Since I operate from the field from time to time, I needed a way to help keep my 2.2aH SLA battery charged while in the field. A friend gave me a surplus solar panel that is rated 3.2 watts max with 18.8 volts @ max power and a max current of 170 mA. This little solar panel will produce over 22 volts with no load, so I figured I had better build some sort of voltage regulator circuit for it to prevent it from cooking my battery. The circuit is nothing more than a standard LM317 3 terminal adjustable voltage regulator mounted on a scrap piece of copper clad board that I cut traces on with a Xacto knife, and a couple of resistors and capacitors. it has one variable resistor to set the output voltage with, and you can set this to 13.5 volts for a float charge, or around 14.4 volts for a quicker charge. Of course, with my panel only rated at 170mA, it would take a while to charge my battery. But if I connect it to the battery before I start operating and then just leave it connected while I do, I can get a few more hours operating from the battery. I built the circuit into a standard QRP enclosure (Altoids tin) and then mounted it on the back of the panel using double sided mounting tape. Since the Altoids tin has a snap open lid, it makes it easy to open up and adjust the voltage or service the circuit if ever needed.

Below is the schematic of the charge controller, and as you can see it is a fairly simple circuit. You will notice that I have also included a diagram of the copper clad board and parts layout to make it easier to reproduce the circuit. Board measurements are not critical, I just used a piece I already had and then drew the lines on it with a pencil and ruler to get it all layed out, then using an Xacto knife I cut the traces to seperate the pads. The heavy black lines in the diagram represent the trace cuts on the board. Just make your board to fit inside an Altoids tin and you can mount it in the tin by soldering the ground pad of the board direct to the tin. To heat sink the LM317, I used a heatsink insulator between the tab of the 317 and the Altoids tin and use the tin itself for the heatsink (probably overkill with my little solar panel). To adjust the voltage on the controller, set the panel out in bright sunlight and facing the sun as square as possible, connect a voltmeter to the output leads and then adjust the pot to get a reading of around 13.5 volts. To use the circuit, set the panel in the sun as before, and connect your SLA battery to the output of the circuit with the positive lead going to the positive terminal of the battery and the negative lead going to the negative terminal of the battery (of course). That's all there is to it, now you can keep your SLA battery topped off while operating QRP in the field.



73 !

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