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Why Does My Car Battery Discharge Overnight or When Parked?

Dan Ferrell writes about DIY car maintenance and repair. He has certifications in Automation and Control Technology and Technical Writing.

Before you replace a discharging battery, perform some simple tests.

Before you replace a discharging battery, perform some simple tests.

Why Is My Car Battery Draining So Fast?

When your car battery discharges soon after shutting down the engine, it's commonly caused by one of three things:

  • Your battery may have reached the end of its service life (often 4 or 5 years).
  • A parasitic drain is depleting battery power.
  • A problem in an electrical system is affecting battery power.

Many battery-related problems can be diagnosed at home with a visual inspection, a digital multimeter, or an inexpensive hydrometer, available at most auto parts stores.
This guide shows you how to apply some simple tests to find the cause of your battery failure. These tests are easy for the average car owner to apply. Still, you'll be advised when it's time to take your battery to a shop, if necessary, to confirm a diagnosis.

So let's find out why your car battery doesn't hold a charge.

Find Out Why Your Battery Won't Hold a Charge

  1. Inspect the battery.
  2. Do a battery terminal test.
  3. Clean the battery terminals.
  4. Do a battery voltage test.
  5. Do a quick charging system check.
  6. Check for battery leakage (and fix any leaks).
  7. Do a battery drain test.
  8. Perform a hydrometer test.
  9. Do an alternator diode test.
  10. Check for other potential battery power drainers.

Instructions and tips for each of these tests are included below.

1. Inspect the Battery

Many times, problems with a battery that will not hold a charge can be traced to poor battery maintenance. For example, you may forget to check the battery electrolyte level, check the terminals for corrosion or looseness, or make sure that the battery is held in place to prevent physical damage.

Start a battery discharge diagnostic with a visual inspection. A simple inspection may help you quickly spot problems that can lead to battery problems.

For a better inspection, it is a good idea to disconnect the battery and check it on a bench or similar surface. However, before disconnecting the terminals from the battery, you may want to use a memory saver to keep the computer memory alive and other electronic device settings (see the Saving Your Computer Memory box below).

If you don't want to buy a memory saver now, you can use a 9V battery instead. Connect the 9V battery to the car battery terminals before disconnecting the cables.

Once you have your car battery on a bench,

  • Check the battery case and posts for damage
  • Check the electrolyte level, if your battery has removable caps
  • Check the case for bulging

If the battery is damaged, replace it. Also, make sure all the hold-down hardware is in place and clean the battery tray.

Why Save Your Car's Memory?

You can use a memory saver (or a 9V battery) to preserve your car's memory. For one thing, this keeps you from losing settings on devices like stereos.

Also, in modern vehicles, the car's computer memory obtains information during engine operation from many different system sensors, including ignition timing and injector pulse width, so that it can improve driveability. If your computer loses this information, the relearning process may take hundreds of miles of engine operation.

Corroded, loose or damaged terminals can prevent the alternator from charging the battery.

Corroded, loose or damaged terminals can prevent the alternator from charging the battery.

2. Do a Battery Terminal Test

The battery terminal test helps you check whether your terminals are making good contact with the battery posts. Basically, the test measures voltage drop between the battery post and its terminal.

A high voltage drop indicates that the charging system has difficulty supplying adequate charging voltage to the battery. This means in turn that the battery has difficulty supplying power to the starting and fuel injection systems and other circuits during cranking and start up time.

  1. First, disable the fuel system by removing the fuel pump fuse. To find the fuse, look for the fuse box (inside the cabin) or the power box (in the engine compartment). Removing the fuse will prevent the vehicle from starting during the test. On vehicle models with a distributor, you can keep the engine from starting by disconnecting the ignition coil wire from the distributor. Then you can use a jumper wire to connect the cable to a good ground on the engine (any unpainted metal part on the engine will do, like a bolt or bracket).
  2. Set your digital voltmeter to a low setting on the DC (direct current) Volts scale.
  3. Connect the negative voltmeter lead to the cable terminal of the positive battery post (post marked with the '+' sign).
  4. Connect the positive (red) voltmeter lead to the positive battery post.
  5. Ask an assistant to set the parking brake, put the transmission in (automatic) or Neutral (manual), and crank the engine for a few seconds, just enough to get a good voltage reading but less than 30 seconds. If you have to crank the engine again, let the starter motor rest for a minute or more before cranking the engine again.
  6. If your voltmeter reads over 0.1V of voltage drop, the battery connections are either loose or dirty. Inspect the connections for corrosion, looseness or damage.
  7. If the terminals are corroded or dirty, refer to the section 'Cleaning Battery Terminals' below.
  8. Repeat steps 3 through 6 on the other terminal of the battery, but connect the negative voltmeter lead to the negative battery post, the one with the '-' sign, and the positive voltmeter lead to the terminal.

