What kind of gps battery can I use for uBlox NEO-M8N GPS module?
4.2 Table 11 ought to imply how short a 5mAh battery will last at 25 to 32 mA @ 3V
Perhaps bigger is better. e.g. 3000mAh secondary with CMOS LDO
https://www.u-blox.com/sites/default/files/NEO-M8-FW3_DataSheet_UBX-15031086.pdf