How to Check Car Battery Water Levels

how to check car battery water levels

It’s important to regularly check the electrolyte level in your car‘s battery for two reasons (it’s really not just water). The first reason is that it naturally evaporates, and the second is because each time the battery is charged, a small amount of the electrolytes split into hydrogen and oxygen. Learning how to safely check and top off the water in the battery is an important aspect of maintaining your car. Start with Step 1 below for a detailed guide on how to check the battery water level while protecting both yourself and your car.

Clean the battery and open the caps

find the battery

In most cars, you simply have to pop the hood of your car to access the battery. Some batteries are lower down, behind the front bumper and in front of the front wheels. Sometimes these need to be accessed from below and may need to be removed for service. Most BMW and Mercedes Benz batteries, as well as a few other manufacturers, are located in the trunk in an insulated compartment. The battery may also be located under one of the rear seats, e.g. in some Cadillacs.

Clean up.

Before checking the water level, remove dirt from the top of the battery and from the battery terminals. This is important because you don’t want any foreign material to enter the battery cells when you open them. It’s also important because a clean battery surface helps slow the corrosion of surrounding metal. For general cleaning of road grime and minor corrosion, use an ammonia-based window cleaner. Spray the cleaner onto the rag – not the battery itself – and wipe away the dirt. You can also use paper towels, but you must replace them before they fall apart. Heavy corrosion can be cleaned with a paste of baking soda and water. Dampen the rag and wipe over it, do not soak the battery in baking soda. Sometimes you have to repeat this multiple times. Then wipe with a cloth dampened with window cleaner to remove the baking soda. Residual baking soda on the outside of the battery will accelerate further corrosion of the terminals and surrounding metal. Don’t rush here – make sure the covers are still on the battery latches. Do not allow cleaning fluids to drip or flow into the battery via the caps. Note: If you prefer, you can remove the battery from the car beforehand and then reinstall it. This may be safer, especially if the battery is in an odd position. However, this resets some or all of the car’s electronics (clock, station settings on the radio, etc.). Being able to service the battery without removing it from the car usually saves you a lot of time as well. You can also disconnect the battery terminals from the battery and submerge them in a cup of very hot water. The hot water will melt away the rust leaving a clean surface. Make sure the clamps are completely dry before reattaching them to the battery.

Open the clasps.

There are usually two semi-rectangular plastic covers on top of the battery, which are used to close the battery cell closures. These can be removed by gently prying them open with a plastic spatula or screwdriver. Try prying the cover-up from various places around the cover if it doesn’t come off right away. Some batteries have six individual, circular covers instead. You can remove this by turning it anti-clockwise and lifting it off. If the battery says “maintenance free” it should not be opened. The manufacturers state that these batteries cannot be refilled with water, they simply have to be replaced when they stop working properly.

Keep cleaning if necessary.

Removing the latch covers may reveal more dirt on the battery. Clean them further by wiping away from the latches with a cloth dampened with window cleaner. Don’t use baking soda here. Use a little window cleaner and be very careful not to get anything (cleaner, dirt, small scraps of paper towel, etc.) into the battery. Don’t be tempted to skip this step – keeping the battery clean will reduce future corrosion. This is an important aspect of maintaining a battery to maintain the integrity of the connections.

Assess the fluid level

Compare the liquid level in the cells.

If you look down into the connectors, you can see the electrolyte level of each cell. The cells should be covered with the same amount of liquid. If it doesn’t, it could simply be due to accidental prior trapping. In this case, simply refilling properly later, after the level has dropped to a normal range, can solve the problem. If the fluid levels are obviously uneven, it’s also possible that the battery has a small leak or crack in the case. If this is the case, the battery needs to be replaced. If there is no obvious leak, fill the battery to the maximum safe level using only distilled water. Check again in a few weeks to see if the levels have remained consistent.

Recognize when electrolyte levels are low.

The electrolytes are too low when one of the plates comes into contact with air. If the plates are not completely covered with electrolyte then the battery cannot operate at full capacity. If the panels are exposed to air, the exposed area will be ruined in a few days. If the electrolyte level is only about 1 cm above the plates, adding enough water to cover the plates can restore the battery to working condition with a slightly reduced capacity. (Tutorials on how to add water come in Part 3 of this article.) Otherwise, you’ll need to consider replacing the battery. A low electrolyte level could be caused by overcharging. If this is the case, you should consider having the alternator checked.

Recognize when the electrolyte level is normal.

The normal liquid level is about 1 cm above the top of the plates or about 3 mm below the bottom of the fill tubes extending down from the ports. If this is the case, it may not be worth the effort to top up the battery now. Just put the latch covers back on and check again in three months.

Know when the electrolyte level is at its maximum.

The maximum safe liquid level is reached just touching the bottom of the fill tubes. Most stuffing tubes have a few slits in the side further down. This results in the meniscus (the bit of liquid at the edge of the tube) having a distinct eye shape when the liquid is touching the fill tube, whereas there is no meniscus when the liquid is below the fill tube. The eye-shaped meniscus was designed as a signal to stop refilling. You may need a flashlight to clearly see the fluid level and whether or not a meniscus is visible.

