Adding water to a car battery

Car batteries can go bad if the electrolyte level is not properly monitored. The battery’s capacity will suddenly drop, and in winter it will be a great chance that it will fail completely. So how can we take care of the battery’s electrolyte level?

Why is the car battery’s electrolyte level important?

The electrolyte in batteries consists of 65% water and 35% sulfuric acid. During battery operation, when recharging, the electrolyte may boil, especially when the outside temperature or under the hood temp is too high, the process is accelerated.

Being non-volatile, acid hardly evaporates. But water evaporates fast. As a result, the electrolyte level drops, the proportion of water and acid is no longer in parameters, and the density of the electrolyte increases.

Why you should add water to a car battery?

The normal density of the electrolyte level is about 1.29 g / cm³. When water boils away, the density and the acid content increase. If this happens, the battery plates will wear out much faster, and if you do not add water, the plates will simply damage.

Thus, adding water to your car battery ensures the efficiency and the performance of the battery when the electrolyte level is in its normal level.

What type of water should you add to the car battery?

The only type of water you will need to add to your battery is distilled water, other sorts like tap water and some sorts of bottled water might contain minerals and other substances that can corrode the battery parts. Distilled water contains hydrogen and oxygen and does not have any excess impurities

When to add water to the car battery?

There are some ways to check out the correct level of the electrolyte in the battery, but before that the first thing to do is to take a closer look at the battery itself.

Some batteries have a dedicated indicator. It shows several of its states:

  • Normal, low electrolyte level. In this case, you need to add water.
  • Low electrolyte density, here you need to charge the battery. 

Some batteries have “min” and “max” marks. These batteries allow you to check the electrolyte level by just looking at it, due to the relative case transparency.

You can also check the level of the battery by removing it and open its small covers. On some batteries, you will find a special indicator that the electrolyte should cover with only 0.5 cm (0.196 inches) and not more. If there is no indicator then you can be guided by the immersion of the plates in the liquid.

You should check the battery’s electrolyte level and its density only when the battery is fully charged at a battery charger!

What is the normal level of the electrolyte in a car battery?

The normal level of the electrolyte should be 15-20 millimeters (0.59 – 0.78 inches) above the edge of the plates. If the plates are not covered, this will indicate a low electrolyte level that should be restored as fast as you can.

Best testing is performed with a hydrometer, used most often, and with a refractometer. You can buy these tools in most car parts stores and they will come in handy not only this check but also other car fluids.

At the same time, by measuring the level you can also check the electrolyte state, which must be transparent. If the liquid is not clear or dark, then the battery is failing and you need to buy a new one.

How much water is added to the car battery?

The quantity of water will always depend on the battery’s capacity, the larger the battery is, the more electrolyte it contains. Moreover, the volume depends on the battery design and may differ from manufacturer to manufacturer. Below there are estimations based on different battery capacity:

Battery capacityElectrolyte levelWater level
55 Ah~ 2.6 L (0.68 gal)~ 1.64 L (0.43 gal)
60, 62 Ah2.7 L -3.1 L
0.71 – 0.81 gal
1.77 – 1.95 L
0.46 – 0.51 gal
65 Ah~ 3.6 l (0.95 gal)~ 2.29 L (0.60 gal)
75 Ah3.7 – 4.1 L
0.977 – 1.08 gal
2.41 – 2.60 L
0.63 – 0.68 gal
90 Ah4.3 – 4.8 L
1.13 – 1.26 gal
2.86 – 3.13 L
0.75 – 0.82 gal
These numbers are just estimations if the water level is as low as possible.

How to add distilled water to a car battery?

To properly fill the battery with water, you should follow the simple steps below.

  • Clean the battery on the surface to avoid dirt and other substances entering the battery when you unscrew the plugs. Before that leave the battery at room temperature for approximately 7 hours;
  • Unscrew the plugs and check the electrolyte level as I described above;
  • If you need to add distilled water, add it carefully. If you added a larger amount, remove it using a syringe.
  • Screw the plugs back and leave the car battery for at least 7 hours because mixing the water with acid takes some time;
  • Charge the battery, but if you see that the electrolyte boils, charging should be stopped;
  • Measure the electrolyte density with a hydrometer. The normal level should be 1.29 g / cm3. 


Adding water to a car battery is a good way to maintain it and prolong its life. You should add only distilled water because other types of water contain substances like calcium, magnesium, salt, and other impurities that can create chemical reactions and accelerate the destruction of the battery’s plates.

Be careful when choosing some batteries and they are labeled as “maintenance-free” because most of them are not. Their caps are hidden under the sticker with the battery label. It is enough to remove it and you’ll see the battery caps.

There are indeed batteries that are maintenance-free, those batteries do not have caps on their surface, and do not require maintenance, just use them until they wear out completely.

Try to avoid adding more water to the battery than is needed, because this will negatively affect its performance. The electrolyte density will be reduced, which means that the battery’s power will decrease. Also, this reduces its resistance to freezing temperatures, and in winter this can be problematic.

Fortunately, the excess water can be reduced in a very simple way as I mentioned above. Don’t worry about the mixture with the sulfuric acid, this process is very slow, and reducing immediately after adding, will not affect the mix.

Image by: איתן טלEtan Tal, CC BY 3.0, via Wikimedia Commons

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