how to test a battery charger

How to Test a Battery Charger Properly?

When your battery charger gradually shows inefficient functioning signs like not charging or heating up quickly, it can be time to have it checked. Knowing how to test a battery charger is necessary to avoid causing any effect on the next charging session when your vehicle falls into a state of sudden battery failure.

With a clean and clear mindset on the charger issues, you can also deliver timely repair options for follow-up treatment. Overall, it is a pretty simple process. So let’s find out now with RVing Trends.

How To Test A Battery Charger: Steps to Follow

Regardless of what battery charger you are going to test, whether it is a deep-cycle battery charger or a small rechargeable unit, grasping the charging process can be extremely helpful for the device to refill batteries to a functional status.

The general procedure for testing a car battery charger may be similar to any battery type you are using or on the market. As you only need to connect the multimeter’s negative and positive test probes to the corresponding contact points on your exhausting or malfunctioning charger. The testing gadget will then give you figure readings displaying the existing voltage put out by the charger.

Here are some quickest and most effective ways to test a battery charger. Take some notes, and your charging gadget will be ready to go.

Testing battery charger with bare eyes

In this outer-round simple eye test, you can turn off the engine, open the capo cover, and gradually check every part of the vehicle battery with your bare eyes. However, the accuracy is not as high as using more mechanical methods.

Check the alternator motor drive belt for corrosion or looseness, and observe the alternator’s connector or fixing bolt. You should also observe the wirings to see if they are broken or worn and whether the bolts are loose. One of the parts that cannot be ignored anymore is the electrodes, the battery, etc. If oxidized or rusted, take corrective action to avoid affecting the trip.

In addition, you can pay more attention to checking the belt section. Check to see if the belt tension is correct and if the belt booster is damaged or showing signs of tearing, cracking, or peeling.

Testing battery charger with multimeter

The most promising tool that will help how to tell if a battery charger is working is a multimeter. Follow the steps below to get the best outcome of your testing process.

Step 1

Plug in the charger. To determine if the charger delivers the expected amount of power, you need to connect the charger to a power source first. Plug the power cord into a nearby AC outlet for the charger to start charging. You will measure this voltage with a multimeter.

  • If the charger has its own On/Off switch, turn the switch to the On position.
  • For the tool, a multimeter (sometimes called a voltmeter) can check the voltage levels of many different electrical devices. You can buy a digital multimeter at any electronics store or online for an average price of $9 to $20.

Step 2

Connect the probe of the multimeter to the corresponding port. Most multimeters come with a pair of red and black detachable probes that measure the current flowing through the battery or charger terminals. Plug the black cathode probe into the COM port, and the red anode probe into the V port on the multimeter.

  • Depending on the design of the model you are using, in some cases, the probe port may be marked by color instead of text.
  • If your multimeter has a built-in probe, you can skip this step.

Step 3

Set the multimeter to DC (direct current) mode. Find the dial that shows the different settings on the tool. Turn the knob to the DC range and stop at the following highest setting relative to the charger you are about to measure. It is a preparatory step for testing the charger with DC mode.

how to check a battery charger
Here are some core multimeter symbols for your reference. Photo: Electronics Hub


  • To test AA batteries with an expected voltage of about 1.5V, you need to select the 2DCV setting.
  • Direct current is the current that flows directly from the terminal to the terminal.

Warning: Using the wrong multimeter can cause overload and even more severe damage such as fire and explosion. To avoid this, you need to ensure the multimeter is in the correct mode for the type of current to be measured and set it to a higher voltage than the device under test.

Step 4

Connect the black probe to the cathode contact on the charger. If the battery charger connects to the battery through the power supply port, connect the probe to the metal pin at the end of the jack. If you are testing a base charger similar to an AA battery charger, touch the probe to the bare metal marked with the negative sign (-) on the side of the charging chamber.

Note that some multimeters have input ports that support the direct connection of certain types of power jacks to the instrument.

Step 5

Connect the red probe to the anode contact on the charger. Insert the probe into the metal pin located at the end of the power supply jack. For docking chargers, you have to insert the probe into the bare metal marked with the plus sign (+) on the side of the charging chamber. If you confuse the two poles, the multimeter may show a negative reading (or no reading at all). At this point, swap the two probes and measure again.

Step 6

Check the number displayed on the multimeter. This number represents the number of volts of the direct current supplied by the charger. The battery charger needs to transmit a voltage equal to (preferably higher than) the level of the battery to be fully charged in time. If you are confused and do not know which number is correct, check the manual that came with the charger, or find the information on the device itself.

For example, a standard lithium-ion battery has about 4 volts. Larger electrical appliances may use batteries/batteries with 12-24 volts. If the test readings are below the recommended level, it is time to buy a new charger.

