In reply to Petarghh: I was going to suggest Solar charger but I think they are quite expensive and if they're not reliable then maybe its not worth it...I've never used one.
I got >1,800 shutter releases off of one battery on one charge last summer whilst on a trip (on a D3100), turn all power saving stuff on, do minimal preview, LCD off etc. etc. and sleep with the battery in your bag!
Car charger is a good idea. Find a toilet/cafe/other public place with a socket you can charge it up occasionally would be my other thought.
The MH-24 charger that comes with the 3100 is actually an american charger I believe, the little snap on bit you attach on purchase is an adapter from 2 pin to 3 pin...although I'm not sure it does anything transistor wise with the voltage. One of my chargers has lost the UK Snap on bit and I've just attached a US-UK adapter to it and it works perfectly...but I've never tested it the other way so it may need an adapter anyway. Perhaps not very useful but interesting nontheless!