A low level of electrolyte fluid will cause the battery to discharge quickly.

Inspect the electrolyte level at least once a week. If it is low, remove the caps and add enough distilled water to bring the level between the upper and lower level (illustration).
Do not overfill.
Examine the specific gravity of the electrolyte with a hydrometer, especially during cold weather. If it is low, recharge the battery.
Battery Maintenance
Battery RechargingMazda 6 Owners Manual: Smart City Brake Support (SCBS)
The Smart City Brake Support (SCBS) system alerts the driver of a possible collision
using an indication in the display and a warning sound when the Forward Sensing
Camera (FSC) detects a vehicle ahead and determines that a collision with a vehicle
ahead is unavoidable while the vehicle is bei ...
Mazda 6 Owners Manual: Electrical parts
Battery cable
Before disconnecting connectors or removing electrical parts, disconnect
the negative battery cable.
Wiring Harness
To remove the wiring harness from the clip in the
engine room, pry up the hook of the clip using a
flathead screwdriver.
...