Large solar panel for the MightyMintyBoost

The MintyBoost and MightyMintyBoost (MMB) are both very cool instructables. I just had to build one and since I am into solar I wanted to go with the MMB project. I decided to use the larger solar cell for my MMB project because I would like to be able to charge my iPhone every day with my MMB and would like to go pure solar if I could. If you have an iPhone you know they are pretty power hungry. I typically need to top off my battery during the day, and recharge every night. The MMB with its large 2000 mAhr battery allows you to do that. It contains over 1 1/2 charges for the iPhone 3GS, which has a 1150 mAh battery. That's enough to get through all but the toughest days with both daily top-offs and an over-night full charge.

The smaller solar cell recommended in the MMB project is great because its nice and compact. But its small size limits the amount of power it can generate. It has an output of about 100mA, which would take about 20 full-sun hours to charge the MMBs 2000mAh battery. A sunny winter day produces about 4.5 full-sun hours and a summer day about 8.5. An average day year-round is about 5 full-sun hours, including cloudy weather. Those numbers depend on the local climate and the figures given are for my area, which is near San Francisco right on the coast. Therefore the small solar cell will take on average 4 days to fully charge the MMB battery. That is fine performance if you only intend to use it for topping off and will charge you iPhone from the grid over-night. But if you want to go pure solar like I do, you need a bit more juice from your solar cell

The big solar cell produces 310 mAh, which can fully charge the MMB battery in just 6.5 full-sun hours. Thats pretty good and should satisfy your iPhone's power appetite on a daily basis much of the year except maybe in the winter when you are playing lots of games or something. I was able to verify that charge time the first time I charged my brand new MightyMintyBoost. But to get that full on performance it helps to have the cell held at the optimal angle to the sun and to turn in occasionally so that it tracks the sun during the day. So I designed a support frame for my solar cell that has three different tilt angles to optimize the cell's angle for different times of the year.

Features:

Enough power generated to fully charge and top-off an iPhone 3GS on solar power alone.
Voltage regulated so you can use cells with more than 7 volts output.
Three tilt angles optimized for Winter, Equinox (spring and fall), and Summer sun angles.

Refer to the MightyMintyBoost Project for the construction of the MightyMintyBoost battery and charger part, which is the stuff in the Altoids tin. This project covers how to make a frame for and how to wire a larger solar cell.

Step 1 Tools and Materials

Just a reminder. This Instructable only talks about making the frame and installing the large solar cell. See MightyMintyBoost for instructions about how to make the rest of the project.

Tools:
An Analemma (see second picture)
Soldering iron
Wire cutters
Wire Stripper
Hot melt glue gun
Wood saw (hand or power)
Miter box (if you use a hand saw)
Carpenter's square (if you are cutting by hand)
Drill motor
7/64" drill bit
Sand Paper

Materials:
Large Solar Cell
Voltage Regulator, 5v
3/4 No. 8 wood screws (qty 8)
Hot melt glue
Rubber cement
Super Glue (optional)
Brush on electrical tape
Finished lumber 1x2 x 18 long (poplar is very nice, but any wood will do)
Birch Plywood, 1/4 thick, 2 pieces at least 5 x 4 each

Notes:
The MightMintyBoost charger will accept up to 7 volts input, but this solar cell puts out 9 volts so it needs to be regulated down to keep from frying the charger. Because the cell produces smooth DC voltage, capacitors that are sometimes used with the 7805 voltage regulator are not required. The voltage regulator will only dissipate about 1.5 watts so a heat sink is not required.

I have both a cabinetmakers table saw and a crosscut saw, which makes this project super simple, quick and very accurate. However if you dont have all that stuff dont worry, it can easily be done with hand tools if you are careful and take your time and maybe make a practice cut or two if you havent done much wood work before. There are only a few cuts to make so you can take all the time you like and still finish the project quickly. Making accurate 90 degree cuts, especially when cutting the 1x2 to length, are important to making the project fit properly so do make use of a miter box. The more precise the lengths are the better the final product, but it will work just fine even if things are not perfect so don't sweat it.

I have been itching to get a chance to use the word "analemma" in text ;-). I used it to estimate when to use the winter, summer and equinox tilts. More on that later.

Comments

[onitreb]

Do you need to be concerned that your panel supplies 310mA to the LiPo charger which only "accepts" 280mA from the DC plug? Does the LiPo charger just convert the 30 or so mA's to heat? Any idea how much current you can throw at the charger before something bad happens?

[hmbscott]

I'm not expert, but I doubt it would be good to over current the charger. However there are a couple of things to consider. The charger probably is voltage controlled and if the panel is not outputting enough voltage it cannot output that much current either. If it does need to limited, an inline resistor of an appropriate value will act as a current limiter, similar to how they are used with LEDs. I hope that helps.

