Hi Dyslexicbloke,
I read this as I am looking for an answer to a question I have, but I find it sounds like your project is very close to what I am working on this very moment (read past weeks, and probably future weeks)!
My setup is PV panels and also my micro-hydro turbine system feeding into a 24 volt battery bank.
I am using a Mega 2560, from freetronics which has already the ethernet and SD built on-board.
A Plamatronics PL40 has been doing the job of charging the batteries so far, and it has been great, but I envisage it will be replaced with this unit I am describing.
For this project I moved away from the Arduino IDE and moved to using Eclipse with the Arduino plugin.
Using Eclipse I find is a far better way to construct such a in-depth project.
In Eclipse I also, at the same time develop the web based interface.
Just, quickly, the web based side is using JQuery, JQery-mobile and a few other libraries to make the whole lot far easier.
The client side (your browser) simply requests JSON data from the Arduino and plugs that into a nice web interface.
I am designing it so it works very nicely on smartphones as well as your standard laptop/desktop.
The is logging of battery data happening now, it will be viewable as a trend on the web interface, with scrolling backwards in time.
I have the basic web interface happening with real data being displayed.
I have the Arduino getting current time from a NTP, instead of need extra hardware of RTC, and it is working well now.
I would be willing to share what I have with you, though I still have some way to go yet.
We could maybe start a new thread in the appropriate place for a work-in-progress type of project.
I have been wanting to start a blog on the project, but that would take me away from the actual doing.
My background has been more with industrial automation and programming PLC's and SCADA systems, typically AB SLC/Control Logix and Citect, so am using some of those concepts I have used in the past into this Arduino based project for my home. (far more energy efficient than PLC's and dedicated SCADA/HMI computer)
Paul