Keep in mind that these levels only apply to lead-acid car batteries.

You should always follow the advice of the battery retailer or manufacturer if it differs from the information in this article. Also, be aware that batteries for golf carts, floor cleaning machines, and nickel-cadmium batteries may require different electrolyte levels.

Adjust the liquid level

Only fill the cells with distilled water.

You can buy distilled water at most supermarkets. If the cells are low on electrolyte (the plates are exposed), fill the cells to just cover the plates. Then charge the battery with a charger or just drive the car in normal operation for a few days. Only fill the cells to the maximum safety level – just touching the bottom of the filling tubes – when the battery is fully charged. Use a clean funnel, sports bottle, or squirter to keep track of flow and final stand accuracy when refilling. Be very careful not to get dirt or detergent into the cells. If you use tap water, spring water, filtered water, or anything other than distilled water, then minerals and chemicals (e.g., chlorine) and other contaminants will get in and shorten battery life.

If the battery is weak or discharged, avoid filling the cells completely.

If you add water because the battery is low or dead, it’s better to add just enough to cover the plates (or leave the level if it’s at a normal level). When charging a weak or discharged battery, the electrolyte level increases, so you should leave room for it. (This will not happen with a fully charged battery.) The electrolyte level can also rise as the battery gets hot.

Wipe off spills and seal the cells.

Make sure all areas are clean and free of debris, then reattach the cleaned tabs to the battery. If you have accidentally overfilled the battery but there is no overflow to the top of the battery, your best bet is to just stop refilling and leave it as is. If something has spilled onto the top of the battery, remember that the liquid contains acid – do not get it on your skin or clothing. Wipe away from the ports with a rag or paper towels. Don’t get the rag or paper towels wet enough that they drip onto other parts of the car or anything else. Rinse the rag or paper towel in a bucket of water. Wear gloves – don’t let the water get on your hands. When this is done, dispose of the rinsed rag or paper towels in the regular trash. Pour the water down the drain, being careful not to splash. You don’t want to risk remnants of the acid going anywhere else. Finally, use a rag dampened with window cleaner to clean up anything the overflowed liquid came in contact with. Inspect an overflowing battery weekly for a month to see if overflowing has occurred again. If necessary, clean up an overflow as described above. The amount of sulfuric acid lost in the battery in the event of an accidental overflow is probably so small that it is irrelevant to the battery’s operation. It’s best not to try to replenish acid to replace this loss. (Too much acid shortens battery life more than too little.)

Take the appropriate safety precautions

Protect your eyes with safety goggles.

The electrolyte in the battery is sulfuric acid so it is extremely important not to get any of this liquid in your eyes as it could cause significant damage or even blindness. Contact lenses do not protect you and may complicate an accident. Ordinary glasses do not offer adequate protection due to the lack of protection on the side. It is therefore important that you wear safety goggles. You can get them at any hardware store.

Protect your hands with disposable gloves.

Choose a type of glove that can withstand sulfuric acid for at least a few minutes. You can also find them at hardware stores. Latex and vinyl gloves do not withstand the acid for long. If you use such gloves, change them immediately after you notice a splash on them. A splash of the electrolyte will eat through the glove and burn your skin. Neoprene gloves offer protection for an hour or more but are harder to find at a regular hardware store. Nitrile is not the same as neoprene. Nitrile gloves offer less protection from sulfuric acid than latex and should not be used.

Protect your skin

Wear old clothes with long sleeves, long pants, and closed-toe shoes to cover as much skin as possible. If the electrolyte splatters on your clothing, in about a week or two, the fabric will rot and puncture the spot. So wear old clothes that you can sacrifice.

Know what to do if the electrolyte touches your skin.

If the electrolyte splashes on your skin, wash the area immediately with soap and running water. If you feel any burning or tingling on your skin, you may have splashed a drop of electrolyte on yourself. Only one drop is needed for a chemical burn. You may not see any redness or injury until it’s too late. So if you suspect you’ve splashed yourself, take a break and wash up immediately instead of taking the risk. Throw away any gloves you’ve worn and any rags you’ve used when you’re done. Allowing these to come into contact with other materials can cause damage.


If you don’t know what you’re doing, take your car to a mechanic. Most auto parts stores will perform this service for free. Keep all areas free of dirt while servicing the battery. Do not remove covers while the engine is running. Make sure your eyes are protected. Battery acid can blind you and is very corrosive. Wear safety goggles when checking and topping up battery cell fluid levels. Pry off the covers with a 1-inch (2.5 cm) wide plastic spatula. You can buy a plastic putty knife at a hardware store or anywhere you buy house paint. Alternatively, you can use a screwdriver with an insulated handle, but be careful not to accidentally touch other metal with the screwdriver’s metal shaft. This could create a spark that could ignite the hydrogen gas in the battery. Clean the battery. Dirt binds moisture and becomes conductive, especially when exposed to battery acid fumes. Current flowing through the dirt on the outside of the battery promotes corrosion of the surrounding metal.