charge controller tests out well so far

We've been testing a 6v charge controller schematic with a small 10w solar panel that students of mine built last year and a small 3 amp hour battery with an arduino attached as a dummy load. 

Here's the breadboarded circuit with battery.

And here's the schematic:

Thanks of course to electroschematics.com.

The LM 317 is a standard adjustable voltage regulator.  Output is regulated with the variable resistor.  Most lead-acid 6v batteries have 6.8 v as a cutoff voltage.   The reverse-biased zener diode 2D starts conducting when load reaches float charge and sends excess voltage to ground.
We've heat sunk the LM 317.  A 1W rated resistor for R3 is too low.  It gets way too hot for this application.  We substituted a a ceramic 10W rated resistor and it stays cool. We'll give it a few more days, and if performance still checks out, we'll solder a permanent circuit.  Some of our remote monitoring sites get at least mottled sunlight for a few milliamps each day.