How to check the electrolyte level of a car battery

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How to check the electrolyte level of a car battery
How to check the electrolyte level of a car battery

The electrolyte level of a car battery (which is not really water) should be checked regularly for two reasons: first, because the electrolyte solution tends to evaporate over time. time, secondly because some of that electrolyte turns into hydrogen and oxygen every time you recharge the battery. Monitoring a battery's levels and adjusting those same levels with water helps keep a car running smoothly. Here is how it is necessary to operate to control the electrolyte of a battery by taking all the safety measures, for you as for your vehicle.


Part 1 of 4: clean and open the battery


Step 1. Locate the location of the battery

Most often, it is enough to lift the hood to see it, because it is placed on the upper part of the engine. Two cables go out, one black, the other red.

  • Some batteries are buried a little deeper in the engine, for example, between the bumper and one of the wheels. Other times, they are only accessible from the underside of the vehicle. In the latter case, they must be removed entirely for maintenance.
  • With some manufacturers (BMW, Mercedes Benz), the batteries are in the trunk, in a separate compartment.
  • On some cars, the battery may be located under the rear seat. This is the case with some Cadillacs.

Step 2. Perform an initial cleaning

Before checking battery levels, thoroughly clean the top and terminals of the battery. You are going to open the battery cells and no dirt should get into them. Likewise, a regularly cleaned battery protects the surrounding metal parts from corrosion.

  • Take a simple glass cleaner (ammonia based) to clean the exterior of your battery. Dampen your cloth and rub. The cleaner should not be sprayed directly on the battery. You can use a paper towel as long as it doesn't fall apart.
  • In case of strong corrosion, take sodium bicarbonate. Prepare a slightly solid dough with baking soda and water. As with window cleaner, your paste should first be applied to a damp cloth. Do not sprinkle baking soda on your battery. If the battery is very dirty, it will have to be repeated several times. At the end, finish cleaning by wiping a cloth soaked in window cleaner to remove the last traces of baking soda. If you don't, corrosion will be more intense at the terminals and all metal parts in the area.
  • Don't put the cart before the horse! When cleaning the exterior, make sure the battery caps are in place. It would be dangerous to introduce any cleaning product into one of the battery cells.
  • Note: you can also take the battery out of the car to clean it. When the job is done, you reinstall it in its place. Even if it takes a little longer, it is better to take the battery out and put it on a practical surface. The downside is that you will have to reprogram certain electrical equipment, such as the clock, the car radio, etc. If you can maintain your battery without removing it, you will certainly save a lot of time.
  • To clean the battery terminals, you can also detach them from the battery and place them in a tall glass of very hot water. Corrosion will be removed with hot water. Then make sure that the terminals are perfectly dry before replacing them on your vehicle's battery.

Step 3. Open the fill holes

On today's batteries, they are sealed by two plastic clips located on the top of the battery. With infinite care, lift these bars, with a screwdriver for example. If they are difficult to remove, it is necessary to gradually leverage around the perimeter.

  • Some batteries do not have strips, but six individual caps. To remove them, we use a simple screwdriver a little large.
  • A so-called "maintenance-free" battery should never be opened. It is also indicated by the manufacturer that water should not be added. If the battery shows signs of weakness, it should be replaced altogether.

Step 4. Continue cleaning

When you go to remove the caps, dirt will inevitably come out and settle on the battery. You should carefully remove dust with a cloth lightly moistened with glass cleaner.

  • During this cleaning phase, do not use sodium bicarbonate! Use very little cleaning product and be careful not to introduce anything (cleaning agent, grains of dust, pieces of paper towel, etc.) into the cells which are now accessible.
  • Do not neglect this cleaning! This will prevent faster corrosion. This is an important phase in the maintenance of a battery which will ensure a perfect connection of the different elements.

Part 2 of 4: Assessing Electrolyte Levels Correctly


Step 1. Compare the different electrolyte levels

Looking over, check the level of the electrolyte solution in each of the cells. Each of them must contain the same quantity of solution and therefore have the same level of filling.

  • If some levels are too high, it is certainly accidental: they were previously overfilled. Don't touch anything! The levels will drop naturally over time. You can then redo the levels correctly.
  • If one or more of the levels are too low, there is a high probability that the battery is leaking or cracked. In this case, the only possible solution is to replace the battery. If the battery body is intact, re-level the cells too low and check those same levels a few weeks later to see what it is.
Check Car Battery Water Levels Step 6
Check Car Battery Water Levels Step 6

Step 2. Know when electrolyte levels are too low

It's quite simple: the electrolyte solution is missing as soon as you can see the top of the lead plates. These must be covered to be able to deliver the right voltage at the right intensity.

  • Lead plates that remain uncovered for too long will be irreparably damaged.
  • If the electrolyte level is one centimeter below the top of the lead plates, add water to just cover them. The battery will then be able to deliver an acceptable level of current (see part 3 of this article on how to re-level). If the battery remains low, it must be replaced.
  • A low level of the electrolyte can be explained by an overload coming from the alternator. It is then simply necessary to check that the latter works well.
Check Car Battery Water Levels Step 7
Check Car Battery Water Levels Step 7

Step 3. Know the normal level of electrolyte

The normal level is 1 cm above the lead plates or 3 mm below the ceiling of the battery (visible through the filling holes). Often the level is visible through transparency and must be between two limits (MIN and MAX).

If the level is good, don't touch anything! Put the cap back in place and remember to check it again in three months

Check Car Battery Water Levels Step 8
Check Car Battery Water Levels Step 8

Step 4. Know how to locate the high level of the electrolyte

As much as possible, the electrolyte solution should reach the bottom of the fill holes.

