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: No.1 air bag system warning light does not illuminate
Diagnostic Procedure
When performing an asterisked (*) troubleshooting inspection, shake the
wiring harness and connectors while doing the inspection to discover whether
poor contact points are the cause of any intermittent malfunction. If there
is a problem, check to make sure connec ...
Mazda 6 Owners Manual: Generator removal/installation
Warning
When the battery cables are connected, touching the vehicle body with
generator terminal B will generate sparks. This can cause personal injury, fire,
and damage to the electrical components. Always disconnect the battery negative
cables before performing the following operation.
...