March 16, 2014

Solar Messenger Bag

Solar Messenger Bag, deployed!Now that I commute via train, my Phone, Kindle, and Bluetooth headphones are getting a lot of use. Since I have a terrible memory and forget to charge them, I wanted some "backup" power. I looked into making a Mintyboost (a device charger that can use AA batteries), but decided to take it a step further and go solar.

Adafruit was two steps ahead of me, and already had a tutorial for exactly what I had in mind:

The tutorial was pretty clear about what to buy and how to assemble, so I won't go into great detail there. The only thing that made my project different was getting it into my messenger bag.

I wanted it to be completely hidden when not in use, so that
- I don't look like (more of) a weirdo with electronics hanging out of the bag
- It wouldn't be exposed to rain
- It wouldn't get snagged on anything.

But I did want quick deployment. If it took more than a few seconds to get the panel out and secure it, I knew I'd never use it.

To accomplish this, I attached the panel to the inside of the large flap of the messenger bag, rather than the outside.

When I'm out in the sun, I fold up the bottom half of the flap, exposing the panel. A carabiner clip holds the flap in a folded-up position. An additional strip of velcro keeps the bag itself closed, even when the flap is half-up.

The solar panel itself is attached with velcro: one side on the back of the panel and the other side sewn to the bag's flap. Additionally, I ran elastic bands (the white stuff in the picture below) from corner to corner on the back posts of the panel, through slits I cut in the inside layer of the flap (note the terrible black stitching around the slits). Then I tucked the posts into the slits, so the panel is flush with the fabric.

The power cable runs back inside the bag, where I have the battery/charger unit in a pocket sewn to the side (note the use of an old sock for material).

A micro-USB cable runs from the charger to the outside pocket via a small hole. This outside pocket is where I keep my Kindle and headphones. Both use micro-USB for charging, as does my phone.

Mistakes I made:

  • Patching the solar panel cable in the middle of the cable. When I fold up the panel, the cable bends right at my patch point. My soldering gave way once already. I repaired it, and the second time I put a plastic strip inside the heat-shrink tube before heating it, to give the cable some rigidity at that spot. It bends awkwardly now, but it's stronger.

  • Putting the panel too close to the velcro strip that keeps the bag closed. They're right up against each other. The panel prevents the velcro from making a good connection with the other strip, so the two sides often separate. This isn't too much of a problem, as it just means the flap dangles down instead of being secured down.

  • Made my hole in the Altoids tin too big. I have a fear that small items may fall in. However, this does have a silver lining: When I look down into the bag, I can see through the hole to the LED that indicates that the panel is properly charging the battery. So I know if my soldering connection comes loose again, or if something else is wrong.

It's been a few weeks now, and the charger is holding up great. I haven't had a critical need for it yet, but it's nice to know it's there. And it was a lot of fun to build.

Posted by Kevin at March 16, 2014 09:28 PM
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