Solar Box #3
11-16-14
Introduction
Here is another little solar box that I've built in 2014. It will charge a cell phone
or help charge a tablet computer. It will also fit in a backpack for trips. This section talks
about how it works and how I developed and built it.
How it works
The box consists of a solar panel which charges a lithium ion polymer battery via a
charge controller. The battery feeds a small boost converter that feeds a USB port at
500ma. The phone plugs into the USB jack for charging. On the front is an ammeter to
monitor the performance of the solar panel.
Justification
I like to try to build cool things that work. It's one thing to build something neat.
It's another to make it work and be practical. Otherwise, in my view, it's just a 5 minute wonder
that takes up space, gathers dust, and has to be dealt with eventually.
One thing that I find useful is a small portable solar powered charger. This
represents the third attempt to build one that will be quite useful.
The first solar box worked quite well. I used it for a year to charge my cell phone.
I even used it on the boat. But it was heavy and inconvenient to lug around. Eventually
I stopped using it and put it away. The built in battery self discharged over time and
sulfated. The particular charge controller used appeared to allow slow self discharge and
required a good bit of light to overcome that.
On the other hand the first box allowed me to understand a few things.
- Solar panel charging characteristics under various lights and shadowing.
- The box needs an ammeter to indicate how well the solar panel is working at any moment.
- A voltage meter with a push button switch is needed to observe the state of charge.
Solar Box Attempt #2
This charger using 8 D-Cell batteries was highly effective. The batteries lasted for
over a week's worth of charging so I could take pictures without restriction.
My second attempt to build a practical solar box was a smaller box
which also used a 12v lead acid AGM battery. Unlike the first box I put the ammeter
in front where it was visible. But it failed to gain traction as a useable device because
it was still too heavy and bulky. It would be best for it to sit at home but I don't have
a south facing window for it. The project stalled. It currently sits waiting to be
repurposed somehow.
This charger using 8 D-Cell batteries was highly effective. The batteries lasted for
over a week's worth of charging so I could take pictures without restriction.
On a side note, when I went to Scotland in 2012 I took my iPhone 4S to take pictures with. I
quickly realized that the little battery in the iPhone wasn't going to work all day long. I
went to a Maplin store in Glasgow which is very similar to a Radio Shack in the States. I paid
30 pounds for 8 D cell batteries, holders, soldering iron, housing, 12 volt socket and wires.
I went back to the hotel and put it all together in about an hour. I plugged the phone into
the car charger and the car charger into the socket. It worked all week without the need to
change the batteries.
All of this lead to the idea for a smaller solar box about the size of the Scotland
charger but solar powered. And maybe a little bit lighter.
I spent some time on the Adafruit website. They have a lot of ideas and useful little
building blocks for projects. I looked at their Minty Boost USB charger and how it was used to
to charge cell phones and iPods. I then looked at their solar Lipoly handbag project. A plan came
together.
Solar Box #3 - Initial Operating Configuration
The USB port. This is the front of the Minty Boost. I built the wood
to snugly fit around the actual USB jack and hold the rest of the Minty Boost so that
it would stay firmly positioned.
I bought a 2500 mAh lithium polymer battery, 3.4w solar panel, and a Minty Boost. I wired
them up as instructed on the Adafruit website and tested it out. I even built a small box to
put it in. But without an ammeter and volt meter I was operating blind. I really need to
know if I need to put it out in the sun to charge and how well it is charging.
I wanted a nice little wooden box to hold everything. I determined the dimensions of
the box from the size of the solar panel and the hight of the ammeter when mounted
vertically. I found the rectangular appearance of the ammeter uninspiring. I prefer a sort
of vintage or steampunk look. To get to that or at least make it not an eyesore I
decided to mount the ammeter behind a small round window with a segmented wooden border.
I put the ammeter in the middle. I also
added a USB charging jack on the side. This jack is also surrounded by a smaller segmented
wooden border.
Over September and early October I did various tests. Basically I determined the following:
- The 2500 mAh battery charges the iPhone 4S once before needing to be recharged.
- When the box sits on the dash board in the car it takes 1 1/2 days to charge up.
- The solar panel works about half as well under the windshield as it does sitting
in direct sunlight.
- The Minty boost has voltage divider resisters for the two USB data lines. This is so the Minty
Boost will work with Apple products. This is discussed in great detail on the Adafruit site. These
discharge the battery over 2 or 3 weeks. I determined that a power switch was needed.
