What Drains A Car Battery While It Is Off?

Have you ever had issues with your car battery? It’s very frustrating when your car doesn’t start and you have places to be. So you ask yourself, what drains a car battery while it is off?
Most common reasons why your battery drains is because you left your headlights overnight, clock, radio, and security alarm that runs after your car is turned off, bad charging, defective alternator diode, and old battery. 
Let’s dive deep into the issue and see what are the common causes of drained battery and how to identify what is draining your battery.

Things That Will Drain Your Car Battery

Almost all electronics on your vehicle can drain your battery. And as you know there are many electric-powered features on your car. Here are the 5 most common things:

  1. Human error: This is probably the most common reason. No matter how good driver you are, sometimes you will simply forget to turn your headlights off. If you have a newer car, you will get a warning when you open your door, but that is not the case on older models.
  2. Parasitic drain: This happens when your accessories continue to run even after you turn your vehicle off. This can be your dash clock, your radio, and your security alarm.
  3. Bad charging: If your battery isn’t charging properly, you can end up calling the tow company. Please note that even though you start your car, that doesn’t mean that your battery is charging properly. As soon as you turn off your car, and try to turn it back on, you might not be able to do that due to the battery not being charged properly ( empty ).
  4. Defective alternator: The mission of the alternator is to charge the battery while the car is running. If your alternator has a bad diode, it will cause your battery to drain.
  5. Old battery: I usually change my car battery every 3 years. Some people do it every 4-5 years, but your car battery might be good to go even after 7 years. It all depends on how much you drive your car, do you live in a country with long winters etc.

How To Identify What Is Draining Your Battery?

I will show you the easiest way to troubleshoot a vehicle that has a battery which drains down overnight, or over a few days of not
using the vehicle. Let’s get started.
  1. The first thing you’re going to do is rule out the battery as being the cause of your problem. As batteries get older, the plates could start to short out, causing the battery to drain down when it sits for hours at a time. So, the thing to do is when you’re done driving the vehicle for the day, the alternator was running charging up the battery, you’re going to disconnect the negative from the battery and you’re going to leave it disconnected overnight. In the morning when you go to connect this back on to the battery. If the vehicle starts right up and you don’t have the problem with the battery being drained down anymore, then clearly there is something in the vehicle that is draining the battery down. If you still have the same problem where the battery is not strong enough to turn the engine over, then definitely you need to replace your battery.
  2. When the wires are initially connected to the battery, you’re going to notice a spark. The reason for that is there’s a surge of current going into the vehicle charging up capacitors, powering circuit boards, and usually that current is under 1 amp (usually 300 to 750 milliamp), and then after a set duration maybe around 15 seconds or 20 seconds, that current should drop to a level between 25 and 50 milliamps.
  3. If the current being drained from the battery is well above 50 milliamps, then the battery can drain down over time from sitting.
  4. When you do this test make sure that there’s no light on your hood. If there is a light on your hood, remove the bulb because that’s going to interfere with this test.
  5. You need an inexpensive digital meter. They’re around $5. Set the meter to DC current, have it on 10 amps.
  6. Disconnect the positive terminal clamp and touch it with your RED plug from the digital meter.
  7. With your black plug touch the positive terminal of the car battery itself.
  8. You should see a reading of 0.01 which is 10 milliamps.
Another test that you can perform:
  1. Disconnect your positive terminal battery clamp.
  2. Touch the positive terminal clamp with your RED plug from the digital meter.
  3. Touch the positive terminal itself (located on the car battery) with the black plug.
  4. Expose your fuse box.
  5. Wait for the reading to set on 0.01 or 0.02
  6. Disconnect all the fuses one by one and watch the readings. If you get a reading above 0.02, there is a problem with that fuse and further inspection is needed.
NOTE: When you are checking the fuses inside your car, your door would probably be open. You need to push the button on the door, so the inside lights will remain turned off. If you don’t push the door button, you will have your lights on, and you won’t know which one of your fuses is faulty.
If the reading on the meter does not drop from that higher than normal current reading, many times in the past I’ve seen a faulty regulator in the alternator caused the problem of draining down the battery. You can go over to the alternator and pull the wiring harness off the alternator as well as any other wires attached. and take a look at the reading.

Car Battery Charging Time

Before you buy a new battery, I suggest you recharge it first and see if that will solve your problem. Do you know how many hours to charge it? Well, it depends on how discharged your battery is. Usually, it takes up to 12 hours to recharge.

NOTE: If your battery becomes really hot while you charge it, disconnect it from the charger immediately! The battery should not have more than 125 Fahrenheit during charging. This means that your battery has malfunctioned and cannot be fixed.

Best Chargers For Car Batteries:

  1. Schumacher SE-4022. This is a charger with options to diagnose battery problems. This is by far the best battery charger on the market. Can work with 6 and 12 volts.
  2. Deltran Battery Tender: Has automatic full charge mode. It’s LED display will show you whether your battery is empty(red light) or full(green light). Comes with a 10-year warranty.
  3. Schumacher SC-1200A/CA: Another Schumacher product. Has a charging display. Works with 6-12 volts.
  4. Schumacher SE-1052 50/10/2A: Simple but powerful product. Has 3 functions: 50amp quick start, 10amp fast charge, and 2amp trickle charge. 2 Years warranty.
  5. NOCO Genius G750: Great for lead-acid batteries up to 30AH. A built-in system to monitor the rate of charge.

See the costs of a new battery below:

Car Battery Price

If you ran the tests that I told you above, and you came up with a conclusion that your battery is just old and needs replacement, here is how much it will cost you.

The price of lead-acid car batteries is from $50 to $300. You will find a decent car battery for $150.

The price of lithium car batteries is a little bit higher, starting from $200 to $400.

The labor for battery replacement is obviously $0 because you will do the work. Just connect the positive terminal with the positive clamp and negative terminal with a negative clamp. Secure your battery with a bolt (if your model has a bolt).

Best Car Batteries

  1. Odyssey PC680 Battery: This battery has excellent efficiency. Has a strong and rugged construction. Can work in harsh environments. Has 3 times more lifespan than regular AGM batteries.
  2. XS Power D6500 Battery: Great product. I have used this one on my 2007 Hyundai Santa Fe. It’s leak-proof, no external vents. Has a 3-year warranty.
  3. Optima 34/78 RedTop: Lifespan twice longer than lead-acid batteries. Loaded with a tightly coiled column of pure lead. Strong 5-second starting burst. 3-Year warranty.
  4. VMAX857 AGM Battery: Heavy duty battery. Best bang for the buck. Very small and easily transportable.
  5. ACDelco 94RAGGM: Has about 80 AMP hours of energy and 800 cranking amps. Can work in harsh environments. IDeal for all types of vehicles.

How To Dispose of Your Old Car Battery

It is very important to properly dispose of your old car battery. Improperly disposing of your battery may lead to fire and dangerous chemical leaks. While waiting on your disposing, keep your battery in a cool and dry place, away from kids and pets.

Take your old car battery to an auto parts retailer or hazardous waste collection site. Car batteries cannot be thrown into trash because they contain lead acid. If you are unable to do this, get in touch with Earth911.com, Home Depot, or Auto Zone. They will be more than happy to help you with the disposal.

Thank you for reading my article. I hope it answered all your questions. If I left out something, please let me know in the comments.

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Hi there. I am a certified Automotive Service Excellence (ASE) mechanic since 2018 and a car detailer for 10 years.

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