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: Solenoid valve removal/installation
1. Disconnect the negative battery cable.
2. Remove the under cover.
3. Remove the control valve body. (See CONTROL VALVE BODY REMOVAL.)
4. Remove the solenoid valve(s).
5. Apply ATF to a new O-ring and install it on the solenoid valve.
6. Install the solenoid valve in the control valve body. ...
Mazda 6 Owners Manual: Dsc on-board diagnosis
On-Board Diagnostic (OBD) Test Description
(See ABS (ABS/TCS) ON-BOARD DIAGNOSIS)
DTC Table
PID/DATA Monitor Table
Active Command Modes Table
...