# Solar power arduino

I'm interested in building a solar powered outdoor light with an arduino as the main controller. I want to be able to output a lot more light than the normal ones you can buy but just for a few hours after the sun goes down to save the battery.

This is a schematic for what I'm thinking of doing. Is the idea reasonable?

The solar cell I have in mind is Small Solar Panel 80x100mm 1W - Seeed Studio

For batteries, I plan to use 2 NiMH AA batteries.

The cell is rated at 5.5v 170mA. But when it's held across the batteries, will that pull down the voltage to a level so I don't hurt the 2.4v battery? I'm worried that 170mA is too high for this, but I believe that's a very optimistic rating from the manufacturer and I'll be lucky to see half that in bright light. They do also sell a solar cell that's half the size and current output.

The temperature has been pushing close to 40 celsius here lately, is that a problem for the batteries?

I plan to read the solar panel voltage directly on the Arduino A0 to know when it's dark out. I know I will need to keep the arduino at low power when it's in standby.

Will the solar panel lose power if I enclose this in a glass jar or plastic box?

Thanks for any input you can give me.

In general, with solar panels, it is cheaper to use a bigger panel than to use "clever tricks" to get more from a smaller panel.

Why not use 3 or 4 AA cells for a higher voltage that more nearly matches the solar panel. Also, there would be no need for the wasteful voltage booster.

The battery will only pull down the solar panel voltage while the cells can take more current than the panel can produce. While that is happening you are just wasting solar energy (170mA at 2.4v = 408 mW whereas 170ma at 4.8v = 816 mW).

When the battery cannot take the full current the voltage will rise to the solar panel voltage and will probably ruin the 2.4v battery.

Do you really need an Arduino? Maybe you could just use a small electronic circuit triggered by a Light Dependent Resistor (LDR).

If a Nano uses more energy than a barebones Atmega 328 I would use the 328 on its own.

...R

The solar cell I have in mind is http://www.seeedstudio.com/1W-Solar-Panel-80X100-p-633.html

Typical voltage: 5.5V
Typical current: 170mA
Open-circuit voltage: 8.2 V

For batteries, I plan to use 2 NiMH AA batteries.

I want to be able to output a lot more light than the normal ones you can buy but just for a few hours after the sun goes down to save the battery.

Oh, my... solar projects are some of the more difficult ones...

The "lot more light" is the BIG gotcha here.

1. you must establish a power budget first. Using the LED technical reference determine the power requirement over the time you intend on supplying light... this will give you the A/H or mA/hour DC requirement needed from the battery pack.

2. next add Arduino mA/hour requirement for On state

3. next add Arduino mA/hour for sleeping state

The total should be your 24/hour mA/H budget.

1. select rechargable + solarcell to match power budget in cloudy weather... Or write your s/w to only work when your batteries are appropriately charged (or as you suggested, monitor the batteries Voltage.)

The better battery for youd project is likely
LiFePO4

Glass and plastic "may" affect the solar cell efficiency somewhat, best to profile the enclosure material with the exact solarcell you intend on using ... the worst-case will be with the sunshine striking the glass/plastic at an angle... so be certain to measure the cell output at non-perpendicular angles.

IMO, 3.2V LiFePO4 with 3.3V Arduino is smarter.

Ray

Robin2:
Why not use 3 or 4 AA cells for a higher voltage that more nearly matches the solar panel. Also, there would be no need for the wasteful voltage booster.

The battery will only pull down the solar panel voltage while the cells can take more current than the panel can produce. While that is happening you are just wasting solar energy (170mA at 2.4v = 408 mW whereas 170ma at 4.8v = 816 mW).

When the battery cannot take the full current the voltage will rise to the solar panel voltage and will probably ruin the 2.4v battery.

Do you really need an Arduino? Maybe you could just use a small electronic circuit triggered by a Light Dependent Resistor (LDR).

If a Nano uses more energy than a barebones Atmega 328 I would use the 328 on its own.

I figured that 2 cells hold enough power to keep my project running for a couple of days without full sunlight, so only 2 of them would keep the size a bit smaller.

From what you say though, it sounds like this will destroy the batteries with my 2 or even with 4 when they are fully charged.

The arduino seems to be the easiest way to keep the lights on for only a couple of hours. Plus if I do use smart LEDs I need an MCU to control them. This part is optional, but I figure when I've got the MCU anyway adding some colour change features will be a nice mode.

