Do you know what brand and model the batteries are? Each battery manufacturer usually puts out a specification sheet, that will tell you what rates you can safely charge your batteries at. If you've got the battery info, try using Google to find the charge info.
Nitro Knowledge Base: http://nitrokb.netne.net
My YouTube Channel: http://www.youtube.com/nitrokb -> Lots of on-road nitro & eletric action + some off-road as well
My Flickr Photostream: http://www.flickr.com/photos/nitrokb/
Kyosho V-ONE RRR WC (x2) \ XRAY T2'007 \ Also owned: XRAY NT1 & Mugen MTX-4