How to check for and fix a battery drain in your car

How to Stop Car Battery Drains: DIY Auto
There are plenty of energy-robbing devices on your car that are supposed to go to sleep when it's parked, and sometimes, not all of them go to sleep. Here's how to give them a tranquilizer.
When your car battery dies once, it may be tempting to just write it off as a fluke. Batteries can die for a huge range of different reasons, and there’s always the chance that whatever went wrong won’t go wrong again. But when your car battery keeps dying over and over again, it’s a pretty safe bet that there’s an underlying problem that needs to be dealt with before you end up stranded somewhere.
Why Do Car Batteries Die?
The list of issues that can cause a car battery to die is so long as to approach neverending, but virtually every battery killer out there can be shoehorned into the three basic categories of battery problems, electrical system problems, and simple user error. Some of these can be dealt with at home, and others will probably require a visit to your mechanic, but there’s no way to know for sure until you roll up your sleeves and dig in.
It’s also important to note that when most people talk about a battery dying repeatedly, they are talking about a situation where the vehicle won’t start after it has been parked for any length of time. If your battery seems to die while you’re driving down the road, it’s more likely that you have some type of problem with the charging system (we’ll cover that situation as well).
What Causes a Car Battery to Keep Dying?
Some of the most common reasons for a car battery to die repeatedly include loose or corroded battery connections, persistent electrical drains, charging problems, constantly demanding more power than the alternator can provide, and even extreme weather. Some of these problems are enough to kill a battery on their own, while others are usually coupled with a battery that is already weak or on its last legs.
Headlights or dome lights left on​​​.
Headlights, or even a very dim dome light, will drain a battery dead overnight.
Make sure to check for any interior lights when it's dark outside.
Some headlights are designed to remain on for a while, but a malfunctioning system may leave them on permanently.
Battery in weak or poor condition.
​A poorly maintained or weak battery may not hold a charge very well.
Even small drains, like the memory function in your car radio, may kill a very weak battery.
Corroded or loose battery connections.
​Corroded battery connections can prevent the charging system from topping off your battery when you are driving.
Loose battery connections can also cause problems.
Other parasitic drains in the electrical system.
​Parasitic drains can be difficult to find, but they are fully capable of killing batteries dead.
Common drains include glove box and trunk lights that come on, or remain on, when they shouldn't.
Extremely hot or cold temperatures.
Hot or cold weather won't kill a battery that's new or in good shape, but a weak or old battery may fail in extreme conditions.
Extremely hot or cold weather can also magnify other underlying issues.
Charging system problems.
If a battery seems to die when you're driving, the charging system may be at fault.
Loose or stretched belts and worn tensioners can prevent an alternator from working.
Checking Headlights, Dome Lights, and Other Accessories
Car batteries are designed to power headlights, dome lights, and various other accessories whenever the engine is off, but they have a very limited capacity to do so. That means if anything is left on after the engine is shut off, the battery will almost certainly die.
Leaving the headlights on can kill a weak battery in the amount of time it takes you run a short errand like shopping for groceries, but even a small interior dome light can drain a battery dead overnight. So if you're dealing with a battery that goes dead over and over again, it's worth checking it out at night when it's dark out when a faint or dimmed dome light will be easier to see.
Some newer vehicles are also designed to leave the headlights, dome lights, or even the radio on for a while after you shut the engine off and remove the keys. When everything is working correctly, you can walk away from a vehicle like this, and everything will shut off on a timer. If you come back half an hour or an hour later, and things like the headlights are still on, that's probably why your battery is dying.