The nano will use more power for the power LED (which I can remove) and for the USB driver chip (which I can't). I do have a few bare Atmega 168's lying around too.

mrburnette:
The "lot more light" is the BIG gotcha here.

1. you must establish a power budget first. Using the LED technical reference determine the power requirement over the time you intend on supplying light... this will give you the A/H or mA/hour DC requirement needed from the battery pack.

2. next add Arduino mA/hour requirement for On state

3. next add Arduino mA/hour for sleeping state

1. 300mA * 2 hours = 600mA/day

2 and 3 are very rough guesses since I haven't done much with Arduino sleep mode yet, but I've figuring 30mA on and 1mA sleep. So 2x30 + 22*1 = 82mA/day.

That makes the total 682mAh/day at 5V. Approx 1.4Ah/day at 2.4V. If I average 100mA from my solar cell, that's 14 hours of sunlight to charge it. So that may be a bit optimistic. Using 4 cells in the battery as Robin2 suggested will bring it to 7 hours of sunlight to charge which is quite realistic.

1. select rechargable + solarcell to match power budget in cloudy weather... Or write your s/w to only work when your batteries are appropriately charged (or as you suggested, monitor the batteries Voltage.)

I'm actually monitoring the solar cell voltage in the current design, though I could monitor the battery voltage as long as I have the boost converter to give me a constant reference voltage. Cloudy weather give so much less power than sunny, I'd prefer to plan for sunny with enough battery capacity for 2 days and accept the fact it's not going to run for long if I have more than a few days of clouds.

IMO, 3.2V LiFePO4 with 3.3V Arduino is smarter.

I thought about that but I do still need 5V for the LEDs. Would it be a good option to use something like the solar shield http://www.seeedstudio.com/Solar-Charger-Shield-v2.2-p-2391.html which takes care of charging an Li-ion battery and providing 5V 600mA? The drawback is it makes the project physically quite large. And is it safe to leave the Lithium battery outside in -30 to +40 temperatures?

Slow down!

The VERY FIRST THING you need to do is find the LED combination that gives you "a lot more light", that is to your satisfaction, in the area to be lighted and measure the current draw at the voltage specified for proper LED operation.

Multiply that current in mA by LED operation hours to get XXX mAh.

The required battery capacity is twice XXX mAh, because battery manufacturers are always overoptimistic in their claims and battery capacity decreases with time and usage.

The rest of a properly designed circuit should consume negligible power compared to the LEDs, and can almost always be ignored in the design calculation.

The second step is to choose a solar panel that will provide about 1.5 times the battery capacity in mAh (at the battery voltage) under assumed lighting conditions, for example full sun. How many hours of sunshine do you expect at that spot, during different times of the year?

Oracle:
From what you say though, it sounds like this will destroy the batteries with my 2 or even with 4 when they are fully charged.

I don't think there would be a problem with 4 cells and 4.8v

...R

Robin2:
I don't think there would be a problem with 4 cells and 4.8v

Do you know if the temperature range will be a problem for the cells?

If the batteries don't get charged for a few days (clouds, etc), the boost converter will act like a joule thief and fully drain them, ruining them. If I use 4 cells, I won't need the boost converter, but will the arduino still drain the batteries to a damaging level? For that matter, will running the arduino at too low a voltage damage it?

jremington:
The VERY FIRST THING you need to do is find the LED combination that gives you "a lot more light", that is to your satisfaction, in the area to be lighted and measure the current draw at the voltage specified for proper LED operation.

Multiply that current in mA by LED operation hours to get XXX mAh.

The required battery capacity is twice XXX mAh, because battery manufacturers are always overoptimistic in their claims and battery capacity decreases with time and usage.

The rest of a properly designed circuit should consume negligible power compared to the LEDs, and can almost always be ignored in the design calculation.

The ring of 8 WS2812's will take at most 480mA, but 300mA is a reasonable figure for actual use. I have a version I've been using for a while with a ring of 12 LEDs and without the solar part (I take the batteries out and charge them inside off the AC power). It also don't have a dark detector, there's just a user-controlled switch. I'm reducing the solar version to 8 LEDs to reduce power consumption.

