Thanks for taking the time to do the nice schematic. It really helps quite a bit.
I'm not really getting the point of the 0.47 ohm resistor. I'm also not seeing the voltage divider setup as working.
The purpose of the 1.0 and .47 ohm resistors was to have the lm317 provide a constant 850 mA or so current (1.25V/1.47Ohm). Everything I have read suggests charging NiMh batteries at a constant current. Is this not right?
My voltage divider on my ugly sketch wasn't drawn correctly. I did have it implemented correctly though...
Ideally you would also use a thermistor to measure battery temperature and make sure they're not getting too hot (<55C or so).
Yeah, that was going to be the next part of my plan. I wanted to get the rest working first. When I was charging my batteries, they never got warm to the touch, but that is definitely something I will implement.
Your design has an op-amp to adjust the voltage. I have never played around with op-amps before and really don't understand how they work. I am going to have to study up on them before going too much farther.
That's a good schematic, very clearly explained, thank you.
But I am wondering, why not go for a dedicated IC, cheaper, tested and true, rather than bother with an expensive (for this purpose) arduino + its programming?
I just started with the arduino and wanted a project that would do something useful. I got started with linux in much the same way. I wanted to install the OS and do something useful with it so I built myself a MythTV DVR back when it was much harder to get it all to work (with modern package management systems like yum or apt-get, it's almost trivial to setup a MythTV box).
RuggedCircuits, have you actually built the circuit you posted?