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: Dtc c1955, c1956
Caution
When attaching the tester lead to the DSC HU/CM connector the SST
(49 G066 004) must be used.
Diagnostic procedure
...
Mazda 6 Owners Manual: Supplemental Restraint System Components
(With Front Passenger Occupant Classification System)
(Without Front Passenger Occupant Classification System)
Driver/Front passenger inflators and air bags
Roll-over sensor (some models), crash sensors, and diagnostic module (SAS
unit).
Seat belt pretensioners
Front air bag sensor ...