Battery will not hold a charge

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. A more complex but good diagnostic test is checking "voltage drop" on the battery ground and power cables. You can find a video on how to do a voltage drop test on youtube. 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. A battery should be checked for static voltage, a hydrometer reading that checks the electrolyte level in the battery and a high rate discharge test which is really a torcher test of a battery and is the most relevant. A load of say approximately 200 Amps is put on the battery the battery voltage is measured as the load is applied and the battery will drop rapidly toward 8 volts if the battery is weal. It should maintain the load for 15 seconds and recover to 12.6 volts if the battery is good.