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Know Your Batteries

In this section you can find answers to frequently asked questions about the usage of traction batteries and chargers.

  1. What actually happens during the charge cycle of the forklift battery or traction battery?

    Very simply put, the industrial battery charger takes AC power from the utility company, changes it to DC power, controls the DC power going into the battery, reverses the electro chemical process that occurred during the discharge cycle. The charger forces the acid that has penetrated into the plates during discharge back out into the electrolyte.

    There is a strong misconception that an oversized charger will simply recharge the battery properly in a shorter time period. A battery is designed to absorb a charge at a controlled rate. If it is charged at a faster rate, the energy that it cannot absorb is converted into heat. Cell temperatures rise, and the battery, in effect, “cooks in it’s own juices”.

    If the charger is not large enough to reverse the electro chemical reaction in the battery within the allotted time, some of the acid will remain in the plates. The battery will not perform in accordance with it’s ampere-hour rating capacity. More serious is the permanent loss of capacity resulting from repeatedly leaving acid in the plates. The results you would experience would be an 840 ampere-hour battery performing like a 400 to 500 ampere-hour battery if it is continually undercharged (50% or more loss of shift time).

  2. What are the fundamental principles of a traction battery or forklift battery?

    A lead acid battery stores electrical power in chemical form, and consists of two or more cells. Each cell comprises two dissimilar compounds of lead, immersed in dilute sulphuric acid.

    In its fully charged state, the active material of the positive plate is lead dioxide, and that of the negative plate, porous or spongy lead.

    A lead acid battery is a “secondary battery” in that it can be discharged and recharged a number of times before reaching the end of its life. This is because the chemical reaction which takes place in a lead acid battery is reversible. (Primary batteries, such as zinc/manganese dioxide batteries widely used in torches, radios etc. cannot be recharged and have to be discarded once discharged).

    When a cell is discharged, the sulphuric acid reacts with the active materials on the positive and negative plates. From the reaction on the spongy lead of the negative plate, electrons are liberated, whilst electrons are absorbed by the reaction on the lead dioxide on the positive plate. Consequently a current flows between the two plates.

    Furthermore, in both reactions the active materials are gradually converted to lead sulphate, whilst the sulphuric acid is converted to water. Sulphuric acid being heavier than water, the specific gravity (S.G.) of the electrolyte therefore drops during discharge, as the acid is being “used up”. In the process, the volume of the electrolyte decreases somewhat, with a resultant drop in the electrolyte level.

    When recharged, the sulphate ions in the active materials recombine with the excess water in the electrolyte and convert back to sulphuric acid. As a result, the S.G. increases back to original top of charge levels. Simultaneously the lead sulphate converts back to lead dioxide on the positive, and spongy lead on the negative plate respectively. Again, the volume of electrolyte increases, and the electrolyte level rises, as the cell is charged.

    As the cell approaches its fully charged condition, the chemical conversion can no longer absorb all of the charging current. The surplus current results in the liberation of hydrogen from the negative plate, and oxygen from the positive plate. This process is commonly known as gassing and is the primary reason why these cells need regular topping up with water. Subject to the condition and age of the cell, gassing normally starts at approximately 2.35 volts.

    Gassing helps to agitate the electrolyte in the cell, thereby mixing the heavy sulphuric acid with the water in the electrolyte. Specific gravity readings during charge should therefore always be taken at the end of charge to ensure proper readings. Illustration 1: While the electrolyte S.G. drops linearly with discharge, it increases meaningfully during recharge only once gassing voltage is reached.

    Because of linear change of electrolyte S.G. during discharge, S.G. is preferred over voltage as a measure of state of discharge. Temperature affects S.G. and S.G. readings taken should be adjusted to 25°C.

    The open circuit voltage of a fully charged cell is approximately 2.13 volts. When on charge, the voltage will rise from approximately 2.15 volt to a final value approaching 2.70 volts. The latter voltage is reached when the charge current has dropped to approximately 3% of the cell capacity. This is called the equalising current and serves to ensure all cells in the battery have reached top of charge.

    Cell capacity is dependent upon the discharge rate. Motive power batteries are rated at the 5 hour discharge rate. This means that a 500 ampere hour (Ah) battery can be discharged for 5 hours at 100 ampere to a minimum voltage of 1.7 volt per cell. Beyond this point, known as the knee of the voltage curve, the cell voltage will drop off rapidly. Refer to illustration 2. Higher or lower rates of discharge have lower and higher voltage cut off points respectively.

    Battery life is dependent on many factors, of which temperature, quality of topping up water, proper charging and regular maintenance are of major importance.

    High cell temperatures reduce life dramatically. Unnecessarily high charge rates and high ambient temperatures are prime reasons for high cell temperatures.

    Poor quality topping up water containing harmful substances will chemically reduce the ability of the cell to deliver to its rated capacity.

    Incomplete charging will lead to irreversible sulphation of cells, and out of step batteries. Overcharging will result in damage to the plates and high cell temperatures.