How to Clean Battery Terminals

Corroded battery terminals won't allow the starting system to function properly, or the charging system to recharge the battery. Cleaning the battery terminals is a simple procedure.

  1. Connect a memory saver or a 9V battery to the cables before disconnecting them to preserve the computer memory and other important settings.
  2. Disconnect the battery cables.
  3. Prepare a mixture of 8 oz of warm water and a tablespoon of baking soda in a small container.
  4. Use a small, soft brush to apply the solution to the battery terminals and battery posts.
  5. Scrub corrosion and dirt off the terminals and battery posts with a battery post cleaning tool. This is a special brush to clean battery terminals and posts. To clean side terminals use a small wire brush.
  6. Wipe the cleaning solution off the posts and terminals using a shop rag.
  7. After you reconnect the terminals to the posts, apply a thin layer of petroleum jelly or white grease to the terminals and posts to keep corrosion off the terminals.
Use a digital multimeter to test your battery and other systems affecting battery charge.

Use a digital multimeter to test your battery and other systems affecting battery charge.

3. Perform a Battery Voltage Test

Free maintenance batteries (those without removable caps) come with a charge indicator (test indicator eye) that tells you whether your battery is charged (green eye), needs a charge (no color visible), or the battery needs replacement (yellow eye).

The battery voltage test is especially useful on maintenance-free batteries because it can quickly tell you the state of charge and battery condition. On regular batteries (those with removable caps) you can do a test using a hydrometer, which can also tell you whether your battery is working (see the Hydrometer Test section below).

Using a Digital Multimeter to Test the Battery:

  1. First, turn on the high beams for ten seconds. Then turn them off and wait for two minutes (you want to remove any charge above 12.6V, usually refer to as surface charge).
  2. Disconnect the negative (black) battery cable from the battery.
  3. Set your digital multimeter to 20V on the direct current (DCV) scale.
  4. Connect the meter across the battery posts.

If you get a reading between 12.4 and 12.6, your battery has a good charge. If you were suspecting a battery problem because you had difficulty starting the engine, or you had problems with another electrical circuit, or the alternator warning light on the dashboard is coming on (charging system problems), your problem is not with the battery.

If you get a voltage reading below 12.4, the battery is undercharged. It's possible you have problems with the battery itself (see the "state of charge" table below). Charge the battery and repeat the test. If you get the same readings (and problems) after charging the battery, you may want to take the battery to a shop for a battery load test. This is one of the best ways to confirm battery condition. Still, the following tests can shed more light on the condition of your battery before you take your battery to the shop. Continue with the following tests.

What voltage tells you about your battery's state of charge

VoltagePercentage Charge

12.6V or higher








Alternator problems can discharge your battery.

Alternator problems can discharge your battery.

4. Do a Quick Charging System Check

Another quick test you want to do before proceeding is to do a quick charging system check. Make sure both negative and positive battery cables are properly connected and clean.

  1. Set your voltmeter to 20V on the DC scale.
  2. Have an assistant start the engine and run the engine at 2000rpm.
  3. Connect your meter leads across the battery posts.
  4. Your meter should read between 13.5 and 14.5 volts (consult the vehicle repair manual for your model if necessary; Haynes is a good aftermarket manual).

Note that you should get a steady reading. If the reading fluctuates, check the charging system wiring for a loose connection; another possibility is a faulty alternator or voltage regulator.

If your reading is lower or higher than your specification, then you may have a bad voltage regulator. Consult your vehicle repair manual; it might tell you how to do some quick alternator checks.

5. Test for Battery Leakage

If your battery seems to be in sound physical shape and has good, clean, tight connections, it is possible the battery is leaking its charge.

This can happen when you allow the battery to accumulate a layer of dirt or acid, especially around the top. This test is simple and quick. And all you need is your voltmeter.

  1. Set your voltmeter to a low setting on the DCV voltage scale.
  2. Connect the black meter lead to the negative battery terminal.
  3. Touch the battery top at different places with the red meter lead.
  4. If your meter registers even a small voltage at any point, your battery is leaking its charge. You need to clean the battery.

