Automotive batteries have a finite life span. Undercharging, overcharging and heat all team up to degrade your battery. Poor electrical connections make it more difficult for even a good battery to do its job.
If the battery shows less voltage or won’t take a charge, it’s time to get out the hydrometer. This device checks the proportions of sulfuric acid and water in the electrolyte, which is a precise indication of the level of charge. Pull up the battery fill caps–if you can. Add distilled water to any cells in which the level of electrolyte isn’t touching the bottom of the fill port. Use only distilled water. The minerals in tap water will eventually reduce a battery’s capacity.
If the level of electrolyte is very low, suspect a charging system that’s generating too high a voltage. The maximum voltage you should see across the battery terminals with a fully charged battery and an engine running well above idle is about 14.6 volts.