sandworm:
Zapro: Yep its a NiMH pack. Super heavy, but it's got a lot of juice.
OK, then it's essentially a 9,6V to 11,2V battery - not 9V.
But those voltages fall well within the limits the Arduino can use, so no problem there. If you are looking to power your Arduino and attached sensors longer, you need a Switching power supply and bypass the Linear V-reg's that is in the Arduino.
// Per.