Cleaning the Battery to Fix a Leak

  1. In a small container, mix about 8oz of warm water and a tablespoon of baking soda.
  2. Apply the mixture with a soft brush to the top of the battery and wipe clean the top with a shop rag. If the battery has removable caps, be careful not to let the cleaning solution seep into the battery, or it may get ruined.
  3. Make sure the top and the rest of the case are perfectly clean.
  4. Repeat the leakage test to make sure the discharge has stopped.
Leaving your headlights on or other electrical device after shutting off the engine will discharge your battery.

Leaving your headlights on or other electrical device after shutting off the engine will discharge your battery.

6. Do a Battery Drain Test

Power leaks are not the only causes of power drain on automotive batteries. Another common source of power drains is parasitic loads. A parasitic load comes from electrical or electronic devices (usually a light or accessory) that remain active after you shut off the engine and remove the key from the ignition.

Practically all modern vehicles use a little power after the engine shuts off, for example for the radio, dash clock, alarm, and computer modules. But these small normal parasitic loads can turn into power drainers if you leave your car parked for a few days without starting the engine. Old batteries are more susceptible to this type of power drain than new ones.

Also, abnormal parasitic loads may suddenly appear. These come from electrical shorts or worn-out devices that cause a circuit to remain live when it shouldn't—a worn-out or bad relay, or a module or switch that causes a load to remain on after you turn off the ignition switch.

If you are not aware of the problem, the parasitic load can drain power off your battery overnight.

You can locate many abnormal parasitic loads using a simple method. During this drain test, make sure to keep the doors and trunk closed so that no light bulb or dome light remains on. Also, loosen the under-hood light bulb to turn it off, if necessary.

If you detect a parasitic load, you'll need to have access to the fuse box or power box. If the fuse boxes are located inside the cabin, keep the front door or doors open, but use a clamp or similar device to keep the door switches in the off position.

You may want to use a memory saver or connect a 9V battery across the battery terminal for this test (see the text box Saving Your Computer Memory above).

  1. Set your digital multimeter to read DC Amps.
  2. Remove the key from the ignition and keep all accessories off.
  3. Disconnect the negative battery cable (the one with the '-' sign next to it).
  4. Connect your negative (black) meter lead to the battery negative post.
  5. Connect the positive (red) meter lead to the terminal of the cable you just disconnected.NOTE: On some recent vehicle models you may need to wait about 30 minutes after shutting off the engine and connecting the ammeter to allow all computer modules to go into sleep mode. If necessary, consult your car owner's manual.
  6. Read the display on your voltmeter. On average, a normal reading should range between 50 and 85 mA (it depends on the particular model, number of electronic modules and power devices that remain on after shutting down the engine). Your car owner manuals may give you the normal parasitic load for your car model.
  7. If you get a reading above 50 mA, it's likely you have a parasitic load. This means a live circuit is draining battery power.
  8. If you suspect an abnormal parasitic load, start pulling fuses. Pull one fuse at a time while watching the meter. When the high reading goes down, you've found the problem circuit. You need to check if the circuit has a short or a failed device causing the drain (light bulb, relay, module, etc.).
  9. If you don't find the source of the parasitic load by pulling fuses, it's possible there's a problem in the starter solenoid connections or circuit. Disconnect the wires at the solenoid (if you need to detach the battery cable to disconnect a wire at the solenoid, make sure to reconnect the cable); check your meter reading. If the extra load goes away, check the wires that connect to the solenoid and look for a short, damaged wires, etc.
  10. One more possibility is the alternator. Unplug the alternator electrical connector. If the voltage reading goes down to a normal 0.01 or 0.02Amps, have the alternator tested.

If you recently installed or upgraded equipment or modified a circuit, isolate that circuit by pulling its fuse and check the amperage draw.

Watch the following video for a visual reference of a parasitic draw test.

A hydrometer can reveal the state of charge and health of your battery.

A hydrometer can reveal the state of charge and health of your battery.

7. Do a Hydrometer Test

Like the voltage test, a hydrometer test can let you know the state of charge (and health) of your battery in a few minutes. This test can also reveal a failure. This test is done on batteries with removable caps.

The test measures the specific gravity of the electrolyte in each cell. It compares the specific gravity (density or weight) of the electrolyte to the water. Since electrolyte is heavier than water, a hydrometer test can be used to tell whether your battery is charged, discharged or ruined.

You'll do this test using a hydrometer, an inexpensive tool you can buy at most auto parts stores or online. Hydrometers come in one of three basic types: ball type, needle type or float type. If possible, get a hydrometer that incorporates a thermometer so that you can compensate your readings to ambient temperature. Follow the instructions that come with the specific tool you use.

