Blender
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Long term battery source

Fri Jul 04, 2014 7:15 pm

Hi guys

First off I'm a complete noob. I have however managed to hook up a digital multimeter to my Pi to do some long term monitoring.

My problem is that I need to power my Pi for roughly 1 week on site where there is no power source or opportunity for solar. I was thinking of a car battery but was just wondering if anyone else has solved or know how to solve this problem. I think my B Pi consumes 500mAh. From this point on I'm lost.

I'm aware of the power banks that are available however these will only power a max of half a day I think.

Many thanks!!

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DaveDriesen
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Re: Long term battery source

Fri Jul 04, 2014 8:11 pm

Even a car battery will only power your pi for a few days. Remember in a car it's mostly the alternator that provides the power. The size of the battery will influence its durability but however big it is, it won't last for ages.

A week? TBH I see no easy wins here...

Dave Driesen
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redhawk
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Re: Long term battery source

Fri Jul 04, 2014 8:13 pm

What about the opportunity to switch between battery packs so you can take one home and recharge it??

Richard S.

Blender
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Re: Long term battery source

Fri Jul 04, 2014 8:18 pm

Unfortunately switching power packs isn't an option. Realistically what max length of time could be achieved? Budget is no option (within reason)!!

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Cancelor
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Re: Long term battery source

Fri Jul 04, 2014 8:58 pm

I run my RPi of a small 12v lead acid battery using one of these**:-
http://www.ebay.co.uk/itm/New-Ultra-Min ... 565cec5c7f

The battery is rated at 12Ah and I can quite reliably get 24 hours use out of it.

Amps x hours = 500mA x 24 = 12Ah :D

To run for a week 500mA x 24 x 7 = 84Ah

A 100Ah battery like the one below should cover it.
http://www.ebay.co.uk/itm/019-100Ah-4-Y ... 27df812d83


** adjust the output voltage before going anywhere near the RPi
Last edited by Cancelor on Fri Jul 04, 2014 9:02 pm, edited 1 time in total.
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ghans
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Re: Long term battery source

Fri Jul 04, 2014 9:00 pm

Perhaps a 250 $ truck battery ? A 225 Ah model should last
over a week.


ghans
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kathyjus
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Re: Long term battery source

Fri Jul 04, 2014 10:02 pm

I like Cancelor's solution. And if you can get a 100Ah plus battery that fits with your car, you can always use it when your current one dies on you. If you have a car, that is...
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drgeoff
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Re: Long term battery source

Fri Jul 04, 2014 11:19 pm

Cancelor wrote:I run my RPi of a small 12v lead acid battery using one of these**:-
http://www.ebay.co.uk/itm/New-Ultra-Min ... 565cec5c7f

The battery is rated at 12Ah and I can quite reliably get 24 hours use out of it.

Amps x hours = 500mA x 24 = 12Ah :D

To run for a week 500mA x 24 x 7 = 84Ah

A 100Ah battery like the one below should cover it.
http://www.ebay.co.uk/itm/019-100Ah-4-Y ... 27df812d83


** adjust the output voltage before going anywhere near the RPi
That is not the whole story!

That calculation does not take account of the car battery being 12 volts and the RPi needing only 5. If the 'conversion' is done with a simple linear regulator then more than half of the energy will be wasted as heat, although the calculation of running time will be basically correct.

Using a switching regulator to drop the 12 volts to 5 will be much more efficient, perhaps 85%. So the RPi taking 500 mA at 5 volts is 2.5 Watts. Assuming 85% for the converter means the battery will need to deliver 3 Watts which is 0.25 Amps from 12 volts. So a week of 168 hours needs a 42 Ah car battery. In practice I'd suggest a 60 Ah one intended for car use.
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ghans
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Re: Long term battery source

Sat Jul 05, 2014 8:31 am

Don't you damage a car bettery if you discharge it over 50 % ?

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drgeoff
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Re: Long term battery source

Sat Jul 05, 2014 8:34 am

ghans wrote:Don't you damage a car bettery if you discharge it over 50 % ?

ghans
I've never heard that before.
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FLYFISH TECHNOLOGIES
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Re: Long term battery source

Sat Jul 05, 2014 8:44 am

Hi,
ghans wrote:Don't you damage a car bettery if you discharge it over 50 % ?
50% of its nominal capacity ? No.

Each battery has just low voltage limit. When the cell's voltage falls below some value, you must stop using it (and then start charging, because the battery will continue to discharge itself even if there is no load attached to it - the discharging rate will be lower, but still significant enough not to be ignored).
The cell's exact low voltage limit value depends on the battery's technology.


Best wishes, Ivan Zilic.
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mikerr
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Re: Long term battery source

Sat Jul 05, 2014 8:50 am

Use a Model A if you can get away with it on anything battery powered
1/3 the power usage (150ma vs 450ma for B), 3x the run time, or 1/3 the size of battery required !


turning off the HDMI / PAL video output also saves around 20ma :

Code: Select all

/opt/vc/bin/tvservice -off
every little helps !
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redhawk
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Re: Long term battery source

Sat Jul 05, 2014 9:05 am

What exactly are you monitoring and how often do you have to collect samples??

