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?

  • \$\begingroup\$ If all you are asking is why a battery would read lower when it is loaded down and higher when it is unloaded, then all I can say is that this is to be expected. If it jumps a lot, like you point out, then this suggests to me that the battery is very depleted. (Or else your load is inappropriately high for the battery.) \$\endgroup\$ Aug 14, 2023 at 14:10
  • \$\begingroup\$ This has been asked a number of times, do any of these answer your question? electronics.stackexchange.com/questions/634986/… electronics.stackexchange.com/questions/25216/… electronics.stackexchange.com/questions/429711/… \$\endgroup\$
    – GodJihyo
    Aug 14, 2023 at 14:12
  • \$\begingroup\$ @periblepsis I don't see how it could be that depleted. The batteries were fully charged when I started. The specific ones I have say they're graded for 2000mAh and I measured a system draw of 27.5mA. I'm still working on bringing current consumption down, but it should have lasted twice as long as it did \$\endgroup\$
    – E.HP.S
    Aug 14, 2023 at 17:33
  • \$\begingroup\$ @GodJihyo Yeah, I think so. Do you know of any way to reduce the effect, or is it just something I have to be aware of for long-term battery-powered operation? \$\endgroup\$
    – E.HP.S
    Aug 14, 2023 at 17:37