  • A fully charged battery will result in a reading between 1.265 and 1.299 of gravity across all cells. The lower the gravity reading, the lower the charge.
  • A discrepancy of between 25 to 50 points between any two cell readings would indicate a ruined battery.

Hydrometer results are a general indication of the state of health of your battery. If you need to confirm or do further tests, you may want to have a shop perform a battery load test. These tests are a better indicator of battery performance.

Alternator diodes (connected to the yellow wires) allow current to flow in one direction.

Alternator diodes (connected to the yellow wires) allow current to flow in one direction.

8. Perform an Alternator Diode Test

Alternators use diodes (in a rectifier) to direct current flow out of the alternator and prevent that current from flowing back.

But diodes can go bad.

Usually when this happens, you'll see the headlights, dashboard lights and stereo lights dim or flicker during engine operation. And the problem may cause battery power to drain back into the alternator when the engine has been shut off, leaving you with a discharged battery.

To test for a bad alternator rectifier:

  1. Start the engine and let it idle.
  2. Set your meter to a low setting on the alternating current (AC) scale.
  3. Connect your meter across the battery terminals.
  4. If your meter detects even a small amount of alternating current, most likely the rectifier in your alternator is bad and you need to replace the alternator.

9. Check for Other Potential Battery Power Drainers

The problems above are the most common reasons a battery refuses to hold a charge. If you still can't find the source of the problem, you may have an issue with another system. For example:

  • There may be a problem in the starting system. A short circuit may cause excessive current draw and drain your battery.
  • Check the charging system for a loose or worn-out alternator belt, problems in the circuit (loose, disconnected or broken wires), or a failing alternator.
  • Engine operation problems can also cause excessive battery drain during cranking.
  • If you recently installed or upgraded equipment, added a new electronic accessory, replaced wires or modified a circuit, start there.
  • If your battery seems to fail under extreme weather conditions, either hot or cold, have the battery checked. Old or faulty batteries are more likely to fail under these conditions.
  • Too many short trips (under one mile) can finally have a toll on your battery, especially an old one. The battery doesn't have time to recharge and its lifespan shortens.

They Don't Last Forever

Batteries do eventually die of old age, but you can prevent your car battery from discharging for lack of proper maintenance: Once a month pop the hood and check the battery terminals for corrosion; check electrolyte level (in batteries with removable caps); make sure the battery is held firmly in its tray to prevent damage; maintain the battery and terminals clean and tight. Following these simple steps can greatly increase its service life and avoid many of the common problems related to early car battery failure.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

Questions & Answers

Question: Why has my new car battery drained overnight?

Answer: Check for a parasitic drain, or a possible fault in one of the electrical systems.

Question: Can I drain all the acid and refill the battery with fresh acid? How would I do this?

Answer: Try using a turkey baster to remove the acid from each cell.

Question: My car's battery drains within minutes of shutting it off. However, If I have it running, it's fine. I can also shut it off and start it again right away. What could this be?

Answer: Have the battery checked. It may not be able to hold a charge anymore. If battery is good, there could be a parasitic drain.

Question: We just changed the alternator in my car. My friend soldered all of the wires except for one. He put one of those sleeves on it and just squeezed it together. Could this be draining my battery overnight?

Answer: Usually, a bad diode or a malfunction allowing current to go from the battery through the field. Winding can cause a battery to drain.

Question: Can the security system drain my car battery?

Answer: If this is a factory installed system, it may not be the cause. Aftermarket systems may cause trouble, though, if they are installed incorrectly. It could be improperly installed, even if it is working, or an insulated wire is damaged and causing trouble like touching a metal component.

Question: How do you test if an alternator is bad?

Answer: This other post may help:

Question: Why does my V8 Dodge Ram show a full battery at night and in the morning will show it has no charge?

Answer: If you haven't found any parasitic load that might be draining the battery, have the battery checked. A faulty battery may be unable to hold a charge. This other post may help too:

Question: Could my car have an earth breakdown that causes a battery drain?

Answer: It could be the circuit itself - if you haven't measured any drains, have the battery checked. It may appear would to a voltmeter, but an auto parts store can check it for you. A bad cell can cause your battery to drain, but it won't be caught by your DMM.

This post might help:

Question: Every morning my car won't start. Is the battery depleted or wires shorted?

Answer: Have the battery checked and make sure it’s still good. The problem could be with the charging system, if the battery discharges after driving, or you may have a parasitic drain. This other post may help you with the last item:

Question: Why is my 2004 Chevy Malibu battery losing charge while driving when the battery and alternator are new?

