Image showing a person adding water to a forklift battery

Forklift battery maintenance, and the process of adding water to your forklift battery, is one of the simplest things you can do to protect your investment.

The performance of your forklift is dependent on the performance of its battery. Poor battery maintenance will impact its ability to provide electrical energy to the truck and is a leading cause of premature battery failure and reduced truck run times.

To protect your investment, and maximise your sites efficiencies it is important to ensure forklift battery maintenance is correctly carried out. And that you are using the correct forklift battery water filling systems.

| Why does the forklift truck battery “lose” water?

The liquid inside of your fork trucks battery cells is called “electrolyte” and is a mixture of sulphuric acid and water.

During charging the water in the cell is lost through evaporation and electrolysis which is a chemical process that splits the water into hydrogen and oxygen, which float out of the battery cells. Overtime this will reduce the electrolyte level in the battery cells and the water will need replacing.

forklift battery maintenance is needed due to water loss through electrolysis
Forklift Battery Water Loss

Deionized Water System – Single PS-300

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Forklift Battery Manual Topping Bottle

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| What water should I use?

It is important to use purified water when carrying out forklift battery maintenance. This is normally distilled or deionised pure water. Mains tap water contains dissolved solids that is damaging to the cell plates and should be avoided.


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You can buy deionized water in containers or alternatively use your own deionized water system.

| When should you add water to a forklift battery?

Water should be added to a forklift battery after it has finished charging.

During a charge and discharge cycle the electrolyte in the battery rises and falls, this is known as a tidal flow. When the battery is fully discharged the electrolyte, level is at its lowest, conversely when the battery is at the top of charge, during the gassing phase, the level will be at its highest.

The extent of the tidal rise is dependent on the quality and age of the battery, the battery cell type and the charging process being used.

If you add water to a forklift battery to the correct level before it goes on charge, then there is an increased risk when the battery is charging and going through the gassing phase, that the level will rise and spill out of the cells.

Water should be added to a forklift battery after it has finished charging.

It is worth noting that new batteries have an increased tidal rise as the cells go through “formation” during the first few chargers, normally around 10. Battery manufacturers advise that the batteries are not topped for the first few cycles, check the battery manufacturers user’s manual for specifics on this.

| How often should you add water to a forklift battery?

The frequency that you add water to a forklift battery depends on several factors:

  • Age of the battery
  • Temperature of the battery
  • Battery technology
  • Quality of the battery
  • Type of charger
  • Frequency of charging

There is no single figure that would work for all.

It is best to either manually check the cells or to use an electrolyte monitor.


Manual Visual Inspection

It is best to periodically inspect the electrolyte level in the battery cells and top when it is needed. To ensure this is done it is advised to set a schedule, for example every Monday morning.

This can be done visually by looking into the battery cells and judging the level of the electrolyte (be sure to wear safety goggles and gloves if using this method, sulphuric acid is corrosive!).

Some businesses set a schedule for battery topping, so that each battery is topped on a set period.


Battery Electrolyte Monitors – Mechanical

If your battery is fitted with a float based battery filling system then it will have a visual indicator on the top of each valve which shows the level of the electrolyte in that cell. This is a very simple system, but the normal movement of the float is only 5mm, meaning the difference from OK to needing topping is only 5mm of electrolyte. This will mean the battery will appear to need topping more frequently that what it actually needs.

Many modern batteries are now fitted with electronic battery electrolyte level monitors.


Battery Electrolyte Monitors – Electronic

Battery electrolyte level monitors are electrical devices where a probe is placed in a battery cell, normally towards the centre of the battery, and they provide a LED indication as to the electrolyte level in the cell. Simple monitors have a green LED to indicate all OK, if there is no the battery needs topping. More advanced smart monitors will provide you with a positive indication, red and green LED, and a “count down” to alert you that the battery will need topping soon.

Battery electrolyte monitors allow the operator to only top a battery when it is needed and provide a very clear indication as to when this is. This minimises the amount of times a battery is topped, saving time and increasing the sites efficiency.

Forklift battery maintenance made simpler with a basic electrolyte monitor
Forklift battery maintenance basic Electrolyte Monitor
SmartBlinky Battery Electrolyte Level Monitor, shown on a battery with red LED
SmatrtBlinky Battery Electrolyte Level Monitor

| What is the correct water level of a forklift battery?

There is no definitive answer as to the correct water level in a forklift truck battery as many factors are involved.It is important to remember to not let the cell plates to become exposed to the air as this will cause damage. Equally you don’t want the level to be too high in order to avoid over topping!

Check with the forklift truck or battery manufacturer what the correct electrolyte should be after charging. If this is not possible, or the information not available, then to be safe top the battery so the electrolyte level is 5mm (1/4” inch) above the cell separators (the separators are the raised layers of plastic that can be seen when you look inside the battery cell). If checking the electrolyte level visually be sure to wear safety goggles and gloves!


Low electrolyte level

Having a low electrolyte level in your forklift battery causes 2 primary issues:

      1. It increases the strength of the acid in the battery. The stronger the sulphuric acid the quicker the corrosion rate of the battery cell plates.
      2. It exposes the cell plates to oxidation.


High electrolyte level

Having a high electrolyte level in your forklift battery causes 2 primary issues:

      1. It weakens the strength of the acid in the battery. This will reduce the performance of the battery.
      2. It can cause the electrolyte to spill out of the battery cells, also known as battery boil-over. This can cause corrosion on the cell tops and in the battery tray. The resultant residue on the cell tops can cause tracking issues (voltage leakage), and consequently self-discharge of the battery
water level inside a forklift battery cell, showing the plates, electrolyte and separators
Water Level Inside a Forklift Battery Cell

| How do you add water to a forklift battery?

There are several forklift battery watering systems available on the market to assist you in your battery maintenance.

  1. By hand! This is where you use a jug or other vessel.
    1. Pros: cheap and simple
    2. Cons: slow, inaccurate, least safe and prone to spillage


  1. Battery Watering Gun. These can be free flowing or ones that work on a pressurised system, such as a manual topping bottle. The pressurised system has a mechanism for automatically shutting of the water flow when the electrolyte is at the correct height.
    1. Pros: cheap, and safe. Keeps the operator away from the battery acid. The systems with auto-shutoff are also accurate.
    2. Cons: slow! Faster than a jug, but significantly slower than a single point system.


  1. Single point battery filling system. All the battery cells are “plumbed” together, each cell is equipped with a valve which is either float based or shuts off under pressure (WaterInjector).
    1. Pros: Quick to fill a battery, especially with the WaterInjector system. Safe, the operator is completely removed from the battery electrolyte.
    2. Cons: As each battery has to be fitted with a filling system, it can be expensive to set up (although they always come with a strong ROI).

| Safety when working on a forklift battery.

As the battery electrolyte is diluted sulphuric acid, precautions need to be taken in order to prevent operatives and the environment form coming in contact with it. Personal protective equipment used, including gloves and goggles.


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Safety Equipment

personal protective equipment for working on a forklift battery including face shield, gloves and apron

| Forklift Battery Maintenance Procedure

Follow the following procedure when carrying out forklift battery maintenance:

  1. Be safe and follow the appropriate safety guidelines and wear safety equipment
  2. Set a schedule for battery maintenance or use an electrolyte monitor
  3. Avoid over topping by only adding water to the forklift battery after it has completed charging.
  4. Add sufficient water to ensure the cell plates are submerged, 5mm above the cell separators is normally sufficient
  5. Do not use tap water, do use quality deionized pure water
  6. Use battery filling systems to make battery maintenance efficient and to improve operator safety

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