Solarize Your Backpack and Power All Your Gizmos

In this instructable I'll show you how to build a detachable solar panel and battery charger for your backpack. This can power or charge all your gadgets (cell phone, mp3 player...) while on the road. HAMs can use it to power small QRP transmitters and receivers on a field day etc.
I was inspired by the excellent instructable "Rain or shine solar charger" by Blondietheblond, but wanted to avoid sewing, since I don't have a machine. So this is the result.

Step 1: Gather the Stuff You'll Need:

0) a backpack (duh):

Most backpacks have enough possibilities to attach stuff. Mine provided these ribbons on the back. No idea what they are supposed to be used for.

1) for the solar panel:

- 4 Encapsulated 2V/200mA solar panels (Velleman SOL4).
- Self adhesive pads for cable ties (3M Scotchlok Ab02).
- Nylon cable ties to fit the adhesive pads above.
- Velcro adjustable cable binders (Tesa 55236-00000, 12mmx20cm).
- Heat shrink tubing.
- Electric wire (capable of carying 0.5A is more than enough).
- Connector to go to the battery box.

2) for the charger/battery box:

Parts list for this is less critical, so improvise. There's an explanation with important details ahead.

- Small plastic project box.
- 4 AA-size NiMH batteries (GP 2700 series is ok, see further).
- Battery holder .
- Two 2-pin panel-mount connectors of some sort.
- Components (see further).

...and some tools:

- soldering iron.
- cutters.
- pliers.
- sharp knife.
- drill for making holes in the project box.

Step 2: Assemble the Solar Panel:

- Attach two adhesive Scotchlok pads per encapsulated solar panel on the back.
- Use nylon cable ties of correct size to lace the panels together. The "head" of the ties goes in the middle of the pads. This fixes the position of the small solar panels.
- Connect the panels in series (red-to-black etc) and solder. Insulate with heat shrink tubing.
- At the end you will have a black and red cable ( minus and plus). Solder a 0.5m cable to this and attach a connector plug to go to the battery box, as in the picture.

Tip: I placed 4 panels in a row. You can also place them in a rectangle or other shape that fits your backpack, the important part is that the small panels have a more or less fixed position relative to each other, so they don't slide over each other.

Step 3: Attach the Panel to Your Backpack:

Now comes the "detachable" part.
Attach the panel using the velcro cable binders and whatever nooses you have on your backpack.
To get the power cable to the inside, I used the opening that's there for headphone cables.

Step 4: The Charger/battery Box:

Some theory about NiMH batteries:

The solar panels used here are rated 2V/200mA in full sunlight. I used 4 in series to that gives me 8V/200mA or 1.6W. Now, I want to use this to charge 4 NiMH AA batteries rated capacity C=2600mAh.
How do we know the batteries will be full?
A decent NiMH charger checks the temperature and also voltage drop at the end of the charging. However, to be able to check for the small drop at the end of charging, the charging current must be something like C/2 (the capacity, divided by two, without the "hour").
In full sunlight I measured the short circuit current of the panel to be 270mA, so about C/10. This is the short circuit current, so at higher voltages (the battery voltage) the panel will charge the batteries at less current than C/10. Constantly charging batteries at low currents (compared to the capacity/hour) is called "trickle charging".
Now, it used to be in the past that NiMH batteries did not handle trickle charging well, if above C/20 or even C/50. However modern NiMH can be safely (for the battery I mean) trickle charged at C/10.
Add to that the fact that the sun won't be up all the time, and we conclude that our charger can be very simple: one diode.
The diode makes sure that the batteries can't discharge back in the solar panel, once the sun is down.

What about the load?

NiMH can take quite a load (up to 2C),but there is one thing that they don't like and that's deep-discharge.
Deep discharge, meaning drawing current from the batteries when their voltage is below a certain point (0.9V ... 1.0V) will shorten their life time considerably.
Obviously, there are two things you can do to prevent deep discharge:
1) use a switch to disconnect the load when the batteries are low.
2) use some electronic circuit to disconnect the load, once the voltage is low.

I used the second method in my battery box to prevent overdischarging the batteries: when the voltage is above 1.2V / cell (4.8V for the pack) the output is connected. Then, when the voltage drops below about 0.9V/cell or 3.6V for the pack, the output is disconnected, until the solar panel charges the batteries sufficiently again. In this way you do not need to wurry about overdischarging.

I attached the data sheet for the batteries used here, which shows the charge and discharge characteristics.

Step 5: The Charger/battery Box (2):

Now build the battery box.

Drill holes for the connectors, one coming from the solar panel and one to go to your gizmos.
I used the same type of connector for both, in hindsight it's better to use different types.
Prepare box for battery holder if necessary.
The batteries go in a battery holder with a slide door, for which a rectangular hole in the side of the box had to be cut.
Glue the circuit diagram in the lid of the box, in case you'll ever have to open it again.
Glue the circuit board in the box with hot melt glue (or use any other method you like).
Label the box "in" for solar panel and "out" for loads.

The circuit is built on a piece of perfboard (note the MOSFETs I used are SMD and placed on the underside of the board). Nothing is crictical in this circuit, for all components you can use replacements:

- The diode can be any Schottky diode capable of something like 30V/ 0.5A.
- The p channel mosfet needs a low on voltage (Vsg<3.9V at currents you like to use it), it should also have low Rdson (<0.2 ohm or so) you can parallel them to increase current handling capability.
- The resistor from base to emitter of the BC547 is 62kohm, I forgot to label it.

Adjustment:
Once the circuit is ready, connect a load (say 100 ohm).
Then using a power supply, go up slowly starting from zero volts. Adjust the trimmer so the circuit trips at 4.8V. Then go down again, the circuit should disconnect the load somewhere between 4.0V en 3.6V NOT LOWER. To adjust the lower threshold, you can also replace the 470kohm resistor with a trimmer if needed.

Step 6: Put the Box in Your Backpack and You're Done!

Some more ideas/hints/tips:

- The solar panels aren't critical, just make sure V> Vdiode + 1.2V*number of batteries and Ishort circuit- A fuse on the output may be interesting, especially if you want to power stuff you built yourself.
- The overdischarge circuit itself uses about 7uA when off and 60uA when on. This is far less than the average self discharge current of the batteries.
- Remember there is no power conditioning on the output: the gadgets are suppose to have their own regulation. check this before you use them with this battery box.
- From the output of the box, you need some cables to go to your gizmos. Some cheap "wall wart" power supplies come with a set of connectors you can use to build these cables.