LiFePO4 chemistry is at about 3.2VDC so a blank/filler will make it a drop-in for 2X alkaline which is what I see a lot of devices use. You can get "AA" size on

LiFePO4 chemistry is at about 3.2VDC so a blank/filler will make it a drop-in for 2X alkaline which is what I see a lot of devices use. You can get "AA" size ones in the garden light section of home centers as they're used for solar path lights. Lithium primary batteries are available in 1.5V nominal — not sure if they have any circuitry.