Why Does a Car Battery Need Water?

Have you ever had someone check your car battery after you’ve had some problems starting your vehicle and they told you that your battery was low on water?

If you’re new to batteries, like I once was, you might be wondering why a battery needs water. A battery has acid in it, right?

Why does a car battery need water?

Both flooded and sealed lead-acid car batteries are comprised of lead plates with an electrolyte solution. This electrolyte is made of about 3/4 water and 1/4 sulfuric acid. When charging or overcharging, water evaporates from the battery. Replacing water in a flooded battery is possible but not in sealed batteries.

Below I’ll briefly cover the difference between flooded and sealed batteries and how they are affected by water loss, and then I’ll cover what type of water, how to add it, and when to add it to your battery.

Let’s get started!

Flooded Lead Acid Batteries and Water

A flooded lead-acid battery is the most common battery for cars that comes to mind. You might have a sealed battery now, but you probably grew up with a flooded one.

Flooded batteries have the removable caps that cover the 6 holes for the cells. If you remove this cap and look down into the fill holes with a flashlight you will see a liquid solution and lead plates within it.

Flooded batteries are the most hardy among the lead-acid batteries in that they can tolerate an overcharge better. This is due to the fact that you can replace water that is lost inside the battery as the electrolyte evaporates out during charging (but especially during overcharging).

If you hear a rapid bubbling sound from your battery during charging, that means it cannot accept the current charge that it is being sent. The excess current from the charger has to go somewhere, so it is released via heat. The head boils off the electrolyte in the form of steam.

It’s not good for a lead acid battery to be overcharged, but it’s not the end of the world if it doesn’t go so long that it boils off enough of the electrolyte to expose the lead plates.

Sealed Lead Acid Batteries and Water

This type of battery usually has a sticker prominently placed on it that reads “Maintenance Free”. That’s well and good, and I appreciate the relative safety of these batteries when compared to lead acid (no splashing of the acid), but I’m a bit old-school and enjoy topping off my batteries when needed to extend their lifespans.

These batteries in the lead-acid family are generally GEL Cell batteries or AGM (absorbed glass mat). The GEL Cell batteries have silica added to the electrolyte that makes it into a gel. The AGM batteries have very fine fiberglass mats that are impregnated with the electrolyte and are stacked between the lead plates.

The fact that they can’t have their electrolyte replaces and that they are more susceptible to damage from overcharging are the primary reasons.

Just because they are sealed doesn’t mean they don’t lose water. They still have vents that allow gasses to escape if the charge is too much. When this happens, your electrolyte levels cannot be replaced.

Overcharging, even by a little, can also damage these batteries by creating bubbles in the gel or between the fiberglass mats. This reduces conductivity and the performance of the battery.

What Type of Water does my Battery Need?

If you are looking to fill your flooded lead-acid car battery, make sure to only use distilled water. It’s only a dollar or less at the local grocery store and there’s no reason to not have it sitting around in your garage for this purpose.

But water is water, right?


Municipal water is chemically treated and contains a myriad of impurities. Just Google what’s in it if you don’t believe me. To prove the point to yourself, boil a cup or two down all the way until it all evaporates in a saucepan. Check out the residue at the bottom.

That residue interferes with the performance of your battery in the long-run.

The same can be said of well water, but the problem is generally minerals at that point. Excess iron or calcium are common culprits and they also inhibit the performance of the battery overtime.

Distilled water, on the other hand, is pure water that is collected by evaporating water and harvesting the steam. The impurities are left behind and only pure water molecules are collected.

How to add water?

Most places online will tell you to carefully poor the distilled water into the fill tube until it reaches the desired height within the cell. I find this method messy, prone to overfilling, prone to splashing acid, and prone to carrying debris, bugs and dirt from around your battery into the cells themselves.

I prefer to take a damp rag and wipe my battery clean around the caps that cover the fill tubes. Then I vacuum with a shop-vac around the cover in case any dirt or bugs are wedged somewhere out of view.

I then gently pry the cap off with a screwdriver (while wearing safety glasses this whole time!) and gently wipe away any dirt with that damp cloth from the fill hole to the edge of the battery. I don’t push dirt towards the fill hole.

Once that’s done, take a peak inside the battery with a flashlight. The solution inside should come up about 1/4″-1/8″ shy of the bottom of the fill tube (which will have a v-notch at the bottom).

You don’t want to fill them beyond that as they need room for expansion.

I then take a plastic syringe that I’ve collected over the years from my kids’ medications (you could use a baster) and I gently add distilled water to the cells that need it. This method eliminates overfilling, splashing, and is precise.

How do you know when your car battery needs water?

It’s a good rule of thumb to check your flooded battery water levels once a season (4 times a year) to make sure the lead plates aren’t exposed. Once they are exposed to air they are permanently damaged through sulfation.

Also, anytime you fear your battery has been overcharged by an actual battery charger or by a faulty alternator/voltage regulator you’ll want to check as well.

If you notice your plates are exposed in one or more of the cells, add just enough water to cover the plates by about 1/8″. Then hook the battery up to a proper smart charger.

After it has reached full charge, add water to the appropriate level just below the fill tubes. I then hook the battery up once more to the charger to top it off.

If you’re looking for a smart charger that won’t overcharge your battery and is sensitive enough for a GEL or AGM battery, I can’t recommend highly enough a 5-amp charger on Amazon. These are perfect for routine maintenance or for leaving hooked up year-round if you plan on not using a battery over a season or two (or more).

Related Questions

What happens if a car battery runs out of water?

Assuming we’re talking about lead acid batteries, this would be a very bad thing. The lead plates would immediately begin sulfating when exposed to air and reduce the efficacy of the battery if it were refilled. If you were to try to charge a battery that lost its water you would overheat the battery and risk a fire.

Do I add water to a car battery before or after recharging?

Both. Before recharging, only add enough water to cover any exposed lead plates by about 1/8 of an inch. Bring the battery to a full charge using an appropriately sized smart charger (6 amps or less). Then fill the battery to the cells to 1/4 to 1/8 of an inch below the fill tubes and reconnect to the charger.

Robert Van Nuck

Robert lives in central Michigan and enjoys running, woodworking, and fixing up small engines.

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