Well, the battery charging boards have arrived. As much as we hate to admit it, this might have been the way to go from the very beginning. The new boards have more than a few advantages. They are specifically designed to charge Li-Ion batteries from solar cells, and it adds the capability of charging the 'bots via USB. As an added benefit, it also allows you to patch into the status LEDs (ready, charging, low) and use them as digital inputs to the Arduino. In short, we can let the charging board monitor the battery voltage without having to write a separate voltage monitoring script for the Arduino. Any chance we can get away with writing less code to debug, the better.
There has been another complication, however. The cheap Li-Ion batteries we were going to use are essentially useless. We assembled the first of the boards, plugged in the solar cell, and set it aside to charge. We were alarmed when it claimed to be 'ready' after just a couple minutes. A quick test with a voltage meter confirmed our fears. The batteries had been allowed to go totally and completely flat, which is a death sentence for Li-Ion batteries. They wouldn't take any charge at all.
This is a major stumbling block. The vast majority of available Li-Ion choices out there are 3.7v. Our choice of components was geared towards 7v. If we dropped the operating voltage we would have to order all new electronics. Another problem with dropping the voltage down was that it limited future versatility. If someone later decides to expand on what we do with the Emergence Project, we would almost guarantee the first thing that will be tried is to bump the processor up to a Raspberry Pi or a BeagleBone Black. Both run on 5v, just like our Arduino. We wanted to make building a Raspberry or BeagleBone based 'bot (or upgrading older, existing 'bots) as easy as swapping out the processor without changing any of the other components.
However, the is a ray of hope. After a quick look on eBay, it seems that comparable new batteries of the type we wanted to use aren't hideously expensive. It does mean going even further over budget (from $3 per battery to $10-15) but at this point, it seems the way to go. Sheesh... More money. What next?