It’s one thing to leave your dome light on overnight and drain your battery—that happens to the best of us, and salvation is usually only a quick jump-start away. But if it’s a regular occurrence and you find yourself asking “why does my car battery keep dying,” you have a bigger problem on your hands. Read on to find out what to do next.
Why do car batteries die?
Whether it’s the battery in your car, your phone or your laptop, any battery can withstand only so many charge/discharge cycles before it degrades and won’t hold a charge anymore. Automotive batteries also work in hostile environments with lots of heat and vibration.
Extreme cold is arguably worse for a battery because it reduces its efficiency. Cold weather makes motor oil thicker and more viscous, requiring more amperage from the battery to turn the engine over at startup.
Another thing that’s really bad for most batteries is a deep discharge—that is, draining a battery beyond what the manufacturer considers fully discharged. This causes excess sulfation in the battery’s cells. A complete discharge of the battery can cut as much as a third off of a battery’s service life, if it can be saved at all.
Aside from that, a failing charging system and alternator can cause your battery to die, as well as a few other factors.
How do car batteries work?
Automotive technology has changed a lot in the last 100 years, obviously, but one thing that hasn’t changed much since the advent of the electric starter is the car battery.
Automotive batteries still use a lead/acid design, with a number of cells that are connected together in a series. Each cell has a cathode and an anode resting in a bath of sulfuric acid. When the battery discharges and electrons are released, the acid reacts with the cathode and changes its surface to lead sulfate (and the acid loses its acidity). When the battery charges again, the chemical reaction is reversed: the acid regains its acidity and the lead sulfate reverts to lead oxide on the anode.
The battery is responsible for providing enough current and amperage to start the engine, and it also provides current for the headlights, radio and other accessories. Another part of the electrical system which hasn’t changed much in decades is the alternator. This is driven by the engine and is responsible for restoring the battery’s charge and providing electrical current while the engine is running.
10 Top reasons a car battery dies
Presuming that your battery isn’t nearing the end of its life cycle, there are several things that can cause the battery to deplete.
Lights left on
This one happens all the time, of course. Many newer vehicles have a timer that shuts the headlights off after a minute or two if you forget and leave them on, but if your car doesn’t have this feature you can easily walk away and leave the lights on for hours until they completely drain the battery.
If this keeps happening, try to get in the habit of making a mental note of whether you have the lights on or not. If it’s daytime and hard to tell whether they’re burning or not, check the switch on the dashboard. Make a Post-It reminder for yourself if necessary, because completely draining the battery for any reason will shorten its life.
This is by far one of the most common causes of a dead battery. The alternator has a finite service life, and on many vehicles it’s going to fail within 150,000 miles. A failing alternator won’t charge the battery while you’re driving, or may work intermittently before it fails completely.
If you suspect the alternator might be on its way out, keep an eye on the dashboard ammeter to see if it shows a charge (or look out for the warning light). Most auto parts stores will do a free check of the alternator. If you’re handy, the alternator isn’t too hard to change on many vehicles.
Your battery cables and terminals can become corroded with time, which will usually show up as a fluffy, powdery-white or greenish-white deposit around the terminals. This is often why many car batteries die.
The good news is that it’s fairly easy to clean corrosion from battery terminals and cables, using a stiff wire brush. You can then prevent the problem in the future by using felt anticorrosion washers on the battery posts and a Vaseline-like dielectric grease on the posts and the cable clamps.
Sometimes it’s just the end of the line for a battery. As the battery’s performance dwindles, it’ll turn the engine over slower. It might not be able to power the radio or other accessories for long before being exhausted, either.
Check the purchase date on the battery (the little month/year cutouts on the top of the case); if the battery is nearing the end of its warranty phase, it’s probably time for a new one. Remember that if your battery can’t go for its entire warranty phase, you may be able to get a credit when you buy a new one.
Too many short trips
An engine needs to run for 30 minutes or more for the alternator to really do its job and start replenishing the battery. If you do a lot of trips of ten minutes or less, or if your vehicle doesn’t get driven often, that can be enough to exhaust the battery or not recharge it sufficiently.
There are a couple of systems in your vehicle that will always put a very slight drain on the battery. For instance, the radio pulls a slight charge all the time so it can store radio station presets.
A parasitic drain, on the other hand, is something that’s enough of a draw on the battery to deplete it. It could be a dead short somewhere in the electrical system. These can be hard to track down and might require a technician.
Otherwise, make sure that things like the glove box light or dome light aren’t staying on and exhausting the battery. If you’re handy, you can use a test light or multimeter to check for parasitic drain yourself.
Extreme cold and heat can take their toll on a battery, especially an older one. When it’s below freezing, that by itself can be enough to cut the efficiency of the battery’s chemical process by as much as 50%.
Obviously you can’t do much about the weather, but on very hot or cold days, try to avoid running the radio or other accessories for long stretches without idling the engine.
Automotive electrical systems have a voltage regulator that’s designed to prevent the battery from over- or undercharging, but it still happens occasionally. An external battery charger can also lead to overcharging, although most feature an internal switch so that won’t happen.
Overcharging means that the battery is “overgassing,” with the electrolyte cooking away and releasing oxygen and hydrogen gasses. On older batteries with vented caps on the cells, this could mean exposing the plates and ruining the battery completely.
Charging system failure
Modern vehicles have a serpentine belt that uses the engine’s energy to power the AC compressor, power-steering pump and other accessories. A loose or failing belt can also cause the alternator to not work properly, although most serpentine belts have a spring-loaded tensioner to keep them taut.
Checking the serpentine belt should be part of routine maintenance a couple of times a year.
Loose battery connections
After removing or installing a battery, it can be easy to not tighten the clamps sufficiently. Anything less than a firm connection can be enough to interfere with the battery’s performance.
If you can grab the cable and clamp at the battery posts and wiggle them at all, they aren’t tight enough and need to be torqued down.
What if your battery dies while you’re driving?
If your battery dies and leaves you stranded, it’s almost always an alternator problem. You’ll be able to tell because the car will be running strictly on battery power until the battery is totally exhausted. The headlights will get dimmer, radio and power accessories may stop working and eventually the car will run more and more poorly until it completely dies.
If this happens, find a safe place to pull over and call for help, because you won’t be able to go any farther until the problem is addressed.
How long should a car battery last?
Regardless of your battery’s warranty, the truth is that most batteries last between two and five years. As mentioned, you may be able to get a partial rebate if your battery doesn’t make it to the end of its warranty phase before it needs to be replaced.
How to maintain a car battery
Extending the life of your car’s battery doesn’t require much maintenance on your part. Just remember the following:
- Try to avoid too many short drives that don’t recharge the battery well.
- Check the cables and posts for corrosion and clean if necessary.
- Have the alternator checked for a good charge if you suspect it may be failing.
- Avoid running the radio, headlights or other accessories for long periods with the engine off.
- If your battery’s cells are not sealed units, check the electrolyte levels from time to time in each cell (you can top them off using distilled water).