All batteries, even those using gel instead of electrolyte lose charging capacity as the time passes. The reason for this is that the chemical breakdown of the connections inevitably takes place leading to the deterioration of the plates and loss of the electrolyte. When your vehicle has start up or charging problems, the vehicle troubleshooting procedure should start with a test of the battery. You do that with a visual check followed by a voltage test. If the battery is ok, other components of the electrical system is probably the reason, but battery problems are by far the most likely reason.
If belt tension is correct, then the next item to check is the alternator itself. You now have two choices - you can test the alternator yourself with a simple test using a voltmeter, or you can have the test performed by a professional. If you choose to do the test yourself, then you will need a DC voltmeter, or a multimeter. Such a unit can be purchased at most auto supply stores, hardware stores, department stores, home improvement stores, etc.
Seeking the Elusive Electrical Problem. Many times electrical problems are the most elusive, because they can occur intermittently. For example, the car engine may not work properly, but only after the engine warms up. Or the ignition works randomly. Perhaps the engine cuts out without notice while cruising down the highway. Then again, your car might have a faulty alternator or a defective wire.
You can buy these at nearly any auto supply store, and most hardware stores. You will also need an appropriate sized wrench with which to remove the battery terminals. Important: Always disconnect the negative (-) battery terminal first, and reconnect it last. This will help to keep from shorting the battery while connecting or disconnecting the terminals and possibly causing damage andor personal injury.