One of the things I’m always focusing on when building things, especially things that are relatively permanently constructed, is making sure they are as flexible as possible. When I’m testing things out, I can easily swap wires, reset configurations and mix things up to change performance, maximize efficiency, and/or change how something operates. When something gets permanently put together, that ability is often lost. With my micro solar array, I wanted to avoid that problem. The charge controller I built is extremely simple. It’s just based on an LM317 and a very basic current shutdown switch when the battery is over a certain voltage (not really necessary for NiMH charge rates of < C/8, but nice to have). Depending on the sun, I will hook up a variety of different battery packs. I have one 4 cell, and one 2 cell, and even a single cell occasionally. The problem here is that I normally have the array configured as a 2*12 arrangement. Two 6V panels in series and tie the two pairs together in parallel. This is ok for the 4 cell, since 4 6V in parallel would be too close to the voltage of the cells to run through the LM317, but it’s not so great for charging a 2 cell pack. All the extra voltage is wasted. I started to wonder if I could build a very simple power block using terminals, a few strips of breakaway male headers, and a handful of jumpers to allow for easy changing of the array power configuration. By just changing the jumpers around, you could switch the array from 6V*4, to 12V*2, to 24V*1 very easily. I spent a long time drawing out different ways of doing it and eventually I came up with this.
The brick is half a standard rectangular perf board. The four terminal blocks on the sides allow the panel wires to be easily connected to the board. Underneath the board are a number of short wires that are soldered to connect the terminal blocks to various pins on the headers. The setup of these connections was the most difficult part because it allows for the positions of the wires to remain the same so the jumpers alone determine the power output. This allows me to consider the sun conditions and the pack I’m trying to charge and set up the array to deliver the maximum possible charge current at as close a voltage as possible. The most complicated jumper configuration is the 6V*4 since it basically requires each negative and positive terminal end up in the same place. But, this allows me to (even on a completely overcast day) squeeze all the power I can out of the array. No sense in wasting power ramping up the voltage by arranging the array in series if we’re just going to be cutting it way down for a 2-cell charge. Also, in the above image, notice the digital display. That is a really sweet panel meeter from Adafruit. This one measures from 0-99.9V using three wires. Obviously in an array like this, power is at a premium, so I’ve hooked up the meter to a switch, so I can just turn it off unless I want to know the panel voltage. But, the great thing about this meter is it only draws a few milliamps, meaning in really good conditions (i.e. perfect full sun) I could leave it on and just push up the charge controller a bit to compensate.