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: Scheduled Maintenance (U.S.A., Canada, and Puerto Rico)
Follow Schedule 1 if the vehicle is operated mainly where none of the following
conditions (severe driving conditions) apply.
Repeated short-distance driving
Driving in dusty conditions
Driving with extended use of brakes
Driving in areas where salt or other corrosive materials are used
...
Mazda 6 Owners Manual: Valve clearance inspection
1. Disconnect the negative battery cable.
2. Remove the tire (RH).
3. Remove the splash shield (RH).
4. Remove the spark plugs. (See SPARK PLUG REMOVAL/INSTALLATION.)
5. Remove the high-tension lead.
6. Remove the oil control valve (OCV) connector.
7. Remove the ventilation hose.
8. Remove th ...