Understanding solar for a small project

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pomtom44

124 posts

Master Geek

#247943 4-Mar-2019 11:17

Hi all
Im setting up a small raspberry pi based project, which will be solar powered, and im trying to wrap my head around what ill be needing to get it to work.

My understanding is you have this as a setup

Solar ------ Controller ------- Battery
I
I
Device

and the charge controller relays power between the components.

so looking at jaycar I have found the following

https://www.jaycar.co.nz/12v-5w-solar-panel-with-clips/p/ZM9050
https://www.jaycar.co.nz/miniature-12v-3a-pwm-solar-charge-controller/p/MP3762
https://www.jaycar.co.nz/12v-2-2ah-sla-battery/p/SB2482

Based on some quick math, I know the battery will last for aprox 4 hours at full load.
Ill be running the Pi about 10 mins per day, so I calculate about 20-25 days usage from the battery alone.

So looking at the solar side, would the panel and charger I got work with the battery?
and assuming minimal usage from the Pi, would the charger be able to keep the battery topped up?

Sorry if its a stupid question, still new to solar and trying to understand it

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#2190618 4-Mar-2019 11:22

In general, a panel, controller, and battery is what's needed. I have a similar setup used to charge some batteries for emergency use. I find that attaching it to a good quality car battery charger occasionally to refresh it helps.

What's the base load of the Pi? If it's on all the time it will draw some power. If there's any kind of internet access / auto update it could use more power than expected.

SLA batteries generally shouldn't be discharged below 50%.

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Aredwood

3885 posts

Uber Geek

#2190619 4-Mar-2019 11:23

Main thing to consider. Is that solar panels only output 10-15% of rated power during cloudy weather.

Would it still work with only 0.5W from the solar panel?

pomtom44

124 posts

Master Geek

#2190624 4-Mar-2019 11:35

timmmay:

In general, a panel, controller, and battery is what's needed. I have a similar setup used to charge some batteries for emergency use. I find that attaching it to a good quality car battery charger occasionally to refresh it helps.

What's the base load of the Pi? If it's on all the time it will draw some power. If there's any kind of internet access / auto update it could use more power than expected.

SLA batteries generally shouldn't be discharged below 50%.

The pi will be on for 10-15 mins per day, with one of these to control on and off
https://spellfoundry.com/product/sleepy-pi-2/
which claims to draw 200uA when idle.

There will be no internet connection, just a camera module (which was included in my calculations)

Should also add, there's no power where it will be deplyed, hence the need for solar.
So it would need to be able to be self sustaining

pomtom44

124 posts

Master Geek

#2190630 4-Mar-2019 11:39

Aredwood: Main thing to consider. Is that solar panels only output 10-15% of rated power during cloudy weather.

Would it still work with only 0.5W from the solar panel?

So you think get a more powerful panel?

wellygary

8171 posts

Uber Geek

#2190647 4-Mar-2019 12:22

If its going to be remote installation, then double the specs on whatever panel or Battery you settle on,

The last thing you want to have to do is go out in the middle winter to trouble shoot it after the battery runs flat...

outdoorsnz

660 posts

Ultimate Geek

#2190692 4-Mar-2019 14:10

Most likely best to look at a older low power consumption raspberry pi 1. I had one of these working off a simple solar panel.

https://www.raspberrypi.org/products/raspberry-pi-1-model-a-plus/

The newer ones draw 2.5A so may drain your battery quicker.

pomtom44

124 posts

Master Geek

#2190698 4-Mar-2019 14:19

wellygary:

If its going to be remote installation, then double the specs on whatever panel or Battery you settle on,

The last thing you want to have to do is go out in the middle winter to trouble shoot it after the battery runs flat...

The problem is I dont understand enough about what I need in the first place, so I wouldn't know what to double.

pomtom44

124 posts

Master Geek

#2190699 4-Mar-2019 14:20

outdoorsnz:

Most likely best to look at a older low power consumption raspberry pi 1. I had one of these working off a simple solar panel.

https://www.raspberrypi.org/products/raspberry-pi-1-model-a-plus/

The newer ones draw 2.5A so may drain your battery quicker.

I was looking at the pi zero
Same power usage with a smaller footprint.

timmmay

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#2190702 4-Mar-2019 14:25

The new Pi's might manage to draw a max of 2.5A, but it will depend on power consumption. My PC has a 500W power supply, but most of the time it draws about 90W, 120W under heavy load.

I guess you need to cater for worst case. Say you get 12H of cloud each day, a 5W solar panel is effectively (according to the post above) going to be 0.75W. Multiply that out, allow for some loss charging the battery, work out the power stored in a day. Then work out power draw and see if you need a higher capacity solar panel.

If you visit the site, another option might be to buy two medium sized batteries, charge one at home, and swap them occasionally.

raytaylor

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#2191077 5-Mar-2019 09:11

You may need a 12v to USB adapter to convert the voltage down if the input is USB

Or a 12v to 5V adapter if the input is a 5v DC barrel connector.