- If the box is charging an iPhone and the battery is depleted and the solar panel is not
putting out enough to feed both the charge controller shuts down. To "reboot" it the solar panel
must be put in the dark for a moment or disconnected. Then it works again. This is a bit disconcerting.
I will take more time to understand this characteristic of the charge controller before deciding what to do
about it. Meanwhile it can be worked around.
Solar Box #3 - Final Operating Configuration
The extra panels plugged in for increased charging
I beefed up the box by doing the following:
- Increased the battery to 6600mAh
- Added a pushbutton operated voltmeter to check battery state
- Added two plugin terminals on the back for extra solar panels
- Installed a switch to shut down the USB port when not in use in order to avoid battery drainage.
- Added a 10K thermistor to reduce charging current if the battery gets too hot.
During some initial testing on the boat the box charged at around 800ma with both extra panels
plugged in. Some later testing in a car showed problems presumably because of the heat buildup. The
car is not the best place to leave this.
Caveats
There are a few things to note:
- Do this project at your own risk!
- This project assumes a solid working knowledge of DC electronics.
- There is the risk of fire from the lithium ion battery
if it is mishandled or wired incorrectly. Be careful.
Parts List
Here is the list of parts I used an why. Most of these came from Adafruit.
- Solar Panels - Adafruit P/N 500. Note that these come with jacks measuring 3.5 x 1.1mm, not 3.5 x 1.3mm. They are quite durable and have small feet on their corners so they can be used as standalone panels.
- Solar LiPoly Charge Controller - Adafruit P/N 390
- Minty Boost Kit - Adafruit P/N 14
- JST 2 Pin Cable - Adafruit P/N 261
- DC Coaxial Jack - Radio Shack #2741569
- Solar LiPoly Battery - 2500mAh - Adafruit P/N 328 (IOC only)
- 10K Thermistor - Adafruit P/N 372
- Solar LiPoly Battery - 6600mAh - Adafruit P/N 353
- Mini 3-wire Volt Meter (0-99v) - Adafruit P/N 705
- DC Panel Mount Jack 1.1x3.5 - MCM Electronics P/N 27-5865
- Panel Ammeter - 1A - Dagatron 7203 or similar
- Momentary pushbutton switch - Radio Shack #2751547
- Miniature Toggle Switch - Radio Shack #2750613
The two external panels plug into these solar power jacks.
The insides are not the neatest wiring at this point.
On the left is a volt meter to measure battery state. On the right is a push button switch to
display the voltage. In the same hole is the switch to shut off the Minty Boost
Assembly
The Adafruit site has detailed discussions on how these parts operate
and how to assemble them. In summary the steps are as follows.
- Assemble the Minty Boost Kit. Insert the toggle switch on
the positive lead. Add the JST cable. Use sufficient wire to run the
switch to where it will be situated on the box.
- Assemble the Solar LiPoly Charge Controller. Then solder in a 1K
resister for 1 amp charging. Add the thermistor to protect against
possible overheating of the battery. Then add the 3 pin panel volt
meter to the battery terminals on the charge controller. Use the
battery positive for the power supply for the volt meter. As it turns
out the volt meter works well down below 3 volts.
- Wire up the main solar panel to add the 2.1MM connector and the ammeter
in series on the positive side. Wire up the external 3.5x1.1 jacks in
parallel. The other solar panels will be plugged in to increase the
charging current. The 2.1MM connector plugs into the charge controller.
Testing
Testing of the box shows the following:
- On the boat it works well and gets a good charge if it sits out all
afternoon. With both external panels plugged in it gets enough charge
in about 2 hours to charge a cell phone. Also shadows don't really
stop the charging as much.
- Under the windshield in direct sunlight it tends to overheat. The
10K thermistor steps in and shuts down charging as described in the
Adafruit website.
- On cloudy days it gets maybe 100mA with no overheating.
- The push button volt meter works as long as the battery is
not completely discharged.
Future plans
- More precise voltmeter. The current volt meter measures the state
of charge in one tenth of a volt increments. This gives only a rough
idea of the state of charge. I have on order a volt meter that looks
the same but displays in one hundredths of a volt. I plan to install
this right after I get it.
- I'm weighing the possibility of building an array of 6 panels that
are held together with cloth hinges and wired to an extender that will
plug into the back of the box. On cloudy days this should give a good
charge.