You could use something like this which I used for a while then it died on me:

http://www3.towerhobbies.com/cgi-bin...&I=LXJC38&P=ML

I'm currently using a charger I bought at harbor freight tools. I paid something like 5.00 on sale for it and it works perfect. I'll try to find it for you and post a link. I just leave it on all the time. It's a battery charger/maintainer.