You may also like: Best Solar Powered Battery Charger for RV

Testing the battery charger’s voltage drop out

In addition to the above direct testing method, you can check the charging system in a different approach through a voltage drop test. To check the voltage drop to master the outcome on how to test a battery charger, you can proceed to follow the steps below:

Step 1

Depress and hold the gas pedal to let the engine run at 1500 RPM. In addition, you also need to turn on other auxiliary equipment such as radio, air conditioner, or headlights.

Step 2

On your power multimeter, move to the DCV scale. Insert the red and black test leads of the electrical tester to the positive terminal of the battery and the B+ pin on the generator, respectively.

The voltage level will fluctuate around 0.2V or less. If the voltage level is above this threshold, it may cause a drop in the charging voltage. Please check the connection of the generator and battery for loose or corroded signs.

Step 3

Repeat the measurement procedure as above. However, put the red probe on the generator case and the black one on the battery’s negative terminal. The voltage drop must be at 0.05V or less. If it is higher, the charging battery may have been damaged. And you should check the battery’s ground connection for timely treatment.

Testing the charging ability of the battery using multimeter

Here is another type of test you can apply within your battery charger and battery inspecting process.

Step 1

Turn on the car’s electrical system. After turning on the key, open the headlights to use and suppress the surface charge that the battery can accumulate. However, do not rush to start the car. Before checking the charging capacity, get the “static” indicator to see the current charge level.

  • You can also turn on the radar, blower, emergency lights, and other electrical components for even more power dissipation.
  • The surface-charge suppression step will help the indicator accurately reflect the charging capacity.

Step 2

Set the multimeter to direct current. Turn the knob on the multimeter to a pre-designed mode to measure DC in the following highest voltage range against a battery. Like batteries for small appliances, vehicle batteries use direct current to power engines, headlights, fans, and other electrical components.

  • Batteries typically provide 12DCV, about 6 times more than household batteries.
  • To avoid overloading the multimeter, check if the device is set to a higher voltage than the battery (most 20 DCV).

Step 3

Connect the multimeter probe to the two battery terminals. The best way to proceed is by inserting the probe into the space between the battery terminal and surrounding metal fittings.

This act will ensure that the probe does not fall off during the measurement. You need to find and connect the probe to the cathode first, and then to the anode. Immediately after connecting both probes, the multimeter should show something in the range of 12.6 volts.

testing battery charger with multimeter
It is the static voltage of the battery that shows the battery is still charged, not the charging capacity. (Photo: Joe Belanger)

Step 4

Start the car. The reading on the multimeter will drop suddenly as soon as the starter uses power from the battery. Let the engine continue to run for about 5 minutes to allow the generator to charge for a while. Notice if headlights or electrical components dim or turn off when starting the car, it can be a sign of a damaged battery.

Step 5

Turn off the device and see if the display reading falls around 13.2 or higher. Moreover, turn off the entire system including lights, radios, and other electrical appliances. At this point, the multimeter will display the new reading. If this reading is higher than the battery’s static voltage, the generator is stable, and the battery is properly charged.

  • If the readings do not change, there is a chance that the generator is damaged. You should arrange to have your vehicle checked at a professional garage.
  • If you are testing an external battery charger, a reading in the same range indicates that everything is fine.

Read more: Best RV Power Inverters

The Safety Notices When Testing a Battery Charger

If you don’t know how to check a battery charger properly, you can gradually learn it later with more practice. However, for the below warning notices, remember to always keep them in mind before getting your sleeves on one mechanical-related project.

  • The liquid electrolyte in the car battery is a mixture of sulfuric acid and water. Sulfuric acid is highly corrosive, so when it gets on your skin, you should immediately wash it off with clean water. If you have to contact the battery often, washing with baking soda may be the ideal solution as soda will neutralize the acid and minimize damage. An important note is that you need to flush your skin with water in high forces. Moreover, sulfuric acid is also corrosive through clothing. So you should wear old-thick clothes, gloves, and goggles when handling batteries or battery chargers.
  • Turn off the engine before removing the battery. In most modern cars, the computer controls the functions of the engine, fuel injection and ignition systems, automatic transmission, and many other devices. So be careful not to disconnect the power suddenly to avoid accidentally damaging the computer system.
  • Always remove the cathode wire from the battery if you want to repair it under the car hood. This act will prevent potentially damaging other electrical components or causing an electric shock.
  • When removing and replacing both cables, remember to remove the cathode wire first and reinsert it later. If you do the opposite, which means removing the anode first, when the wrench touches some metal part, it can melt that part as an electric welder.
  • Re-plug the cable as you work with the battery. Do not allow any metal objects to come into contact with the end of the battery terminal. If the cable connects to the terminal when something interferes, the computer on the control panel will be severely damaged.

We hope these above notices on how to test a battery charger can give you some helpful insights to apply to your real successful process.

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