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: Engine Coolant Temperature Gauge
Displays the engine coolant temperature. The blue gauge indicates that the engine
coolant temperature is low, and the red gauge indicates that the engine coolant
temperature is high and overheating.
CAUTION
If the high engine coolant temperature warning light (red) turns on, there
is a pos ...
Mazda 6 Owners Manual: Dtc c1148, c1158, c1168, c1178, c1233, c1234, c1235, c1236
Caution
When attaching the tester lead to the ABS (ABS/TCS) HU/CM harness
connector, the SST (49 G066 001) must be used. (See ABS (ABS/TCS) HU/CM
INSPECTION.)
Diagnostic procedure
...