It can be hard to figure out what causes a car battery to drain. A dead battery, old battery, or bad alternator can cause battery charging problems.
You're late for work and rush out to your car, only to find that it won't start. The headlights are dim and the engine simply refuses to turn over. You realize that your battery is dead. But how did this happen? There are five main causes of car battery drain or failure.
You’ve probably done this at least once in your life – you come home from work, tired and not really thinking, and left the headlights on or didn't completely close the trunk. Overnight the battery drains, and in the morning your car won’t start. Many new cars alert you if you’ve left your lights on, but may not have alerts for other components.
Parasitic drain is due to components in your vehicle continuing to run after the key is turned off. Some parasitic drain is normal – your battery delivers enough energy to keep things, like your clock, radio presets, and security alarm operational at all times. However, if there's an electrical problem, parasitic drain can exceed what's normal and deplete the battery.
If your charging system isn’t working properly, your car battery can drain even while you’re driving. Many cars power their lights, radio, and other systems from the alternator, which can make the battery drain worse if there's a charging problem. Then you’ll be stranded at roadside with a stalled car that needs a boost. And even then, bad charging will drain your car’s battery. A professional mechanic will be able to diagnose what's causing a charging problem.
Defective alternator diode
A car alternator recharges the battery and powers certain electrical systems. If your alternator has a bad diode, your battery can drain. The bad diode can cause the circuit to charge even when the engine is shut off, and you end up in the morning with a car that won’t start.
If your battery is old, it might not hold a full charge. If your car consistently won't start, it’s possible that the battery is worn out. You should generally replace your car battery every 4-5 years. If your car isn’t starting consistently, and your battery is old, battery replacement should be the first thing you consider.
Having a battery that won't hold a charge is frustrating, and figuring out what's causing the problem can be tricky. Assuming that the cause of the battery drain is not human error, you will need the assistance of a qualified mechanic who can diagnose your car's electrical problems and determine if it is the battery or something else in the electrical system.