drifterf
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Run a pi3 on battery for a week ?

Thu Apr 11, 2019 12:35 pm

Hi guys I would like you thoughts on running a pi 3 for a week on battery

I can use a leasuire battery for this project so it's not a problem.

Could people let me know any calculations for running the pi on a battery so it will last a week. This project would involve using the WiFi but nothing else except a python script

MarkTF
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Re: Run a pi3 on battery for a week ?

Thu Apr 11, 2019 1:31 pm

1) Measure or estimate the average power consumption of your system in Watts. You need to provide this many Watts for 7x24 hours so multiply to get 1 week power consumption in units of Watt hours. This link provides some guidance: https://www.pidramble.com/wiki/benchmar ... onsumption. As an example 2 W for a Pi3 implies 336 Watt hours.

2) Battery banks are typically a different voltage than required by the Pi, so a suitable power converter will be required. There will be a conversion efficiency factor for this, typically on the order of 75% for a switching converter. Divide the Pi power by that efficiency to get the power at the battery. For the example above we have 336 Wh / 0.75 = 448 Wh minimum at the battery.

3) Batteries are specified in units of Amp Hours. Multiply this by the nominal voltage of the battery to get Watt hour capacity of the battery. You'll need one that can provide at least the capacity from step 2.

4) Recognize that batteries lose capacity with age, temperature, etc and that there is some (sometimes a lot of) shading of the nominal battery specifications and double the capacity from step 3.

5) If this absolutely positively has to work, test and measure the system components carefully to see if they match the specified performance under your specific operating conditions.

LTolledo
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Re: Run a pi3 on battery for a week ?

Thu Apr 11, 2019 1:35 pm

Hmmm.... 2 x 8D batteries(connected in parallel) and a buck converter might cover it.....
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W. H. Heydt
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Re: Run a pi3 on battery for a week ?

Thu Apr 11, 2019 1:35 pm

1. Determine the current requirements for your set up (best done by measurement).
2. Multiply by 5v to get power in Watts (W).
3. Add a "safety" margin, say 20%.
4. Multiply by 168 (the number of hours in a week. This gives you the total Watt-hours (Wh) you need.
5. Find a battery whose Wh capacity exceeds the number computed in step 4.

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NGC6543
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Re: Run a pi3 on battery for a week ?

Thu Apr 11, 2019 1:39 pm

I typed this up while making coffee etc, and folks beat me to it. You can have it anyway...


Have a look at these pages for some power consumption figures:
https://raspi.tv/2018/how-much-power-do ... asurements
https://www.pidramble.com/wiki/benchmar ... onsumption

Let's assume a 3B+, and a current draw of 500mA. That gives a power consumption of:

5V * 0.5A = 2.5W


Run that for a week and you need:

2.5 * 24 * 7 = 420 Watt-hours of energy.

Do you know the efficiency of your power supply? What are you using? Let's assume 85%, so you really need:

420 * 100/85 = 494 Wh

Convert your leisure battery Amp-hours into Watt-hours if necessary, assuming a 12V battery:

12 * Battery_Ah = Battery_Wh

How does the Battery_Wh stack up against the required Wh? Is anything else running from the battery? You should allow plenty of 'extra'; no batteries like being discharged deeply. Your 'average' leisure battery is generally 100Ah or more I think, so you will be fine unless it's also powering a fridge, stereo...

hippy
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Re: Run a pi3 on battery for a week ?

Thu Apr 11, 2019 10:51 pm

^ or you can condense that down to -

5 * Pi_current * 7 * 24 / 0.85 = 12 * Bat_current * 7 * 24

Bat_current = Pi_current * 0.5

hours = 2 * Bat_Ah / Pi_current


So, if a Pi is drawing 0.5A, a 100Ah 12V battery will give 400 hours of use, ~ 16 days.

If the Pi is drawing 2.5A, the same 100Ah 12V battery will give 80 hours of use, ~ 3.5 day.

Bat_Ah = hours * Pi_current / 2

So for 7 days use, 168 hours, at 0.5A you need a 42Ah 12V battery.

For 7 days at 2.5A you need a 210Ah 12V battery.

Or thereabouts.

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Imperf3kt
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Re: Run a pi3 on battery for a week ?

Fri Apr 12, 2019 12:53 am

You forgot to calculate voltage drop over time.
As you drain the battery the voltage will drop, and so the current draw will increase.

You probably also do not want to drain the battery to 0v, so should consider a much larger battery capacity - a 12v battery should never be drained below about 10v
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davidcoton
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Re: Run a pi3 on battery for a week ?

Fri Apr 12, 2019 10:38 am

Imperf3kt wrote:
Fri Apr 12, 2019 12:53 am
You forgot to calculate voltage drop over time.
As you drain the battery the voltage will drop, and so the current draw will increase.
True, but the starting voltage will be over 12V so the overall effect is small. Covered by the general allowance for conversion inefficiendcy and other losses.
Imperf3kt wrote:
Fri Apr 12, 2019 12:53 am
You probably also do not want to drain the battery to 0v, so should consider a much larger battery capacity - a 12v battery should never be drained below about 10v
Battery capacity is measured to a safe endpoint, not 0V, so this is entirely unnecessary.
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