Forklift Battery Operation


A cycle is a discharge which is followed by a charge. In the duration of the charge the electrical energy that is supplied by the charger causes an electro-chemical reaction within the forklift battery. This then restores the active materials to a fully charged condition

The positive and negative plates, or electrodes, are separated from each other and immersed in electrolyte. In the fully charged condition the active material of the positive plate is lead dioxide and that of the negative plate is sponge lead. The electrolyte is a solution of sulfuric acid and water that normally varies in specific gravity from 1.275 to 1.295.

The combination produces a voltage of approximately 2 volts on open circuit. This voltage potential results from the fundamental characteristic of a storage forklift battery which dictates that when two electrodes of dissimilar metals are immersed in suitable electrolyte and a circuit is closed between the two, electrons begin to flow. A fully charged cell will normally have an on-charge voltage of from 2.45 to 2.70 volts when charging at the finish rate.

The Discharging Cell or Forklift Battery

While a forklift battery is being discharged or used, lead dioxide and sponge lead combine with sulfuric acid to form lead sulfate within both plates. This action then causes the specific gravity of the electrolyte to decrease. As the discharge progresses, individual cell and forklift battery voltage declines, generally in direct proportion to the rate of discharge.

The Discharged Cell or Forklift Battery

As the depth of discharge increases more sulphuric acid is removed from the electrolyte so the specific gravity decreases and may drop below 1.100 as it approaches the specific gravity of water. Almost all of the active material of both positive and negative plates is converted to lead sulfate, and an effective electrochemical reaction is no longer possible. At this point the forklift battery has reached its discharge limit.

The Charging Cell or Forklift Battery

The charging action begins when the terminals of the forklift battery are connected to an external source of direct current. The electro-chemical reaction is reversed and the positive plates, negative plates, and electrolyte start returning to their original charged condition.

Charging causes the forklift battery voltage to rise as active materials are restored. A cell being charged may have a voltage of from 2.12 to 2.70 volts depending upon charging rate and time.

It can be seen that storage forklift batteries do not actually store electrical energy. Instead, they accept the electrical energy delivered to them during charging and convert it into chemical energy. During discharging, this chemical energy is reconverted into electrical energy to be used as needed.

As an operating guide, to obtain the best performance and life from a motive power storage forklift battery, the depth of discharge should not regularly exceed 80% of the forklift battery’s rated capacity in ampere-hours. It should be charged after each shift of use or whenever the specific gravity of the electrolyte falls below 1.240. It is very important that proper ventilation is provided during charging to make certain that-

(1) the hydrogen gas, given off toward the end of the charging process, is dissipate

(2) that individual cell electrolyte temperatures, during normal operations, do not exceed 110 degrees F.


Ampere-Hour Capacity

The electrical capability of a storage battery is mostly expressed in ampere-hours. The ampere-hour capacity is the number of ampere-hours which can be delivered under specified conditions of temperature, rate of discharge and final voltage.  Ampere-hours are determined by multiplying the number of amperes which the forklift battery will deliver by the number of hours during which the current is flowing. Total cell or battery capacity then is determined by the size and number of plates which make up the element. Due to the variety of job requirements forklift batteries are produced with many different sizes of cells.


With reference  to storage batteries, many “voltage” conditions have been recognized. The most important of these are:

A. Open Circuit Voltage – This is the voltage of a cell or forklift battery at the terminals, when no current is flowing. The nominal open circuit voltage of an individual fully charged cell is 2 volts. This is true regardless of cell size. The voltage of an 18 cell lead-acid battery is stated, therefore, as 36 volts.

B. Initial Voltage – The initial voltage of a cell or forklift battery is the closed circuit voltage at the beginning of a discharge. It is usually taken after current has been flowing for a sufficient period of time for the rate of change of the voltage to become practically constant. This usually occurs within a matter of minutes.

C. Average Voltage – The average voltage of the cell or battery is the average value of the voltage during the period of charge or discharge.

D. Final Voltage – The final or cut-off voltage of a cell or forklift battery is the prescribed voltage at which the discharge is considered complete. It is usually chosen so that the useful capacity of the forklift battery is realized without subjecting it to harmful over discharging. Final voltage will vary with the rate of discharge, cell temperature and the type of service, but for motive power applications it is considered to be 1.70 volts per cell.

Voltage conditions B, C, and D above are monitored when conducting test discharges. They are essentially academic as regards normal forklift battery usage in a truck.

Rated Capacity

The rated capacity of a storage forklift battery is the number of ampere-hours or watt-hours which it is capable of delivering when fully charged and under specified conditions of temperature, rate of discharge, final voltage and specific gravity. United States industry standards for motive power batteries always specify this to be at the 6 hour rate of discharge. The total capacity available from a forklift battery is greatest at low rates of discharge over a long period of time. Discharging at high current rates reduces the total ampere-hours or watt-hours available.


Sulfation occurs when conditions within the cell cause sufficient accumulation of abnormal lead sulfate at both the positive and negative plates to permanently effect the normal chemical reactions.