[dmhjah]

So I got a solar cell around the same size as the one your using. On a sunny but overcast day it put out 7.06v when connected to the voltmeter. On a clear day I don't know. It is supposed to have a Voc of 6v so I don't know where the 7 is coming from. I guess the safest thing I need know is how much over 7v can the charger take. Thanks David

[hmbscott]

The overcast-day voltage you measure should be a accurate value for Voc, unless the light level is very very low. The Voc measure should be independent of illumination intensity if its reasonably bright out. I am guessing here but I think you probably can get away with using that cell without a voltage regulator because of the quiescent load the charger would present to the cell. Plus it seems very unlikely that the charger is going to be fried by going 60mV over its max input limit. If you want to make very sure that your cell will not fry your charger you could add a resistor wired across the cell leads to load it down just a tiny bit. I am guessing here but I expect a 1K ohm resister would be enough to drop the max cell voltage below 7V. It would draw about 7mA at 7V. That small current draw from the cell should pull its voltage down a bit from Voc and protect the charging circuit. You can check cell voltage, which must be done in full sun, with a 1K resistor wired across the leads, that will tell you if this resistor value will work. If not, you can adjust the resistor value. A lower resistance value will pull the voltage down more and vice-versa. So fiddle around until you find a resistor that pulls the voltage down to just under 7V in full sun. Then add that resistor across the cell leads in your circuit. It basically will act as a poor-man's voltage regulator.

[dmhjah]

So if I have a solar cell that puts out 6v 200mA and it bright sunlight it hits, for a brief moment 7.2 v. Will this automatically fry the charger? Or should I use the voltage regulator to make sure that doesn't happen? Thanks David

[hmbscott]

These are great questions. It has made me think about this a bit and really improved my understanding of how this gizmo works. If the cell is rated for Voc (voltage open circuit) of 6V and Isc (current short circuit) of 200mA, then it will not produce more than 6V no matter how bright the sun. You could even use mirrors to increase light intensity to greater than a full sun and the Voc voltage would not increase (current does increase in that case and can exceed Isc). Voc voltage is related to how the cell is wired internally and the wavelength of the photons it converts, but not light brightness. Note that under a real load, actual cell voltage will increase with increased light and can approach Voc, but Voc is the maximum possible voltage the cell can produce. So if you Voc is <7V it would be safe to use that cell without a regulator. However if the Voc rating is much greater than 7V, I would consider using a regulator even though the cell may actually operate a few volts under the Voc rating when connected to the charging circuit - and when there is charging load. If you stop charging, either by disconnecting the battery, or switching to the trickle charge mode when the battery is fully charged, the voltage may float up since the load is reduced, which may cause damage. My cell has a Voc rating of about 9V, which is why I used the regulator. Note that voltage regulators need a couple of volts higher input to work properly. The Fairchild data sheet for there 7805 has a minimum input voltage requirement of 7V. So don't try to hook a 6Voc cell to a 5V regulator. You need a cell with at least a 7V output, preferably a couple of volts more than that to accommodate the real cell voltage droop under load. The 9V cell in this project is a near perfect match for the 5V regulator for that reason. It has enough voltage headroom to produce near maximum current under load and drive the voltage regulator. The 9V cell should in fact operate pretty near its Mpp (maximum power point) running into the 5V regulator. If the cell rating were closer to the regulator's minimum voltage input requirement, I expect the cell voltage would be forced up to the regulator’s minimum voltage input, which would cause the cell current to droop. That might be pretty substantial actually if the Voc rating were near the minimum input voltage for the regulator. That implies that a cell with say a Voc of 7.3 V might be a poor choice for this project using a 7805 regulator since the cell might not put out anywhere near its Isc current rating in that configuration. This relates back to you first question. Which the short answer for my cell is there is not much current reduction, probably less than 10mA, but in the case of a cell with a Voc rating nearer to say 7V, the regulator may considerably reduce cell current. Thanks for asking, Scott

[dmhjah]

Thanks, So if I went out and used a volt meter and it was greater than 6v then that would mean it would have to actually be a Voc > 6 since I got up to 6.8x which is not high enough for a 5v regulator. So the next question where do I get a solar cell like the one your using?:) Thanks David

[hmbscott]

David, You should be OK with a cell that has a Voc of 6.8V. The Sparkfun LiPoly Charger used in the MightMintyBoost project will take a 7.0V input. So that cell should be fine without a voltage regulator. If you want to get the same cell I used. Its the Large Solar Cell from Sparkfun. There is a link to its product page on the Step 1 page of my instructable. Scott

[dmhjah]

Hi, I see how to install the voltage regulator. My question is does it still push the same amps even though the voltage dropped down to 5v? Thanks David

[hmbscott]

According to the data sheet the device consumes 5mA typical with a max of 8mA. The charging load will have more effect. Solar cells have a characteristc IV curve, so the circuit impedance the cell is connected to will determine its output voltage and current. The data given for the cell are the open circuit voltage and the short circuit current. The probable operating point will be closer to the short circuit current end of the IV curve in this case. The following link on page 10 of the document shows a typical IV link. This cell may be running at 6-7 volts in this circuit. Note also that the IV curve is only accurate for full sun illumination. The endpoints don't change with reduced light but the middle of the curve droops. IV Curve