The original batteries I intended to use (AA NiMH cells from Eneloop, AmazonBasics, or Ikea) are well tested to provide their rated capacity beyond 1000 charge/discharge cycles. If I use an Li-ion or Li-polymer battery, I'll have to do my own testing on the battery, but so far with Li-ion 18650's, all the ones I've received have been good for their 2.2Ah. I suppose there are fraudsters out there selling factory defects or cells pulled from used and dead batteries, but you wouldn't design a project around the possibility of defective batteries.

I've never used the Li-Polymer packs that are commonly available, but I would do my homework on the actual power they provide.

jremington:
The second step is to choose a solar panel that will provide about 1.5 times the battery capacity in mAh (at the battery voltage) under assumed lighting conditions, for example full sun. How many hours of sunshine do you expect at that spot, during different times of the year?

I'm not sure what you're saying here. For 2.2Ah batteries, I should use a solar panel that will provide 3.3 amps? That will cook the batteries.

This time of year, I expect 14+ hours of good sun a day. In the winter, 6 hours would be a good day.

Hi,

If local temp is 40DegC, then panel temperature will be much higher, this means panel efficiency will be much less than quoted specs.

You need to read up on PV cells operation. They are a current source not a voltage source.

Tom...

For 2.2Ah batteries, I should use a solar panel that will provide 3.3 amps?

DO NOT confuse current (A) with battery capacity (Ah). They are completely different quantities.

1)300mA * 2 hours/day = 600mAh/day (corrected)

A solar panel that produces 365 mA current at some voltage, when perpendicular to full sunlight for one hour, produces 365 mAh. To charge a 2.2 Ah battery (2200 mAh) at a rate of 365 mA would take 6 hours. For a panel in fixed orientation, producing the appropriate voltage at 365 mA at noon, it would take much longer.

"Full sunlight" is approximately 10 AM to 2 PM, for a panel perpendicular to the sun at noon. Outside of those hours, the cosine of the angle from the sun to the panel perpendicular must be considered. On sunpower websites, you will find tables that estimate the effect.

It would be wise to have some means to disconnect the drain from the batteries before the voltage gets too low.

Another solution to that problem is to use batteries with a very much higher capacity (maybe 10 or 20 times more) so that they are unlikely ever to discharge by a large percentage if the sun continues to work as normal.

As someone else said it is VERY important to understand the difference between mA and mAh (and how they relate to each other).

...R

it would seem that you have a complete charging control system with safties for things line no use and over-charging the batteries. excessive sun, too much use and discharging the batteries.

as with any control system, everything can be covercome, you just have to put in the effort.

my thoughts have been to buy a pair of cel phone solar battery charger power supplies. these are cheap and self contained.

then just switch between them, let one charge, use one for power. that isolated me from doing the engineering needed for a well thought out and well designed system.

Robin2:
Another solution to that problem is to use batteries with a very much higher capacity (maybe 10 or 20 times more) so that they are unlikely ever to discharge by a large percentage if the sun continues to work as normal.

I came to the same conclusion.
To have a 24/24/365 solar power supply, I plan to use a solar panel that delivers in winter during daylight enough power to cope with about 2-3 times the total regular consumption of my device.

That means @50° latitude roughly the solar cells should nominally provide at least 30-50 times the device power consumption, just to provide an approx. power balance under winter conditions!
Then provide battery mAh for 5 to 10 days (to cope with a longer rainy period).

Least but not last, also provide a circuitry to dissipate adequately the excess of power during the hot summer days, where the solar panels will deliver around half to 3/4 of the nominal power.
Ideally send the excess of power to fans on order to cool the cells and device down.

Oracle:
Do you know if the temperature range will be a problem for the cells?

If the batteries don't get charged for a few days (clouds, etc), the boost converter will act like a joule thief and fully drain them, ruining them...
... The original batteries I intended to use (AA NiMH cells ...

NiMH cells are not ruined by an evtl. deep discharge, that applies for LiPo cells (and lead batteries).

The most common boost converters on eBay need about 10mA for themselves.
If you circuit needs 100 mA (or more) that solution should be OK.

@RIN67630, I think you are 12 months late for this party.

...R

Robin2:
@RIN67630, I think you are 12 months late for this party.

...R

I just thought it may be good to reopen the discussion and it might interest some other tinkerer as well.
You haven't got that much deep discussions about feeding 24/24/365 solar power on that site.

The challenge is quite a different one than to just charge batteries.
The optimization circuitry is also a complete different one, you have FIRST to get the best of bad light flooding conditions, e.g. have the least losses.