Simple LM317 short-term solar charger

Why?

Having attended music festivals for quite some time now, I took up the habit of bringing a car battery with me, to have some electrical lighting in and around my tent at the festivals' campsites. Usually, such battery lasts sufficiently long for two nights, but ever since I took a battery with me I feared it might run out of power too soon. Therefore, I had always wanted a solar panel to recharge the battery at least a little bit during the day to extend the time until the battery has insufficient charge for my lights.

The solar panel I use

Some weeks ago I finally bought a small solar panel, a SCHOTT Solar ASI-F 10/12 with 10 W_p designed for 12 V applications, measuring about 60x30 cm² and weighing about 2 kg – so it's reasonably portable.

The problem

Unfortunately, you can't just connect the solar panel to a lead–acid battery, as their voltages don't match (on a bright sunny day the panel delivers more than 16 V, but a lead-acid battery's charging voltage shouldn't usually exceed 14.4 V). Therefore, some circuitry is needed to combine the solar module and the battery.

My simple solution

There are some schematics for solar chargers available on the internet, with some of them based on a LM317 adjustable voltage regulator. Nearly all of them incorporate an additional transistor (and some more resistors) to reduce the charging current when the battery's final charged voltage is reached (trickle charging), but I didn't want to throw too much resources into a device I'll only be using three times a year for at most three days each, so I had to use components I already had lying around – fortunately a LM317 was one of them :)

So I stripped down the circuit to the bare minimum:

As you can see, only the LM317 is left with two resistors to set its output voltage (13.75 V). This circuit is not suitable for standby use, but for a three days' festival where the battery is getting drained more at night than it's going to be charged at day that's fine.

For robustness I quickly made a PCB using the following layout; clicking on the picture should give you a PNG file that, when printed at 600 dpi, you can instantly reuse to create your own PCB (both images show the view from top/the component side):

I could have designed the board much more compact, but I had a piece of base material left over from a previous project and decided to be lazy :) and just use the space readily available. Here's a picture of the assembled circuit:

You might notice some differences between the schematic and the final board: I didn't use 10 μF capacitors on the input and output. Given the battery is a very large capacitor of some sort and the solar panel also has some capacitance, I just soldered a small 20 nF capacitor I extracted from an old defective radio into the circuit as output capacitor, to prevent excessive ringing in case the battery is not connected. For this board I also added a 1N4004 diode towards the battery and used an 11 kΩ resistor instead of the 10 kΩ one, so that the battery will be charged to 14.3 V (the LM317 is set to output 15 V, and the voltage drop across the diode is about 0.7 V).