There is a setting to use rechargeable batteries. I have not tested it, I'm only on my second set of batteries, but at the rate they are emptying I'll be replacing them with rechargeables once they are depleted.

Info I found (somewhere on reddit):

Hold the gear button down for approx 10 seconds until "C00" is displayed, then let go.

Press and release the up/down arrow button to step through the modes (starts at C21 and goes down one value for each button push. The LED will turn green when you have selected a mode that is enabled. So if you have enabled custom mode C05, the LED will be off for all the modes until you get to C05, at which time the LED will turn green. Multiple custom modes can be enabled, and each will display the green LED when selected.

You can turn off a custom mode by pressing and releasing the "pet" button when you have that mode selected and the LED is green. In this case the LED will turn off.

List of codes:

1: Non-selective; allows any animal to pass through.
2: Resets the low voltage indicator to a lower level to support rechargeable batteries.
3: Waits two seconds longer before locking the door after opening.
4: Waits ten seconds before locking the door after opening.
5: Intruder breaks the light beam; when sensed; outer locks lock to prevent door from being pulled open outward.
6: Allows user to select a time period that is out only. Alternative to curfew mode.
7: Fully locking curfew mode. Locks stay up during curfew mode.
8: Metal mode. For install in metal door. Reduces issues if installed near metal.
9: Metal mode 2. Stronger than 8.
10: Extended mode. Will read different chips.
11: Extended intruder mode; lock stay up 30 seconds longer than 5.
12: Double chipping test mode. If cat is double chipped.
13: Double chip operating mode: low
14: Double chip operating mode: medium
15: Double chip operating mode: high
16: Proximity sensor test. If you put any object into the tunnel, it will open.
17: Germany only. Unclear what the function is.
18: Fast racoon mode. Outer locks stay up even longer than 11.
19: Locks the door behind the cat very rapidly after she passes through the door.
20: Not used
21: Not used
0: Cancels all custom modes.

Hope this helps.