I have use a 7.8v upgrade on my drr and it works well and i have run maybe 3-7 nimh charges on it and no sign of performance decrease. I just wired a 2 AAA batt holder to my car and voila! 7.8v pack the easy way. btw, I used two regular alkalines for the extra batts, that makes it 7.8v. I have direction on my site here
I have run several charges through it, and it's perfect, not even hot. a 7.2v batt pack, when peak charged, have more volts than it says like 8v or 8.5v, that's what I heard.