Assumptions for battery
- 2.5amps draw at 5v thats approx 1.25 amps at 12v.
- 10 mins run time per day is 0.25 amp hours on the battery.
- The battery should not have a current draw of more than 10% of storage. So that means even though we only draw 0.25 amps per day, the current being drawn during the runtime is 1.25 amps. 1.25 amps is 10% of 12.5 amps. So the battery needs to be at least 12.5ah of capacity. Based on that we can use a 14ah battery because I am pretty sure you can get them at that size.

Solar Panel assumptions

- We have a 5 watts per day current draw (12v x 2.5amps x 0.1666 hours = 5 watts)
- On a cloudy day the solar panel will output 10% of its rating if it is at the correct sun angle and conditions are good, short cable run etc. We shall assume 5% if the panel is mounted flat.
- 5 watts load is 5% of a 100 watt panel. Over 6 hours of overcast cloudy light thats only 16 watts. I think you could get away with a 10 watt panel which is only $60 from jaycar or a 20 watt panel that is $100 from jaycar.

Solar controller
- A 20 watt solar panel at 12v = 1.66 amps
- A 2.5 amp load
- Minimum solar controller size is therefore 2.5 amps, the greater of the two.
- Jaycar MP3750 is a 10 amp solar controller but is very cheap - less than $50 so will do the job well. Although it has a USB output, its only 1.25 amps so you will need a seperate one to power the device.

Summary
- A 14ah battery gives you a month+ of daily usage at 10mins per day
- A 20 watt solar controller fully charges the battery on a cloudy day
- A good quality 12v 14ah SLA battery (UPS type/Alarm) should last a couple of years+ if kept between 20-25 degrees.

Ray Taylor

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timmmay

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#2191082 5-Mar-2019 09:17

I got a 12V to USB converter from ebay - something like this. The bare wire ends will usually go into the "load" output of the controller, but otherwise you can solder large crocodile clips onto it and connect them direct to the battery.

pomtom44

124 posts

Master Geek

#2191111 5-Mar-2019 09:38

raytaylor:

You may need a 12v to USB adapter to convert the voltage down if the input is USB

Or a 12v to 5V adapter if the input is a 5v DC barrel connector.

Assumptions for battery
- 2.5amps draw at 5v thats approx 1.25 amps at 12v.
- 10 mins run time per day is 0.25 amp hours on the battery.
- The battery should not have a current draw of more than 10% of storage. So that means even though we only draw 0.25 amps per day, the current being drawn during the runtime is 1.25 amps. 1.25 amps is 10% of 12.5 amps. So the battery needs to be at least 12.5ah of capacity. Based on that we can use a 14ah battery because I am pretty sure you can get them at that size.

Solar Panel assumptions

- We have a 5 watts per day current draw (12v x 2.5amps x 0.1666 hours = 5 watts)
- On a cloudy day the solar panel will output 10% of its rating if it is at the correct sun angle and conditions are good, short cable run etc. We shall assume 5% if the panel is mounted flat.
- 5 watts load is 5% of a 100 watt panel. Over 6 hours of overcast cloudy light thats only 16 watts. I think you could get away with a 10 watt panel which is only $60 from jaycar or a 20 watt panel that is $100 from jaycar.

Solar controller
- A 20 watt solar panel at 12v = 1.66 amps
- A 2.5 amp load
- Minimum solar controller size is therefore 2.5 amps, the greater of the two.
- Jaycar MP3750 is a 10 amp solar controller but is very cheap - less than $50 so will do the job well. Although it has a USB output, its only 1.25 amps so you will need a seperate one to power the device.

Summary
- A 14ah battery gives you a month+ of daily usage at 10mins per day
- A 20 watt solar controller fully charges the battery on a cloudy day
- A good quality 12v 14ah SLA battery (UPS type/Alarm) should last a couple of years+ if kept between 20-25 degrees.

Good math, but a few corrections.
Pi will be running of 12v not 5v as the sleep controller runs of 12v.
Pi wont be running at 2.5 amps, reading online i see it will draw no more then 300mA

pomtom44

124 posts

Master Geek

#2191112 5-Mar-2019 09:39

timmmay:

I got a 12V to USB converter from ebay - something like this. The bare wire ends will usually go into the "load" output of the controller, but otherwise you can solder large crocodile clips onto it and connect them direct to the battery.

Pi controller runs of 12v so wont be needing a voltage converter :)

raytaylor

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#2191625 5-Mar-2019 20:25

Awesome so we could go with a 7amp hour battery.

*IF* you do that, it is important that you use no more than a 10 watt (0.8amp) panel so you dont overload the battery when recharging. It really shouldnt be charged at more than 0.7 amps (7ah / C10) so a 5 watt panel would probably be much better (0.4amps), though I wouldn't advise going smaller.

Ray Taylor

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pomtom44

124 posts

Master Geek

#2191902 6-Mar-2019 07:58

raytaylor:

Awesome so we could go with a 7amp hour battery.

*IF* you do that, it is important that you use no more than a 10 watt (0.8amp) panel so you dont overload the battery when recharging. It really shouldnt be charged at more than 0.7 amps (7ah / C10) so a 5 watt panel would probably be much better (0.4amps), though I wouldn't advise going smaller.

Isn't that what the controller is for? to stop overcharging?

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