That is no surprise, but it won't tell you whether the battery is good or not.
I recently used mine, it told me to make an adjustment. It illuminated the scale, and the needle pulsed with the cylinders firing. After I was done I rechecked, and it told me I needed to adjust more. This repeated 2-3 times until I got suspicious and had a closer look at the connections and so on. Didn't find a problem, but all the time the TwinMax was on, and by now long enough to finally drain its 9V battery to the point where the light on the scale went out. That gave it away. Installed a new battery, and had to undo all the adjustment back to where I started.
I thought it would be nice to be able to use a 9V adapter. I didn't want to drill and insert a socket in the tool, so I came up with this: get a 9V terminal and female barrel plug at Radio Shack and solder them together to make an adapter (marked). The polarity matters, but with a multimeter it is easy to figure out the order of wires.