Had a look at my options in Bunnings ( a large Hardware chain in Australia). No rocks :-( . But I found one solar sensor light that said it has a Lithium Ion battery, and a larger solar cell than the average garden light. Surely this is a step up in the charging department compared to a garden lamp that uses a Ni-Cd battery..........

Weeeellllllll...........its a small step up....as you'll see.

None of the solar products actually specified their power output, so without going overkill, I went with the sensor light with the Lithium Ion battery for a whole $20.

So what do you get for $20? Time for a teardown ( cue Dave Jones!)

First thing I looked at was the battery, as its in a separate compartment. Its labelled as a 3.2V, 400maH, 14500 size. This is the same size as an AA. Interesting though, seeing as most 14500's are 3.6 or 3.7V.......

Multimeter says a smidge under 3.3V, but I'm not sure of its state of charge. Weekend test I think. 

On to the main electronics enclosure. After unscrewing the back, it was pretty obvious the build quality left alot to be desired. Not only that, the wire connecting the solar panel to the small circuit board was snapped off at board level - meaning the light would have never charged! I'll have to fix that if I ever want to use the light for its original purpose. The electronics consists of the back of the main power switch, a small circuit board, and wires connecting the PIR, the LED, the Solar Panel Connector and the battery. After unscrewing the circuit board, you get to see the 5 components that comprises the charging circuit. 

1 x SS14 diode( probably something like this)

1 x 431 ( cj431?) Regulator

3 SMT resistors. 47K, 20ohm, and 100K.

As you can see, the soldering is terrible! So terrible, another of the wires from the PIR was loose. Talk about dry joint!

Anyway, I'll read the datasheets to work out the charge rates etc.

Time for the solar panel. It's about 18cmx18cm. As it's night here, I tested the panel under a halogen floodlight to get an idea of the specification. Turns out it's about 5V, and short circuit current straight through the multimeter was about 350ma! Whew- just enough to power the rock! And a bit spare to charge the battery.

So after all this, I still haven't found the right pet rock for this project, so I'd better start trawling the local eBay sellers and see what can get here in a week or so.

Til next time :-)