I have been long fedup by 9V batteries which were discharging quickly and providing lover voltages than I needed so I used the 9V UPS from Aliexpress ( Type-C 15W 3A 18650 Lithium Battery Charger Module DC-DC Step Up Booster Fast Charge UPS Power Supply / Converter 5V 9V 12V ). I have also modified the PCB by removing the red LED, which indicates ON status of the device (so it doesn't drain batteries during storage) and also installed switch to connect pin 5 of the XR2981 chip either to where it is connected originally or to ground. This pin is the EN (enable) pin of the DC-DC Step Up Booster chip, which if connected to ground plane, disable the chip and consumption (which in conjunction with removed power indicator LED) drops to arround 68uA - great for storage.

The top part of the enclosure has place to install connectors from used-up 9V batteries. There is also is a cover to prevent short circuiting connectors (they are short circuit protected electronically, but when short circuited you would need to connect the device to charger to unblock that protection) when not in use.

The top part snaps in but was a little loose for me so I have fixed it with some tape (didn't have time to do new revision and new print)

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