I'm sure there are alot of people here that already know this, but last night I was doing some battery research while considering lightweight alternative powering scheme's for my Oade m118 and discovered that there's not a 9V battery chemistry (the transistor type batteries) out there that actually delivers 9V. They actually put out 8.4V nominal. In my case, 16.8v would probably power the m118 for awhile, but it wouldn't be long before they dropped down to below 16V which is too low...meaning that 9V transistor batteries just aren't a good alternative. In my case, the important discovery is that there's never a situation where using two 9V in series would ever give me a full 18V, but I can't even expect decent run-times between 16-18V before voltage dips down too low using two 9V.
Nevertheless, I thought it might interest people to know that you don't actually get 9 Volts out of a 9V battery, regardless of whether the chemistry is alkaline or any of the various rechargeable chemistries...I checked them all.