  • At the bottom of the filling holes, you will see small slits on the sides. They are there to give a concave shape to the surface (we speak of the meniscus) of the electrolyte, when it reaches the bottom of the hole. If you are below the hole, there will be no meniscus.
  • As soon as you see the concave meniscus, you can be sure that you have reached the maximum level. The filling must be stopped. If you can't see very well, feel free to use a flashlight.
Check Car Battery Water Levels Step 9
Check Car Battery Water Levels Step 9

Step 5. Only “lead-sulfuric acid” batteries can be refilled

Always follow the advice of the manufacturer or the dealer, he is the authoritative on this article.

Certain particular batteries (nickel-cadmium, those fitted to cleaning machines or golf carts) may have different filling marks

Part 3 of 4: top up electrolyte levels


Step 1. The cells should be filled with distilled water

The latter can be bought in any drugstore or supermarket. If you find that some cells are too low (plates exposed to air), just pour water to cover the plates. Then you can either ride to recharge the battery or charge it at home for a few hours. If the battery is well charged, but some levels are too low, do not exceed the maximum level, located just at the bottom of the fill hole.

  • To pour precisely the amount of water needed, use a precision utensil, such as a funnel, a mechanic's pipette or a sports bottle. Be careful not to introduce any foreign object or product into the cells.
  • Avoid using any other water (tap, well, etc.) than distilled water. Indeed, these waters contain elements (salts, chlorine, pesticides…) which, because of certain chemical reactions, reduce the life of your battery.
Check Car Battery Water Levels Step 11
Check Car Battery Water Levels Step 11

Step 2. If the battery is fully or partially discharged, avoid filling the cells to the top

If necessary, just cover the plates with lead.

  • When charging a discharged battery, the electrolyte level rises, so do not overfill. If the battery is fully charged, none of this will happen.
  • The electrolyte level also rises when the battery is called upon, during start-up for example.

Step 3. Wipe up any spills and close the fill holes

The entire surface around these holes must be thoroughly clean before replacing the plugs.

  • If you add too much water, but the solution does not overflow, stop everything and leave it as it is. If any liquid spills out, remember that it is acid and that it is out of the question to be removed without care.
  • Sponge up the excess with rags or a paper towel. Your rags should not be too wet, otherwise you will scatter acid on other parts of the engine, which is not good. Rinse your cloth in a large bucket of water. Obviously, this operation is done with gloves.
  • Once the job is done, throw the rags or paper towels in the trash, gently empty the contents of the bucket into the sink, taking care not to spray everything. Acid can corrode many surfaces. Finally, wipe it down with a rag soaked in glass cleaner.
  • If the cells are too full, monitor your battery at least once a week for a month to identify possible bleeding. If so, wipe off immediately as instructed.
  • Sulfuric acid that has boiled over does not need to be replaced. The quantities lost are negligible and do not affect the proper functioning of the battery. No need to reopen the caps to redo the levels! For a longer battery life, know that it is better to be below the levels than above.

Part 4 of 4: take certain precautions


Step 1. Wear eye protection

Since the electrolyte solution is in fact more or less diluted sulfuric acid, you must wear protective glasses in the event of unexpected splashes. You simply risk blindness if you damage your eyes.

  • In any case, contact lenses can not offer any protection, on the contrary! A pair of prescription glasses does not protect better since there are no protections on the sides.
  • That's why you have to wear real safety glasses that you will find in any good DIY store.

Step 2. Protect your hands with disposable gloves

Go to a DIY store and ask the salesperson for gloves that are resistant to sulfuric acid.

  • Latex or vinyl gloves are not acid resistant for long. If you notice any splashes on these gloves, remove them quickly. If you keep them, the acid will quickly puncture the glove and then attack the skin.
  • Neoprene gloves are the most durable (about an hour), but they are not very easy to find on the market. Neoprene is not the same as nitrile. Nitrile rubber gloves offer even less protection than latex, so it is best not to use them!

Step 3. Protect your skin

Wear old clothes that cover you completely (long pants, long-sleeved shirt) and safety shoes or at least closed. The acid droplets form holes in the tissue within a few days. It is therefore advisable to wear clothes that you do not plan to put back on later.


Step 4. Know what to do if the electrolyte solution gets on your skin

If unfortunately this happens to you, know that it is necessary to wet the affected area abundantly while soaping.

  • Any burning or tingling sensation is caused by acid splashed onto the skin. Even a tiny drop can cause a serious burn.
  • Respond quickly. Don't wait to feel a burn! If you are the victim of splashing, even minimal, stop everything and rinse the affected area thoroughly.
  • Don't throw your rags and gloves anywhere! Take them to the recycling center. Indeed, if this waste were to come into contact with other materials, this could trigger unexpected reactions.


  • If you don't feel like a mechanic, take this task to a professional. Auto supply stores offer this service, often for free.
  • Work in a clean space.
  • Do not open your battery while the engine is running!
  • During maintenance, wear protective glasses. You risk going blind if you get acid splashed.
  • Wearing protective glasses is more than recommended when checking and filling cells.
  • Thoroughly clean the outside of the battery. In fact, dust tends to retain moisture, which is conductive, especially as acidic vapors emerge from the battery. Under these conditions, if an electric current were to flow outside the battery, electrolysis would begin, systematically attacking all the surrounding metal parts.
  • To remove the closure strips, use a plastic spatula 3 to 4 cm wide, available in any good DIY store, in the paints department. You can also take a screwdriver with an insulating handle. Be careful not to touch any metal parts with the metal part of the screwdriver. A spark could ignite and start a fire by igniting hydrogen vapors from the battery.

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