What we really need is a tiny UPS, of sorts. We'll be hooking up a solar cell and a battery to a single board computer. Currently, that SBC is a custom Pic32 device, and it does it's own UPS and voltage monitoring duties.
I've been tasked with trying to replicate all of its features with off the shelf products... and for the most part I've succeeded. But I don't currently have any way to switch between two sources of juice, or monitor when they're getting low.
These guys have something:
http://www.mini-box.com/picoUPS-100-12V-DC-micro-UPS-system-battery-backup-system
I really like it, the price is well within the budget. We might even work it in though it does 12V and I'll probably be using 5V... there are enough engineers on hand to figure out something. But I'd still have no idea what the voltage was for the PV or battery. I was hoping that there was some simple little USB multimeter thing that I could use to monitor this with, but I can't seem to come up with anything. I've found all sorts of cool hardware, but nothing that will help us.
Does anyone know of anything?
How about an arduino ? It's got A-to-D ports on it and a pile of inputs as well as outputs. A little more expensive than what you pointed out (though not much, and I think the more highly-integrated ones get cheaper). Plus, the hardware is open source so available from all over the place.
I think you're on to something with the picoUPS. It seems like a good product, and if I'm understanding you correctly, you now need a device to monitor your battery and bus voltages. pjz's suggestion about the Arduino boards looks pretty good too. The Arduino Duemilanove looks like it will be about right. It has six analog inputs which you can use for monitoring your DC voltages. The downside is that it isn't a fully-integrated solution. You will have to experiment with programming it, and then you'll need software on the computer it's attached to for reporting or logging the results. You will probably find software available which is "almost right," which should save a lot of heartache. Good Luck!
It looks as if this place offers a 4 channel voltmeter (or will soon):
http://www.digital-measure.com/html/prices2010.htm and voltmeter.htm
Still a bit larger than I'd like, but not much.
Too bad I can't find one of the micro-UPS boards that actually monitors its own voltages via USB.