Habitual over-discharging below final voltage, prolonged operation in an undercharged condition and extended stand periods while in a discharged state are major causes of sulfation.
A servicing schedule should be followed to provide frequent monitoring and adequate charging.


Operating Cycle

An operating cycle of a storage battery is the discharge during use and subsequent charge to restore its initial condition.

Service Life

The service life of a storage battery is the period during which it provides useful power while being discharged. It is usually expressed as the time period, or number of cycles, which elapses before the ampere-hour capacity falls below 80% of its rated value. In order to obtain maximum service life it is recommended that a forklift battery should be restricted to one full cycle per 24 hour day or fewer than 300 cycles per year.

Other factors which most often adversely influence service life are:

A. Abnormally high or low electrolyte temperatures.

B. Frequent overdischarging.

C. Failure to add water regularly.

D. Frequent overcharging.

E. Poor, or high, resistance, connections or contacts.

Effect of Temperature

The normal operating characteristics of a storage battery are modified by unusually low or high cell temperatures.

Low Temperatures

Available forklift battery power is reduced by low temperature because electrolyte viscosity and resistance is increased and diffusion throughout the pores of the active material is retarded. For example, a fully charged forklift battery (1.275 to 1.295 specific gravity at 77 degrees F.), when its electrolyte temperature is about 32 degrees F., will deliver only 75% of the capacity which would be available at normal room temperature. This drops to 40% at 0 degreee F. The electrolyte could freeze if a discharged forklift battery was exposed to very cold temperatures for several hours.

In addition to the discharge related problems, the charge acceptance of a lead-acid forklift battery is impaired when electrolyte temperatures drop below 60 degrees F. As a result, forklift batteries should always be kept fully charged, especially in cold weather. They should be heated, even during operation or storage, if exposure is severe enough to cause the temperature of the electrolyte to approach 32 degrees F.

High Temperatures

Although high temperatures of up to 110 degrees F, do not cause a reduction in available capacity, battery operation is adversely effected. Because most chemical reactions are accelerated at high temperatures, the rate of corrosion of the positive grid is increased and the active material is shed more rapidly. Even electrolyte temperatures above 90 degrees F. will cause some reduction in service life and should be avoided whenever possible. Cell temperatures should never be allowed to exceed 110 degrees F.

In the past it was believed that, when batteries were to be used in the tropics, the specific gravity of the electrolyte should be reduced to approximately 1.225.
The forklift battery industry no longer recommends such action. Any advantages which can be related to reducing the specific gravity are more than offset by the problems of

(1) electrolyte adjustment,

(2) identifying such reduction to all forklift battery service personnel so the forklift batteries are properly charged,

(3) greater internal resistance,

(4) reduced cell capacity and

(5) assuring that the higher gravities are again restored if batteries are reshipped to a cold climate where freezing could be a problem


Safety Procedures While Charging Forklift Batteries

Specific areas should be designated for charging forklift batteries. These areas should be equipped with overhead hoists, conveyors or cranes for handling forklift batteries.

Charging areas should be adequately ventilated. The actual amount of ventilation will depend upon such factors as number and size of forklift batteries being charged at the same time, room size, ceiling height and air-tightness of the building. Hydrogen concentrations above 4% can be explosive.
Smoking, open flames, and sparks should all be prohibited in the charging area. Post placards “Hydrogen”, “Flammable Gas”, “No Smoking” and “No Open Flames”.

Facilities should be provided for flushing and neu- tralizing spilled electrolyte, for fire protection, including hand operated fire extinguishers, and for protecting charg- ing equipment from damage by trucks, tractors or cranes.

Fresh water should always be available in case electrolyte is splashed on skin, clothing or into eyes. The kinds of equipment available for eye-wash and acid neutrali- zation vary widely but either an eye-wash fountain or deluge shower and chemical burn station (squeeze bottle containing a buffering solution for relief of acid burns) should be located in the immediate work area. These should be clearly identified and readily accessible.

Before connecting a forklift battery to, or disconnecting it from, a charger, the charger should be turned off. Live leads can cause arcing and pitting of forklift battery connector contact surfaces.

Make sure that all electrical connections are tight and mechanically sound to prevent any arcing or loss of power.

Wear a face shield or goggles, rubber gloves, apron and boots when checking, filling, charging or repairing forklift batteries during periods of possible exposure to acid or electrolyte.

When forklift batteries are charged on racks, the racks should be insulated to prevent any possibility of shorting.

When charging an enclosed or covered forklift battery, always keep the forklift battery tray cover, or compartment cover, open during the charging period. This will help to keep the forklift battery cool and disperse the gases.

Keep vent caps in place at all times except while servicing or repairing cells. This minimizes the loss of electrolyte and prevents foreign matter from entering the cells.

Shut off and disconnect both input and output connections to the charger before repairing charging equipment.

When taking specific gravity readings, use a face shield or goggles and read the hydrometer with eye at about the same level as the electrolyte. Return all electrolyte to the cell.