When does a car battery need to be replaced?

The battery is what powers every electrical component in a car. At some point, the car battery needs to be replaced, but when? Is there a way to tell that the battery is failing? How long does it last? Let’s start with why the battery needs to be replaced. Most cars have lead-acid batteries that are made of six cells with lead and lead-oxide plates filled with an electrolyte (dilute sulfuric acid). Car battery Over time, a lead-acid battery loses some of its capacity, mostly due to sulfation. At some point, the capacity will drop so low that the battery won’t be able to start your car. How long can it last? An average car battery lasts from 5 to 7 years. Usually the battery lasts longer if the vehicle is driven daily. If the vehicle is parked for extended periods of time without starting, the battery will need to be replaced sooner. The battery lasts longer when it’s kept fully charged, although overcharging can also damage it. Therefore, problems with your vehicle’s charging system can also cause the battery to fail prematurely.
Is there a way to tell that the battery is failing? Not always, but there are some telltale signs. When the battery is failing, it loses its capacity, meaning it can hold less charge than when it was new. If your battery has low capacity you may notice that when you are starting the car, the engine turns over slower than before, even if the battery is fully charged. It sounds like the starter motor is “lazy” and not as eager to turn over your engine. You may also notice that the interior lights flicker and go dimmer than usual when you are turning over the engine.
Battery tester Is there a way to test the battery condition? Yes, your mechanic can test your battery with a battery tester. Car dealers and many independent repair shops have a computerized battery tester (in the photo) that not only can tell the state of the battery, but can also test the charging and starting system.  A battery and charging system test costs from $35 to $60, or it could be offered for free as a part of a seasonal promotion.  You may also get the test printout that indicates the current capacity of your battery and measurements of the starting and charging systems. Usually the test will show if the battery needs to be charged or replaced. If your battery is very old, you might consider having it replaced rather than paying for the test. A battery also needs to be replaced if it is leaking or has a mechanical damage.
How expensive is it to replace a battery? A new car battery costs from $60 to $130 and your dealer may charge another $30-$80 to have it replaced. Replacing a car battery as a do-it-yourself project is not very difficult but the battery is quite heavy. Lifting it out of the car can be hard on your back. An old battery could be covered in electrolyte, which is very acidic and can burn your clothes or skin. Here is the DIY battery replacement guide from Pep Boys.
If the battery is replaced without using a memory saving device, the automatic power windows and idle control will need to be reset. Read more: Common problems after replacing the battery and how to avoid them.

Corroded battery terminalDoes the battery need any maintenance? Most modern batteries are maintenance-free. You can verify this in the owner’s manual for your car. Typically, the battery is checked visually during your regular oil changes. Your mechanic may recommend servicing your battery terminals if there is some corrosion at the terminals (in the photo), which is not uncommon. Servicing battery terminals is not very expensive ($30-$50).

Does the red battery warning light on the dash mean the battery is bad? The red warning icon on the dash depicting a battery indicates a problem with your vehicle’s charging system. If it comes on while driving, it means your that battery is not charging. The battery may still be okay, but the charging system needs to be tested. The main component of the charging system is the alternator. Your mechanic can test the alternator with the same computerized battery tester we mentioned above.

What may cause a car battery to drain quickly (e.g overnight)? Either the battery has an internal defect or some electrical component in the car stays on and drains the battery. A battery test will usually show if the battery has an internal defect. If the battery tests OK,  your mechanic may recommend a parasitic current draw test. The test involves  measuring the current draw after all electrical devices are turned off, and comparing it to the specifications. Typically, the current should be less than 50  mA (milliamps) after the car was shut off for 20-30 minutes. If the parasitic current draw is higher, the electrical component that draws extra current needs to be identified and replaced. The diagnostic process involves disconnecting various circuits and re-checking the current draw. This process could be time consuming.

Read next: Common problems after replacing the battery and how to avoid them