Hi!
I have a question:
I want to power my project with a NiMH Battery, but I want to charge it while my project "is driving around" 
That's because I want to add solar panels on the top and connect them directly to the battery which is already connected to my arduino board. But I need to "cut" the connection or stop the charging when its fully charged, right?
So how can I solve this?
Thanks!
I would use a charge controller/regulator kind of setup to hold a certain voltage level to charge the cell(s) appropriately from the PV panel(s). Just from looking at my MH-C9000 charger which I use to charge my Eneloops (low self-discharge NiMH cells), something around 1.47V per cell (fully charged), or perhaps slightly lower should be good. You won't need to disconnect the "charge" if it's balanced at the right full voltage level. This basically makes it a "float" charger, just maintaining a full charge in the cells without over charging. Perhaps not ideal for your cells (though maybe your project), but it can work reasonably well, and is better than just connecting the PV panel(s) directly.
I would do some basic research on chargers for Nimh cells and on your solar panels as well. At what latitude are you located? If your Winters are long there isn't really a lot of insolation available and I would make sure that you have about 150 to 200 % 'excess capacity available. A Nimh charger isn't really hard to make, Maxim makes some nice charge controllers as well as several others and you might find a charger on Ebay. Basic advice is to limit the charge to .1C(apacity) unless the data sheet for the batteries indicates otherwise. It's been my experience with Nicd as well as Nimh that you need to replace about 120% of what you've used for a full charge. If you look carefully at the data sheets for the chargers you can get a good insight for charging the batteries.
Bob
Are the panels capable of sourcing more power than the batteries/system can use? In other words are you using the solar panels for charging or range extending? You could use your arduino (or an additional one just for charge control) to monitor the battery level and to turn off the connection to the solar panel when the batteries are charged and then turn it back on when the charge level drops. If the panels can't supply more power than the system uses then you could probably get away with just using the battery and other circuits as load and not worry about overcharging. Put 2 switches - one for the solar panel and one for the battery. That way you could use the solar cells to charge when the gizmo is not running.