... some assembly required.
Warning: slightly technical and much text. Just skip and vote generously if you can't be bothered to read all this. It's a great article and quite unlike shitposts. Promise.
10W solar panel
When I started taking photographs, there weren't any digital cameras yet. Most cameras for film have batteries, but they don't have to do all that much, and with a few spares you were good to go for months on end. Digital cameras will drain their batteries in a few hundred shots, and then you recharge them. But what do you do when you're out and about for weeks or months, away from the mains?
An obvious answer would be to bring a huge amount of spare batteries, but even those will all be drained eventually, and it is also very expensive. What I did was buy a USB charger for my camera's battery and a power pack with a capacity of about 30Wh. Such a power pack will charge the camera and cell phone a few times and provide some light in the tent.
Battery packs also only last for so long, so I started searching for a solution that could keep my camera and telephone charged indefinitely.
You can buy battery packs with a solar panel, but to keep the package small, manufacturers use a small solar panel that doesn't deliver enough power for my needs. On the other hand there are professional kits available that expeditions use in their base camps. These are way over the top for my needs and also very expensive. Goal Zero has some kits that are quite good and the larger ones come close to what I wanted, but they are expensive.
So, I decided to design and build my own; after all, I'm not an engineer for nothing. I wanted something that would fit in a side pocket of my back pack, with about 60Wh capacity, intelligent solar charger electronics with MPPT, and a solar panel that could charge its internal batteries in one (cloudless) day. Here's what I came up with, affectionately known as "the black thing":
The solar panel plugs in on the left, the USB charger cable on the right. It will also accept charging from most notebook chargers. Under the lid I store an AA-battery charger, a charging cable, and some spare AAs:
Deeper down are the interesting bits, the electronics. There are two 10Ah LiFePO4 batteries in series with protection and cell-balancing electronics, a solar charger, a DC/DC converter for making 5V @ 2A for the USB output, and some electronics that disconnect the USB port when the internal batteries are being discharged too far.
For the electronics-minded: it is built mainly around the LTM8062 and MAX1626. I can't solder BGAs and the like at home, so I used and heavily modified evaluation boards from the manufacturers; there's only one small PCB in there that I made myself from scratch.
The solar panel I used, shown at the top of the post, is a very standard 10W panel that snugly fits inside my back pack. You can buy foldable 10W panels that are even smaller.
I have been testing this set-up for over a year now, and it has kept my camera's, telephone's and flashlight's batteries charged all that time, with power to spare for other things, and I am very happy with it. It is also cheaper, lighter and more efficient than the near-equivalent Sherpa 50 Solar Kit, but also less drop-proof and larger, because I used an off-the-shelf enclosure and three separate circuit boards.
I am now designing a Mark II, with the same specifications but even more efficient and about half the size using an LT3652, a single, custom PCB and a custom-made, 3D-printed enclosure.
That was a long read, now wasn't it?