A car battery usually doesn’t hold charge due to the following reasons
Bad Cell: Batteries will eventually die no matter what. They will develop a bad cell that prevents them from holding a charge and will need to be replaced. This is part of normal vehicle maintenance. Have your battery tested every Spring and Fall to ensure that you’re able to avoid being stranded by a failed battery. Batteries most commonly “die” during hot and cold weather.
Low Water: Some batteries have fill holes that allow you to maintain them, although most modern batteries are maintenance free and don’t allow you to add water. If you have a maintainable battery, topping off the water level may solve the problem.
Bad Alternator: If the alternator is not charging the battery while the engine operates, the battery will not hold a charge. This is most noticeable if you jump the battery and the engine runs for a while, only to eventually die. It is also observable by watching your headlights – while the engine is running, check the lights. If they start bright but slowly fade, chances are good that the problem is the alternator, not the battery.
Corroded Battery Cables: Battery cables and terminals can suffer from corrosion. As it builds up, this corrosion prevents solid contact, and can limit the charge being supplied to the battery from the alternator. Regular maintenance of your battery can prevent this.
Loose Alternator Belt: If the alternator belt is loose (old and stretched), it will not operate the alternator properly, which means that the charge the alternator produces may not be enough to recharge the battery.
Parasitic Drain: It might be that your battery and alternator are just fine, but there’s something else draining the charge. This could be something as simple as leaving the lights on, or having an accessory plugged in to an “always on” power outlet, or it could be a stuck relay or something else that’s pulling power from the battery when the engine isn’t running.
Old battery: one the reasons cold be that your battery has just gotten old and has completed its life span, so the best thing to do is simply replace it.
How to check the current battery charge