Answer: This is usually caused by a problem in the charging system; if the alternator and battery are good, check the drive belt tensioner and the belt (worn or damaged). Modern serpentine belts won’t show signs of wear. Check your manufacturer suggested service schedule. Also, you might want to check the voltage drop for the charging circuit. This other post will give you an idea about the test:

On the charging system, check between battery positive and alternator battery connection; and alternator case and engine ground. Also, you may want to go through the steps for the voltage drop described in the link.

Question: New battery, but the battery is taking over an hour to go to sleep. After 30 minutes, it's measuring 1.40-1.60; after about an hour it measures 0.40. Why is this?

Answer: It's probably one of the modules that are taking time to shut down. Check the car owner's manual or vehicle repair manual. On newer cars, a module may take 30 minutes or more to go to sleep. Hope this helps.

Question: Can l recharge my 12V battery with a 60W solar panel in order to reach 12.6V that shows 100% charge? If not, what are the effects on the battery?

Answer: I don't see any problem but I think you'll need a controler. I've never used one myself. It'll be better to read the manufacturer's documentation for the best set up and recommendations.

Question: I have a hybrid Lexus, and my 12V battery discharges if the car is not used for more than three days. Lexus put the new 12V battery in, checked the hybrid batteries, and said they are good. This problem has lasted for five days. Do you have any ideas?

Answer: One of the modules or another circuit is possibly drawing too much current and discharging the battery when not in use. You might want to check for parasitic draws. This other post may help:

Question: I have to remove the positive terminal at night when it is cold or my car won't start the next morning. Why?

Answer: This could be a possible explanation. During cold weather, the chemical reaction inside the battery slows down considerably, and engine power goes up. Disconnecting the battery might be saving some energy the starter motor desperately needs to fire up the engine. This other post may help you deal with this issue:

Question: Why have you made the system of checking your car battery so comprehensive? You will scare most people off. You and I are aware that it is probably a failing battery. For example, dirty terminals will not cause a battery to discharge overnight. I suggest revising your very comprehensive list and making a very quick and simple checklist that an amateur can follow.

Answer: I try to make a point of covering most situations to help people deal with different scenarios. Still, each situation is, hopefully, described in a simple way so that most people can understand.

Question: What is a parasitic drain?

Answer: This is like an unusual electrical load that remains after you turn off the ignition switch. This other post may help:

Question: I have a carbureted engine in my Honda SX8 1997. Battery drains overnight. The issue started happening after passing a few flooded streets (knee-deep flood). It killed some of my wrings (some relays had been checked and replaced). Could it be that my alternator is draining my battery? The battery is 2 years old?

Answer: Have the battery checked to make sure your battery is still good. If there’s something wrong with it, the alternator may have something to do with it. Check for a parasitic drain and the alternator, if necessary. These other posts may help:

Question: Very recently, I changed my fan belt after it tore while moving, but I replaced it with a standard one and tighten it. Often, when I start the car I detect the battery warning light, after cranking the engine, most of the time when cold. Engine rpm’s go more than 2000 and still the warning light is there. The moment you rev the engine more than 2000, the light goes off until the next start. What might be the problem?

Answer: If it only happens when starting the engine, the belt might be slipping or an idling gear or tensioner (depending on your particular model) might be getting stuck and getting "unstuck" when revving the engine. Check these components. If possible, have somebody crank-start the engine and rev while you watch under the hood what's going on.

Question: The battery holds its charge while out of the car. As soon as it's put in to the car the voltage drops from 12.8 to 12.2 in a matter of minutes. Any ideas?

Answer: Have the battery tested, make sure it's working right. Check the charging system:

And check for a parasitic draw:

Question: I have two yellow top batteries for my audio system. I charged both batteries over the weekend but I'm still getting a low voltage. Now I know a few weeks ago the negative side of the battery touch a pole in the back of the car. Could that have done something to the ground wires?

Answer: If voltage output is low, have the batteries checked. Also, try checking the voltage drop in the ground side, if necessary. This other post may help:

Question: I have a 2001 Yukon that I have bought two different batteries and two different alternators for. They have checked it for a problem and thought it was the alarm system and it wasn't so. When I get out of the truck I just unhook the battery so it will start the next day. What is the problem with my Yukon's battery?

Answer: There could be a parasitic draw, draining the battery. This may include a module that doesn't go to 'sleep' as it should after a period of time. This other post may help you:

© 2017 Dan Ferrell