On October 4, at noon, I sat down at my breakfast table, and plugged in my soldering iron. I was going to build the circuit shown in Figure 1. I had one hour to put it together, which was enough time, and then I was going to drive down to the airport to fly to Kathmandu.
As the soldering iron was warming up, I looked for the collection of parts that I needed for this circuit and had shoved into an envelope. Rats! Where were they? I knew I had left the parts in a safe place. I searched in every reasonable spot, every pocket of my briefcase, and all around my house. After 10 minutes, I gave up and unplugged the soldering iron. (Later, 10 miles up the trail above Namche, I found the parts in an envelope in my trousers' left front pocket, which, of course, was a "safe place.")
Fortunately, the circuit that I was going to build was just a spare, a back-up, and we never needed it. So, it wasn't a big deal that it didn't get built.
About eight years ago, I explained in "What's All This Battery-Powered Stuff, Anyhow?" that I used a gear-motor with a hand crank, at about 40 RPM, to charge up the batteries for my new Sony camcorder when I was off backpacking or trekking. That was better than nothing. But the gear-motor's maximum output—barely 2 W!—was limited NOT by the motor, nor by the strength of your arm, but by the gears' maximum allowed torque, which was NOT a lot. So a couple of years later, I bought a small solar panel that could put out much more charge on a typical sunny day. Next, I bought a bigger, yet lighter panel. Then when I was in Kathmandu, I discovered that one of my panels had apparently quit (really, it hadn't), so I bought another panel from Lotus Energy (see the table).
Because my camcorder batteries have been mostly NiCads, I used a simple circuit and just let the solar panel's photocurrents flow into my batteries. The circuit shown in Figure 2 is merely a simple scheme with a Schottky rectifier to connect the solar panel's output to the battery, plus a detector to show if the battery isn't connected. If the battery is below 8 or 9 V, the LED will NOT turn ON, and that's GOOD. That means the battery is getting charged, and holding the voltage low. But if the voltage is above 10 V, the LED will turn on, indicating that the battery is NOT getting charged. This is a bad thing, so the LED signifies bad news. It's time to re-adjust the rubber bands! I mount the LED right near the banana plugs, which I keep outside of my pack's back pocket, while the battery rides inside the pocket. The solar panel is lashed on top of my pack.
The number of 1N4002s in series at D2, D2.5, D3, D3.5 should be perhaps two or three, but maybe more, depending on your actual battery. I recently found that one of my batteries has six NiCad cells in it, not the usual five, so I had to use a couple of extra diodes in series, or the LED wouldn't have gone out!
Many SONY and RCA camcorders have a simple flat interface to the battery, where it was easy for me to set up a couple of small blunt bolts or pins, to be pressed against the recessed terminals of the battery. The connector should be arranged and keyed in such a way that it cannot be applied BACKWARDS to the battery. The sketch of how I did mine is shown in Figure 3. I used tin snips to cut copper-clad 1/16-in. epoxy material into thin strips, such as 3/8-in. wide. And I used a hacksaw blade to saw a dozen gaps in the copper. I soldered three of these thin strips (at the places marked with S) to make a triangular frame, which is easy to strap to the battery with a few rubber bands. I used the isolated foil areas to solder up circuit nodes, such as the LM334 and various diodes.
How do I know how much charge to put into a NiCad battery? I have several two-hour NiCads (2000 mA-H). If the battery gets low, and the camcorder shuts off because it's low, then I can put in well over 1 A-H, or 0.3 A × 3 or 4 hr., before I need to taper off. Usually, if I'm charging up one battery, I'm using another one to record with, so rather than worry about EXACTLY how full it is, I just swap batteries and fill up the one I was using.