Why Is My Car Battery Charger Draining My Battery?

A car battery charger is a handy tool to help recharge your drained battery. But you might wonder, why is my car battery charger draining my battery? Do not fret. We researched the answers for you, and this is what we found out. 

Your car battery can drain if you use an unreliable car battery charger. The charging system might be defective. If the voltage from the charger is too strong, it can damage the components of the car battery. Make sure to use a compatible charger for the car battery. Be careful not to overcharge your car battery, as it can also lead to draining. 

A car charging system needs regular inspection and maintenance to avoid battery drains. The cables connecting to the battery must be clean and properly connected. Check for any loose wiring connections or faulty voltage regulators. Read further to understand and prevent the car battery charger from draining your car battery.

Causes Of Car Battery Charger Draining A Battery

Selective focus charging car with electricity trough cables, Why Is My Car Battery Charger Draining My Battery?

If you think that your car battery cannot go longer than expected, the battery level is low after a charge. Sometimes, the charger can drain the battery instead of adding more power. Read some of the causes below.

Unreliable Charger

The main reason for the draining is a faulty charging system. If you use an unreliable car battery charger, it can deplete battery health. Check to see if the charger works properly. Before testing, you should have a voltmeter, an ammeter, and an ohmmeter. Then, test the charger by following the process below.

Testing Car Battery Charger

Man holding battery capacity tester voltmeter for service maintenance of industrial to engine repair

  1. Put safety first. Wear appropriate personal protective gear before you test the charger.
  2. The first process is to test the charger's output with a voltmeter. Connect the voltmeter's positive and negative terminals to the charger. Set the voltmeter DC to 20V.
  3. Turn on the charger and start your reading. The required range is around 12-14 volts.   
  4. If the voltage is out of the range, adjust the charger output. 
  5. After achieving the correct voltage, turn off the voltmeter and disconnect it.
  6. The second process is to check the current flow from the output terminals with an ammeter. Connect the charger to the reader and check the total amps. A car battery charger amps depending on the amperage of the battery. For example, if the battery rating is 100 amps, you should use a 10 amp or less charger.
  7. For the last process, you need the ohmmeter. This step will check the resistance of the charger. The values derived from the tests should be within the range. If not, the charger will not work as intended. Adjust the outputs as needed.

It is also possible to test the charger without a battery. Instead of a voltmeter, use a multimeter. Check the current with a load tester and use an ohmmeter to check any shorts. If the charger is not working, ask the manufacturer for assistance. 

Check ou this Voltmeter on Amazon.

Incompatible Charger

Another reason why the battery drains is because of an incompatible charger. You should know the type of battery your car has. If the battery is lead acid, buy a lead acid charger. The same applies to other car batteries. Once you have the charger, check its manual to follow the instructions. Using a different car charger may not work, and you compromise the battery's health.


Your battery may drain if you overcharge it. In that case, the battery gets more power than its capacity. A defective or wrong car battery charger will send out more voltage than needed. 

The effects of overcharging are the following:

  1. Evaporation of sulphuric acid and distilled water electrolyte solution
  2. Heat build-up
  3. Melting of interior battery components
  4. Spilling of battery acid
  5. Corrosion
  6. Hydrogen explosion

You must know when to stop charging to avoid swelling the battery. If the pressure inside is too high, the battery can explode.

You can use smart car battery chargers that automatically adjust the current. It prevents overcharging by cutting off more current as it nears full charge. Moreover, cyclic chargers overcharge more likely than float chargers. If you are unsure about the car battery charger, you can ask trusted dealers to help you.

You can read further about charging time from this post: How Long Does It Take To Charge A Car Battery?

Other Reasons

Listed below are other causes of car battery charger draining:

  • Incorrect charger settings
  • Loose or corroded connections
  • Plugged-in devices and other parts that are on (e.g., front lights, phones car adapter)

If you know what causes the battery to drain when using the car battery charger, you should also know how to prevent this from happening. 

How To Prevent A Car Battery Charger From Draining The Battery?

Car battery without brand surrounded by other batteries

For the reasons mentioned above, you can still avoid the battery drain if you use a car charger. Read below some tips you can do:

  • Always securely connect to the correct output terminals 
  • Check the electrolyte level of the battery and top-up if needed
  • Follow the instructions for the charger
  • Keep battery posts clean
  • Monitor when you need to stop charging the battery
  • Set the charger to the lowest charge rate
  • Turn off all electrical components in the car, such as lights

It is essential to know how to properly use the charger to avoid damage to the battery and injuries to yourself. 

What Type Of Car Battery Charger To Use?

Selecting the compatible charger will prevent battery draining. Yet, there are different battery chargers available in the market. To help you decide, below are the charger types and their descriptions.

Battery charger isolated on white background

Battery Maintainers

This type keeps the battery full even if not in use. It automatically charges the battery when it needs power and keeps it alive. You can attach this for long periods without any harm to the battery. There will be no issue with sulfuration after leaving the battery idle.  

Quick Chargers

Car battery charge

This type provides constant current and voltage for the battery. The charger can take minutes to a few hours to fully charge a battery. Yet, you only use this charger if the battery is in good condition.

The downside of this charger is the possibility of overheating and off-gassing if sulfuration is present in the battery. You must quickly disconnect the charger if the battery is fully charged and the indicator light is on. 

Trickle Chargers

Also known as float chargers, this type of charger helps improve battery health. This charger makes the batteries get slow and have low currents. It also replenishes the active materials in the battery. You also don't need to worry if you charge the battery for hours because overheating is not a problem. Choose this charger if your battery has sulphuric acid.

Smart Chargers

In addition to the basic charger types, smart or intelligent chargers can detect if the battery is already full. The charger uses battery temperature and voltage to know if the battery is at 80%. The charger will switch to trickle charging when the battery reaches that level. This charger will never overcharge the battery. Hence, its life extends. 

Below is a smart car battery charger you can buy if you don't want to drain the car battery.

Click here to see this product on Amazon.

The chargers are distinct, so you must understand and differentiate the types. When choosing the charger, make sure it is compatible with the battery. You cannot benefit from the chargers if the battery cannot use it. Carefully check the specifications of the available batteries. If you are unsure which to buy, consult your battery manufacturer or its manual. 

In Closing

Although uncommon, the car battery charger can drain the battery. If the charger is unreliable or incompatible, it will cause the draining. It would help to buy a suitable charger for your vehicle's battery. 

Overcharging not only drains but damages the battery. You must know how to use the battery charger by following the manuals, so it works as intended. Choose the right charger that will keep the optimum energy level of your battery without damaging it. 

There are ways to avoid draining the battery. Always follow the charging instructions and secure the connections. You can also prevent draining if you test the charger's output and adjust it to be within range. 

If you know how to use the car battery charger, you can maintain the battery's life.  

Learn more about car battery chargers from these posts:

How Long To Charge A Car Battery With 12 Volt Charger

How Long To Charge A Dead Car Battery

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