    Regular and proper maintenance is important to identify and correct such problems before permanent harm is done to a battery. The following chapters serve as a guide to help users perform this maintenance.

  3. What do I need to know about charging a forklift battery or traction battery?

    To ensure maximum performance, both in terms of battery capacity and expected life, correct charging is essential.

    In general, a battery may be charged at any rate in amperes that will not produce excessive gassing or result in electrolyte temperatures above 45°C. A discharged battery can accept high rates of charge initially but, as the gassing stage is approached (2.35 volts per cell), the current must be reduced to avoid excessive gassing and high temperatures which can permanently damage the plates and separators.

    A battery should always receive the ‘correct’ amount of charge – sufficient to fully charge it and/or maintain it in that condition, but no more. In other words, undercharging or overcharging should be avoided to the extent that it is practical under conditions of usage. An insufficient charge, even to the smallest degree but continuously, will cause gradual sulphation of the plates, with eventual loss of capacity and a reduction of battery life.

    Excessive charging will tend to ‘form up’, or corrode, the positive spines into lead dioxide, weakening them physically and increasing their electrical resistance. If overcharging is at comparatively high rates, gassing will also be excessive and this tends to “wash” the active material from the plates. All of these factors reduce the capacity and shorten the life of the battery.

    With any type of battery operation there are reasonably simple checks to determine whether or not the amount of charge is correct. If the proper amount of charging is being given, the specific gravity will reach its fully charged value at the end of recharge and will remain at that value. In addition, the amount of water required by the cells will be minimal.

    If the specific gravity does not reach full charge value and/or tends to show a continuing decline it is evident that the battery is not receiving sufficient charge.

    On the other hand, if the specific gravity reaches or remains at full charge level and an excessive amount of water is required, the battery is receiving an excessive charge and the rate should be reduced.

    To keep the battery in good condition it is necessary to use a charging regime that takes into consideration the operational function of the battery. To provide the correct charge regime the current and voltage of the charger must be correctly adjusted and the duration of the different charging stages carefully monitored. Volt and ammeters should be calibrated at regular (six-to-twelve monthly) intervals.

    Charging regimes must comply with the following requirements:

    • The duration must be acceptable for the duty cycle
    • Ampere hour and Watt hour efficiencies must attain maximum values to ensure total transformation of energy
  4. What do I need to know about the specific gravity measurement of a forklift or traction battery?

    Determing the state of charge of the battery

    Dilution of the electrolyte during discharge and re-concentration during charge is a useful guide to the state of charge of a lead acid cell. The less acid there is in the electrolyte, the less dense it is, and the lower its specific gravity (S.G.). One cubic centimetre of water has a mass of 1 gram. In other words, its density is 1 gram per cm 3 at 4°C . Specific gravity is the ratio of the density of the substance measured to the density of water. Density of water at 4°C = 1.000 g/cm 3 but only decreases slowly as temperature increases. The specific gravity of water is therefore usually expressed as 1.000.

    Specific gravity could also be expressed as the mass of the liquid divided by the mass of an equal volume of water. Specific gravities are usually measured with a hydrometer. This works on the principle that a float will sink further into a liquid of low specific gravity than of high specific gravity because the latter is denser and provides greater support.

    Using The Hydrometer

    From one of the cells, draw acid into the barrel until the hydrometer float moves freely. Now read the specific gravity at the point where the scale emerges from the acid. Afterwards, make sure the acid is returned to the cell from which it was drawn, or the specific gravity and acid level of both cells will be altered.

    When taking a reading:

    • Make sure that there are no air bubbles under the float
    • Bring your eye level with the acid surface in the barrel of the hydrometer
    • Ensure that the hydrometer is in a vertical position
    • If necessary, shake the hydrometer gently to enable the float to move freely
    • If the ambient temperature differs from the battery temperature, draw in electrolyte to warm the hydrometer first
    • Read off against the lower meniscus

    The change in specific gravity of the acid is directly proportional to the ampere hours taken out of the cell on discharge.

    Where extreme accuracy is required, a reading should be taken after a stabilisation period of several hours with the battery on open circuit. The minimum recommended specific gravity is 1.160. Select a different pilot cell each month to serve as a useful, general indicator for the condition of the battery during charge and discharge.


    • The specific gravity of a cell will decrease with any increase in temperature
    • Always record the electrolyte temperature when specific gravity readings are taken
    • Cells should not as a rule be discharged more than 80% of their nominal capacity

    Temperature Correction

    All specific gravities quoted relate to temperature at 25°C and have to be corrected if read at other electrolyte temperatures. Specific gravities decrease as the temperature increases. To provide a standard, the specific gravity can be corrected to 25°C.

