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How to Test Your Battery for Current Leakage

Dan Ferrell writes about do-it-yourself car maintenance and repair. He has certifications in automation and control technology.

A car battery leak can discharge your battery overnight.

A car battery leak can discharge your battery overnight.

Any abnormal electrical draw on your car battery can potentially discharge it and leave you stranded. One type of abnormal electrical draw on your battery is a parasitic load: electrical devices that continue to use current even after you shut off the engine.

This article is about a different kind of draw; current that leaks out of your battery because of an acidic layer that accumulates over time around the battery case, usually due to little or no battery maintenance.

The average car owner can easily diagnose this kind of battery leakage and fix it. In fact, you can complete this simple task in about an hour. You'll need some simple tools and cleaning supplies, and although you don't need to know much about electricity, you'll need a digital or analog voltmeter.

NOTE: If you want to test for a parasitic drain, or this current leakage test comes up negative, read the article Car Battery Problems: Parasitic Battery Drain Tests to check for a possible power drain from a load. You can do this test at home using a digital voltmeter, too.

Also, take into account the age of your battery. Usually, car batteries have a useful life of four to five years. If your battery holds power for short periods at a time, and your tests don't point to any other problems—including the charging system—your battery may have reached the end of its service life. This article on using a hydrometer will help you find out the state of health of your battery.

Tools You'll Need

  • Digital or analog voltmeter (multimeter)
  • Wrench
  • Pliers
  • Baking soda
  • Tablespoon
  • Warm water
  • Small bowl
  • Soft brush
  • Paper towels
  • Battery post cleaning tools (if necessary)
  • Distilled water (if necessary)

How to Do a Battery Leakage Test Using a Voltmeter

This simple test will help you detect any electrical current escaping across the top of your car battery case.

A visible layer of dirt or any type of foreign residue on top of the battery, or around the battery case, could lead to a power leak. Even if your battery case seems clean, but you've experienced low battery charge or battery discharge problems for no apparent reason, continue with this test anyway, to rule out a power leak as your main problem.

1. Plug in the black meter probe into the jack marked COM, and the red meter probe into the jack marked DCV.

2. Set your digital voltmeter to a low range on the direct current (DC) voltage scale and turn the meter on.

3. Touch the negative battery post with the negative probe of your voltmeter. The negative battery terminal has a minus sign (-) next to it. Your voltmeter's negative probe has black insulation.

4. Now, touch the positive (red) voltmeter probe to different points across the top of the battery without disconnecting the negative probe from the negative battery post. If your meter detects even a small amount of voltage during your power leak test, you've found a power leak and need to clean the battery case.

Proper car battery maintenance can prevent power leakage.

Proper car battery maintenance can prevent power leakage.

Warning

Keep your battery away from appliances with open flames, like dryers and water heaters, to prevent an explosion from hydrogen gas.

How to Clean Your Battery Case

To thoroughly clean your battery, it's a good idea to remove it from your car and set it on a workbench or similar work area.

Warning

Protect your eyes when cleaning your battery. Whenever you need to handle a car battery, it's a good idea to wear goggles and chemical-resistant gloves. The sulfuric acid inside your car battery can blind you if your eyes come in contact with it. Even the small amount of acid that can build up on the top of your battery case may cause permanent damage.

1. Use a wrench and a pair of pliers to disconnect your battery from the tray and cables.

2. Disconnect the hold-down mechanism to free the battery. Check that the hold-down is working correctly; if it lets your battery shake as your car moves down the road, it will reduce its service life. If shaking has already caused damage, it may explain your battery's failure to hold a charge.

3. To service your battery, use a space in your garage or work area away from appliances with open flames like dryers and water heaters. The chemical reaction inside the battery produces hydrogen gas that can suddenly ignite. And if the gas ignites, your battery can explode. So don't take a chance.

4. Prepare a solution of 1 tbsp. of baking soda in 8 ounces of warm water in a small bowl.

5. Thoroughly clean the battery case and posts with the solution, using a soft brush, and wipe the case with paper towels. Don't let the solution seep through the battery vent caps (if you have a non-maintenance-free type battery). The solution will ruin the electrolyte and your battery.

6. As you clean the battery case, check it for physical damage like cracks, loose battery posts, and bulging spots. Any of these problems may lead to battery charge drain. Replace the battery if you find any signs of physical damage.

7. Then, check the terminals on your battery cables. Do they seem oxidized or corroded? Clean them as well, using the solution and a battery post cleaning tool.

8. Clean the battery tray as well with the solution, the soft brush, and the paper towels.

Checking Battery Electrolyte

Many car batteries are now sold as "maintenance free" and don't require a periodic electrolyte level check. However, if you have a battery with removable vent caps, you need to check the acid level after cleaning the battery case.

  • With your battery still on the workbench, carefully remove the vent caps from the top of the battery using a standard screwdriver.
  • Using a flashlight, if necessary, look through the filler cap openings and check that the electrolyte covers the battery plates and their separators. If the filler cap openings have a filler ring indicator, the electrolyte level should reach the bottom of the filler ring. Otherwise, add some distilled water through the filler opening using a small funnel. Don't use tap water to refill your battery; the chemicals and impurities in tap water will settle at the bottom of the battery and can short out the plates and ruin the battery.
  • Once you've replaced the vent caps, repeat the battery leak test, using your voltmeter, to confirm that electrical current has stopped draining out of the top of your battery case.
  • Install the battery on its tray and secure it with the necessary hardware.
Even a car BAT-TERY needs proper care.

Even a car BAT-TERY needs proper care.

Now that you know how to test a car battery for current leakage, check it regularly and remove dirt and corrosion buildup as necessary, even if you don't detect current draw. Poor maintenance can drastically reduce your battery efficiency. This simple procedure will help extend the service life of your battery, says James E. Duffy in Modern Automotive Technology. Plus, it will help you avoid some issues associated with common dead battery problems.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.