I'm working on a device that uses a proximity sensor to count the number of times something passes through its field of view at a certain distance. It is connected to an Arduino Nano and a battery pack that uses 8 AA batteries (specifically, these rechargeable Nickel-Metal Hydride ones from Energizer), bringing the peak voltage to about 11V2 and the nominal voltage to about 9V6. This is then regulated down to 5V, using (currently) an L4940V5 linear regulator.
The device is meant to run continuously for at least 168 hours (one week). While testing it, I noticed that the battery pack was reading around 5.1V after about 36 hours. After switching off and on though, the battery voltage sprang back to 9.1V
Can anyone explain why this would happen?