How to refill battery acid and water

Battery fluid, a mixture of sulfuric acid and distilled water (called electrolyte), creates the electricity that makes a modern battery work so efficiently. Depending on the type of battery in your vehicle, battery fluid can evaporate and over time will need to be topped off as part of regular battery care.

Índice Show

What Type of Battery Is in My Car?
Why Do Some Batteries Need to Be Topped Off?
How to Tell if My Battery Needs to Be Topped Off
How to Top Off a Battery

What Type of Battery Is in My Car?

The most common battery designs in use today are maintenance-free and non-maintenance-free (NMF).

Non-maintenance-free batteries (also called a “wet-cell” battery) are easily identified by the removable filler caps on the top of the battery. Battery fluid in NMF batteries needs to be regularly checked and topped off. Non-maintenance-free batteries come with six individual fill caps or two caps covering three cells each.

Maintenance-free batteries are also easily identifiable because they do not have filler caps. A large, plain unremovable flat cover encases the battery housing and may be labeled with the words “Maintenance Free.” Today, virtually all new cars come standard with maintenance-free batteries. While maintenance-free batteries do not need to be topped off, you may still want to test your battery from time to time to ensure good performance.

Non-maintenance-free batteries for automobiles are still available for purchase online and in auto parts stores. However, NMF batteries are more commonly used in marine, motor sport and utility vehicle applications.

Both non-maintenance-free and maintenance-free batteries contain sulfuric acid that can cause serious burns. Always wear gloves and eye protection when working around a car battery or jump-starting a car. If you come into contact with battery fluid, flush with plenty of water and seek immediate medical attention.

Why Do Some Batteries Need to Be Topped Off?

Without getting too technical, it all depends on a battery’s chemical makeup or design. As an NMF battery discharges and recharges, battery fluid changes to a gas and evaporates. This requires the battery fluid to be refilled from time to time. Conversely, maintenance-free batteries are engineered to maintain battery fluid levels without refilling and cannot be checked, even if you wanted to.

Temperature also has a major impact on why NMF batteries need topping off. Summer heat accelerates the evaporation process. In winter, cold weather lowers battery power. Coupled with motor oil thickening from lower temperatures, the battery works harder to start your car. To quickly restore battery power, the alternator charges the battery at a higher rate for a longer time. Excess charging overheats the battery, causing the battery fluid to decompose, dry up and require topping off on occasion.

Not topping off battery fluid in an NMF battery ultimately leads to premature battery failure.

How to Tell if My Battery Needs to Be Topped Off

Some batteries have a clear battery indicator “eye” on the top that glows green if the water level is good and fully charged, and goes dark if the battery needs fluid or is discharged. If it’s yellow, it usually means that the battery fluid level is low, or the battery is defective. (Battery manufacturers recommend replacing maintenance free batteries that have low battery fluid levels.)
A battery with low battery fluid levels also gives signs you shouldn’t ignore. Slow crank/no crank starting condition, dimming lights, alternator or battery light flickering on, other electrical problems or even the Check Engine Light illuminating can point to battery problems.
Non-maintenance-free batteries can also be checked by opening the fill caps on the top of the battery and looking inside. The fluid should be about 1/2- to 3/4-in. above the internal “plates,” or about 1/2- to one-inch from the top of the battery (just to the bottom edge of the fill hole). If the fluid is below that, it needs to be topped off.

A 12-volt battery contains six “cells,” with each cell containing a set of “plates” (electrodes). Looking down the fill hole of each cell you can see the top of these plates. Usually grayish in color, they resemble a row of heavy-duty construction paper folded accordion style. A chemical reaction between the battery fluid and plate material produces the electricity that flows out of a battery.

How to Top Off a Battery

If you’ve determined that your battery fluid is low, follow these steps:

Make sure the battery is completely charged. If you’re unsure of the state of charge and there is no indicator eye, use a voltmeter to check the voltage across the terminals (12.6 volts with the engine off). Or check the battery fluid specific gravity (pH) with a battery hydrometer. (A fully charged battery should have a pH reading of 1.265 or higher.)
Make sure the engine is off and cool.
Clean the case with a brush and weak solution of 1/4-cup baking soda and one-quart clean water, then flush with clean water before removing the filler caps. This helps keep dirt and gunk out of the battery.
Remove the filler caps. Use a flat plastic scraper to help pry off a three-cell cap — never use anything containing metal.
Carefully and slowly (do NOT splash) pour distilled water into each cell fill hole as needed. ONLY use distilled water because tap water contains minerals that will harm your battery. You can use a small, clean plastic measuring cup or turkey baster to control the amount and flow of the water into the battery.
Measure carefully. Overfilling will dilute the electrolyte, causing it to expand, damaging the battery. Plates that dry out become overly acidic and form sulphate crystals that accumulate in the battery. Sulfate build-up restricts current flow and shortens battery life.
Re-cap the fill holes.

Remember, even well-maintained batteries do not last forever. Depending on the climate and driving conditions, expect to replace your battery every three to five years. Be sure to dispose of your old car battery safely.

Originally Published: August 25, 2020

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Most batteries you use are dry cell batteries; examples include regular AA alkaline batteries or rechargeable nickel cadmium batteries. However, your car or golf cart batteries contain wet cells, generally referred to as lead-acid batteries. Each cell in a lead-acid battery contains two lead plates submerged in sulfuric acid. You need to ensure the acid level in your battery is occasionally topped up, as once the acid level falls below a certain level, the cells go dead.

Wear rubber gloves as a precaution before adding fluid to your battery. The acid contained in the cells is dangerous and burns if it gets on your skin.

Use a screwdriver to unscrew the caps on the top of your battery. Most lead-acid batteries have slotted caps so insert a suitably-sized screwdriver in the slots and turn counter clockwise, then lift the caps off using your hands. If your battery has push-on caps you need to insert the end of the screwdriver under the side of each cap and pry them off.

Check the acid level in the battery cells. There are two lines on the inside of the cells representing the minimum and maximum acid level. If any of the cells are below the maximum level, you need to add fluid. Charge your battery, if necessary, before adding fluid.

Pour distilled water into the acid reservoirs. A bottle that has a small spout is best so you can accurately and slowly pour in the distilled water. Don't exceed the maximum level, because the acid expands when a battery gets charged. Never use water from your tap; it damages the plates in the cells.

Use your fingers to replace the caps. Either screw them or push them in place. Use a screwdriver to tighten the caps if they are threaded.