if they're the same voltage (which I presume they are), no problem -- 7.2ah makes a lot of sense for a d8/sbm1 combo, but is total overkill for just a d8 -- I assume you have one sitting around. If not, my recommendation is to skip external batteries alltogether and run the d8 off of nimh aa rechargables. A set of good ones will run the d8 for a good 4 hours or so.