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Q: Why wont my battery keep a charge. on 1999 Mercury Cougar

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We had alernater rebuilt and had battery tested. The alternater is putting out power. We hooked a tester to the battery and alternater and it reads 12.22 but the longer the car stays running the numbers start dropping.
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It could be a defective battery. The battery is a reservoir of electricity but if it sulfates and goes bad it just will not hold a charge. Have the battery charged and tested. Make sure the battery connections are clean tight and secure. The alternator replenishes energy taken out of the battery once the engine is started. The alternator generates electricity and recharges the battery. Turn on the ignition switch to the on but not run position do you see an icon on the instrument panel representing a battery, if you do not see this battery icon the alternator is not seeing power to allow it to charge. Start the engine does the battery icon go out. Put a volt meter across the battery terminals with the engine off, at rest the battery voltage should be 12.6 volts if the battery is fully charged. Now get a friend to start the car while you watch the voltmeter, the voltage reading across the battery should read approximately 14.6 volts if the alternator is charging correctly. If the alternator is charging properly and the battery tests OK then you may have a "parasitic draw" on the battery. A "parasitic draw" on the battery is something that stays "live" stays switched on even through the car is turned off and the keys are out of the car.
A battery may appear OK but was a hydrometer and High Rate Discharge test done? To check the alternator I like to look at the diode pattern on an oscilloscope to know it is truly OK. I wonder have you a voltage drop or dirty electrical connections?
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