear how fussy the Pi is when it comes to powering off-grid

DEOK LM2596 DC 5/12V Voltage Regulator Adjustable Power Buck Converter+LED Voltmeter
Link:-
http://www.amazon.co.uk/DEOK-Regulator- ... +regulator
...............................
Specific:
Module Model:LM2596 Buck Converter+Voltmeter
Cutting Size: 66.04mm x 35.05mm
Input Voltage: DC 4.0~40V
Output Voltage: 1.25V~37V (continuously adjustable, the input voltage must be 1V higher than the output voltage)
Output Current: 2A(normal and stable), 3A Max.
Ripple Frequency: About 150Khz
Ripple Peak: About 100mV
Display Color: Red
Voltage Meter Error: + / - 0.1V
Measure Range:4~40V (for measure accurately, please ensure that the input voltage is more than 4V)
IN+: Input Postive
IN-: Input Negative
Out+: Output Postive
Out-: Ouput Negative

Features:
A key switch input and output voltage
green light: measuring the output voltage
red light: measuring the input voltage
with terminal and retain the wire bonding wiring point
If remove the P3 jumper cap, the voltmeter shutdown
Lower Power Consumption
...............................

The specs look to be ok to me but im no pro il be honest, im still learning :)
Im a little hesitant to connect direct to the power port il be honest i can't afford to replace the Pi at the moment & in desperate need to power the Pi by batteries/solar soon as possible as i maybe forced homeless very soon ...
I have 6x 2000Ah 3.7v laptop batteries made into a 7.4v 6000Ah pack, & also 2x small scooter 6v 12Ah batteries from Maplins & have had the Pi working off-grid via a cheap in-car 12v to 5v USB regulator recommended from a youtube video but too much consumption needing 12v (both batteries) to just power the in-car regulator to get a 5v output. its a nice fast easy off-grid set up but i would like to power the pi from just one 6v 12ah battery as im charging the other, have the laptop battery pack as a back up or alternative if there is alot of sunshine out to charge both 6v's, the in-car USB regulator doesn't work on 6v but the LM2596 DC 5/12V Voltage Regulator does, and i have two regulators ... does anyone think im all set to go as it is & ok to connect the LM2596 regulator directly to the power port on the Pi without mods???
Oh & would i need a little resistance as:- Output Current: 2A(normal and stable), 3A Max. or would this not be an issue, i can power radio equipment & neon lights from the regulator without resistance, im thinking there might not be need for resistance, help HaHa

Thankx for your time reading people

User avatar
rew
Posts: 428
Joined: Fri Aug 26, 2011 3:25 pm

Re: Running RPi for 20 mins on battery

Sun Dec 15, 2013 8:34 pm

yeah, those LM2596 converters work. I've had a 'pi running on one of those for about a year now. It's powered from a 12V wall-wart.

I would recommend getting a 200+ mAh 20C 2S LIPO battery. That will put out 7.4V nominal, allowing for the inefficiency of the DCDC converter. The raspberry pi will use about 400mA average, allowing up to 30 minutes of battery time. In theory you could go even smaller. The 20C batteries you can get at the RC hobby shops. forget the "you need 3C" argument. Just get some margin. :-) Use 20C.

Chose a model-A if you can. A big chunk of the power on the raspberry pi is used by the USB/ethernet chip. Buy one without that chip and it will use less power....
Check out our raspberry pi addons: https://www.bitwizard.nl/shop/

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