Richard S.

ghans
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Re: Long term battery source

Sat Jul 05, 2014 9:08 am

OK , 50 % is more of a rule-of-thumb to achieve the
best compromise between battery lifetime and economic
utilisation of nominal capacity , it seems.
A lead-acid battery would still work if discharged deeper if you
keep the terminal voltage above 11,8 V or so.

http://www.mpoweruk.com/life.htm


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Cancelor
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Re: Long term battery source

Sat Jul 05, 2014 11:04 am

Interesting web site, looks like temperature is the big killer, so keep it out of the sun. If it's out doors then the RPi might get upset if it's to humid.

Between 500 and 800 cycles should be achievable (10 years plus).

I wonder how mission critical is this set-up? If it runs out of juice after 6 days is that the end of the world?

If you spend £50 on a battery only to find it's not up to the task, do you loose your job?
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kfalconspb
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Re: Long term battery source

Mon Jul 07, 2014 6:50 am

Can you place it in a tile ceiling above a emergency exit sign or emergency light? Tap the power to recharge a smaller battery(maybe run off batteries internal to the emergency light). Cover it with insulation and it will be less likely to be discovered until you can safely return to collect the device. Depending on the objectives and sensitivity of it being found later, you could collect the data via wifi or usb cellular modem (tmobile prepaid ;) ) and leave the Rpi as a disposable tap/collector.

Blender
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Re: Long term battery source

Mon Jul 07, 2014 11:02 am

Thanks Guys.

The Pi will be left on the underside of a bridge, monitoring a potential probe within the concrete in mV to find out if the reinforcement is corroding or not.

I currently have it hooked up to a 12V 110Ah battery for a trial. I have connected a cigarette lighter socket to the battery, then a standard usb stepdown to 5V connected to the cigarette lighter.

Seems to be working great so far.... I let you know how it goes.

Thanks

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Re: Long term battery source

Tue Jul 08, 2014 8:59 am

Just read an interesting project using a tiny 15W turbine for wind power and pi:

http://www.switchdoc.com/2014/07/wind-p ... pberry-pi/
http://www.switchdoc.com/2014/01/small- ... open-loop/
We got about 60 – 90ma of current at 15MPH

a bit short of the 150ma needed for a model A
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Cancelor
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Re: Long term battery source

Tue Jul 08, 2014 9:06 am

Blender wrote:Thanks Guys.

The Pi will be left on the underside of a bridge, monitoring a potential probe within the concrete in mV to find out if the reinforcement is corroding or not.

I currently have it hooked up to a 12V 110Ah battery for a trial. I have connected a cigarette lighter socket to the battery, then a standard usb stepdown to 5V connected to the cigarette lighter.

Seems to be working great so far.... I let you know how it goes.

Thanks
*makes a mental note to check under my local bridges for free RPi and car battery ;) *

Mind you the anti terrorist squad might beat me to it!!! :twisted:
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AndrewS
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Re: Long term battery source

Tue Jul 08, 2014 12:25 pm

Blender wrote:I have connected a cigarette lighter socket to the battery, then a standard usb stepdown to 5V connected to the cigarette lighter.
A cheap cigarette-lighter -> USB adaptor may well be using an inefficient linear regulator (and even then the output might not be very well regulated if it's only designed to be a 'phone charger'). Much better to use a proper 12V -> 5V switching regulator, which will be more efficient and therefore give you longer battery life.

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redhawk
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Re: Long term battery source

Tue Jul 08, 2014 12:33 pm

To follow up from previous poster I would suggest you carefully take apart the ciggy power adapter and inspect the circuit.
Some cheap and nasty regulators are linear, if however you find a IC with 8 legs (sometimes with #34063 markings) then it's safe to say you have a switching mode regulator.

Richard S.

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Re: Long term battery source

Tue Jul 08, 2014 1:34 pm

redhawk wrote:To follow up from previous poster I would suggest you carefully take apart the ciggy power adapter and inspect the circuit.
Some cheap and nasty regulators are linear, if however you find a IC with 8 legs (sometimes with #34063 markings) then it's safe to say you have a switching mode regulator.

Richard S.
I was thinking just about all the 12V to 5V (USB) car adapters would be switchers. A linear would generate too much heat for that application.
I look for the switching transformer to id a switcher.
Unless specified otherwise my response is based on the latest and fully updated RPiOS Buster w/ Desktop OS.

Blender
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Re: Long term battery source

Sun Jul 20, 2014 8:38 am

Hi guys

Just an update on the Pi and car battery set up. I managed to get just over 9 days out of the Pi which I was well pleased with.

Thanks for your help

faraday
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Re: Long term battery source

Thu Jul 24, 2014 10:04 pm

mikerr wrote:Just read an interesting project using a tiny 15W turbine for wind power and pi:

http://www.switchdoc.com/2014/07/wind-p ... pberry-pi/
http://www.switchdoc.com/2014/01/small- ... open-loop/
We got about 60 – 90ma of current at 15MPH

a bit short of the 150ma needed for a model A
Hi, just saw this post. The 60-90ma is off the 50W wind turbine, not the 15W wind turbine. Sad but true.

Best regards, Faraday

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