Battery Equivalents and Replacements

Top Off Car Battery: Battery Acid vs. Battery Water

Despite technological advances, especially lithium batteries, many road vehicles still use lead-acid batteries as starting/cranking batteries.

Some lead-acid batteries are not maintenance-free batteries, and to prolong car battery life, one must add battery water to the battery acid to maintain the battery electrolyte in proper condition.

Published: November 21, 2022.

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Types of Starting Lead-Acid Batteries

It is very important to know that there are several types of starting lead-acid batteries, including wet/flooded, Absorbent Glass Mat (AGM), and Gel-Cell batteries car starting batteries.

Wet/flooded lead-acid batteries are the oldest type of lead-acid batteries.

These car batteries feature 6 (six) openings on top (fill caps), a cap for each battery cell, allowing the user to open the cells and check the electrolyte level.

Note: some non-maintenance batteries also come with battery indicator "eyes" that allow the users to check the electrolyte level without opening the cells.

Battery electrolyte consists of pure water mixed with sulfuric acid - battery electrolyte is still very acidic, despite the acid being diluted.

Nonetheless, many users refer to the battery electrolyte as "battery acid."

As the battery is used (charged or discharged), water from the battery electrolyte evaporates and, as such, gets lost.

Also, when the battery is being charged almost fully, water is decomposed to hydrogen and oxygen, which are then vented out - another process causing the batteries to lose water from the battery electrolyte.

AGM and Gel-Cell batteries are Sealed Lead-Acid (SLA) Valve Regulated Lead-Acid (VRLA) batteries that are designed to prevent water loss from the battery.

However, both AGM and Gel-Cell batteries can, under certain conditions, lose water via safety valves (hence "Valve Regulated"), which prevent batteries from swelling when they are being overcharged with strong currents. Or they at least try to prevent battery swelling.

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Under unfavorable conditions, AGM and Gel-Cell batteries may lose water, and when that happens, the only thing a user can do is recycle the old battery and replace it with a new one.

Distilled Water vs. Tap Water

When the lead-acid battery loses water, its battery plates can get dry, and formed crystals of lead sulfate can become very hard, causing a decrease in performance, and in the end, warping the battery plates and short-circuiting the cells, which effectively kills the whole lead-acid battery.

Thus, when checking the cells for potential water loss, be sure to have distilled water ready in order to top off the battery fluid.

Why distilled or demineralized water?

Well, tap water is not pure water - it also contains plenty of minerals dissolved in water, and if such water is added to the battery electrolyte, these minerals will react with sulfuric acid and other chemicals, interfering with normal battery chemical processes and decreasing the performance of the battery.

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How To Top Off Car Battery Fluid?

If You have AGM/Gel-Cell lead-acid battery, there is nothing one can do regarding electrolyte battery levels - these are maintenance-free batteries.

In order to top off wet/flooded lead-acid batteries, the first thing is - safety first. If possible/available, the use of rubber gloves and goggles is highly recommended.

If they are not available, proceed with topping off battery fluid only if absolutely necessary, and do it very carefully.

safety glovesSo, very carefully open all six filling caps and check the electrolyte level.

If the plates are completely submerged in the electrolyte, there is no need to add distilled water - close the filling caps.

If the plates are partially above the electrolyte level, very carefully add distilled water to the "problematic" cell, but don't overfill the cell.

Note: in order to find the recommended maximum electrolyte height, check the battery manual/Owner's Guide. If You can't find it, filling the cells to about half an inch above the plates is a good starting point. But, if possible, verify this information before filling in the cells.

After filling the cells, close the filling caps.

Depending on the local climate conditions, it is recommended to check the electrolyte level at least twice per year, especially in areas with hot summers (increased evaporation) and cold winters (battery works harder).

Also, when maintaining the battery, it is good practice to check the battery terminals and to clean them from battery corrosion, if present.

Battery Acid or Distilled Water

As one can see, there is absolutely no reason to add any other fluid to the lead-acid battery but distilled water, except in two instances:

When working with sulfuric acid, be very careful - goggles, rubber gloves, protective clothing, and rubber boots are must-haves.

Also, be sure to prepare chemicals (baking soda, for example) to neutralize battery acid as quickly as possible if something should go wrong.

Lead-Acid Battery Electrolyte Frequently Asked Questions (FAQ)

Here are some of the most common Frequently Asked Questions about lead-acid electrolyte and lead-acid batteries in general:

What fluid is in a battery?

The lead-acid battery is filled with diluted sulfuric acid - it is mixed with the distilled water. The exact ratio depends on the battery model, battery plates' material, and similar.

What fluid should you use to top up the battery?

Lead-acid batteries should be topped up with distilled water. Demineralized water is also popular since it is somewhat cheaper, but distilled water is, IMHO, still a better choice.

Can I use distilled water instead of battery acid?

When topping off your battery, You MUST use distilled water since topping off with battery acid is done very rarely in very specific situations.

Can you put regular water in a battery?

No, if regular or tap water is added to the battery, it may change the battery chemistry and decrease its performance.

What happens if the battery has no water?

If the plates are partially dry, there is a great danger of irreversible sulfation, which degrades the battery performance and can even cause the plates to buckle and twist, causing internal short circuits.

If the battery has no fluid at all, it is either not activated yet, or completely dead.

How do you make homemade battery acid?

Battery acid should not be "made at home" - one can buy battery acid in various local hardware stores or even gas stations, or one can order it from online shops.

Can battery acid burn You?

Yes, battery acid can burn You causing skin acid burns, making holes in everyday clothes, and destroying common boots and similar.

Long Story Short: If your battery is 3-5 years old, and it is a wet/flooded battery showing signs of dying battery, replace it using a new maintenance-free AGM or Gel-Cell car battery and recycle the old battery.

When topping off the battery, be very careful, especially if You are not wearing safety goggles, rubber gloves, protective clothing, and boots - yes, even when topping off the battery with distilled water since there is always a danger that a droplet or two of acid end flying out of the battery. Always safety first!

If unsure what to do, take the car to the certified mechanic (or shop) and let them check the battery and do whatever has to be done.