    Take the temperature of the electrolyte and add 7 points (0.007) to the specific gravity reading for every ten degrees above 25°C

    Or subtract 7 points (0.007) from the specific gravity reading for every ten degrees below 25°C

    For example:

    A reading of 1.255 at 35°C corrected to 25°C gives 1.255 + 0.007 = 1.262

    Or a reading of 1.275 at 15°C corrected to 25°C gives 1.275 – 0.007 = 1.268

    Specific Gravity When Fully Charged

    With the electrolyte level correct, the fully charged specific gravity should be in the range of 1.290 -1.300 at 25°C exept for new cells. For new cells the fully charged specific gravity will be 1.260 – 1.270. After a few weeks of service the specific gravity will rise and will typically stabilise between 1.290 -1.300.

  5. What does a traction battery do?

    A traction battery:

    • Powers the motor that drives an electric vehicle such as forklift truck
    • Powers accessories like headlights on an electric vehicle
    • Provides power for a specific purpose on an electric vehicle such as the lift on an electric forklift truck
  6. What happens if the battery is over-filled with water?

    Over-filling and overflow of electrolyte will cause a battery to have loss of capacity and in turn, lost man hours will occur due to forklift down time. The electric motor’s (LIFT & DRIVE) as well as contact tips are the items that show damage.

  7. What happens when the water level is too low?

    When the water level in a cell drops to the point that the plates are being exposed to the air, a process known as sulphation begins. The active material in the plates drys and becomes very brittle; somewhat like concrete curing. Permanent damage will occur if the water level is allowed to drop to the point of exposing the plates to the air.

  8. Where does the water in an industrial forklift truck battery go?

    Proper watering of an industrial lift truck battery is the most important factor in establishing a solid battery maintenance program. The addition of water is usually necessary at weekly intervals by manual installation of an automatic system.

    Water (H20) is transformed into gasses during the charge cycle. Hydrogen bubbles are produced at the negative plates and oxygen bubbles at the positive plates. The Hydrogen and Oxygen bubbles are released into the air, and gassing (which is perfectly normal and indeed a requirement) occurs in the range of 2.35 to 2.38 volts per cell (this value changes as temperature changes).

  9. Why should a battery not be discharged more than 80% of its capacity?

    The specific gravity of the electrolyte reflects relatively accurately how many ampere hours have been taken from a cell on discharge.

    If the battery is grossly over discharged the temperature will rise dramatically in the latter stages. The battery should be allowed to cool before recharge.

  10. How do automatic time relays work?

    These devices work on the principle of a voltage sensing relay sensing the predetermined set voltage of the battery being charged. This starts a timing mechanism which, after the preset period, terminates charging.

    Most of these devices have L.E.D. indication for charge conditions showing first rate, second rate, equalizing rate and charge complete.

  11. How big is a traction battery’s capacity?

    A traction battery’s capacity will decrease toward the end of its life. Unless a specific problem occurs, this will be a gradual decrease and ample warning of reduced capacity will be evidenced by the slowing down of the vehicle toward the end of the day’s work.

    A traction battery is considered to be at the end of its useful life when its capacity decreases below 80% of its rating. However, it can sometimes be transferred to a smaller job to provide additional life and service.

    As a motive power battery usually discharges every day in the course of its regular performance, it is seldom necessary to conduct a formal test of its capacity. Most users do not have the facilities to do this conveniently or accurately. If any such testing is desired, consult the Dart Traction service engineer.

  12. What are the features of the water used for topping up traction batteries?

    Water used for topping up batteries must comply with the following specifications:

    • It shall be clear, colourless and odourless
    • The pH shall be between 5 and 7
    • Impurities shall not exceed these limits
    • It shall, wherever possible, be stored in a glass or plastic container
    • Conductivity shall be less than 30 µS/cm
    Impurities Milligrams per litre
    Dissolved solids 25
    Arsenic (AS) 1
    Chloride (Cu) 5
    Copper (Cm) 0.1
    Iron (Fe) 0.2
    Manganese (Mn) 0.1
    Nitrogen (as NH4) 5
    Nitrogen (as NO3) 5
    Heavy metals (as Pb) 5
    KMnO4 reducing substances * 10
  13. What is a charge cycle of a sulphated battery?

    Where cells are so severely over-discharged that they have a high internal resistance due to sulphation, the charger control could interpret the resistive voltage drop as cell voltage, switch to second rate and under-charge the battery.

    The elevated voltage in the second rate will allow the current to climb as the sulphation is broken down and repeated charges could ultimately recover the battery.

  14. What is constant current charging?

    The principle of constant current charging, the three current rates being represented as straight lines, ie constant current. The first rate charge is continuous until 2.4 volts per cell is reached. This current rate should be:

    Battery Ah

    5 = Amperes

    As with the taper charger the charger control senses 2.4 volts and activates the ‘second rate’ for a period of 3 to 4 hours, at a constant current rate of:

    Battery Ah

    15 = Amperes

    At the end of this period the unit usually switches automatically to ‘equalize rate’ and continues to charge at:

    Battery Ah

    30 = Amperes

    for 3 or 4 hours before automatically switching off.

  15. What are the basic principles of DC battery chargers?

    There are three types of traction chargers commonly in use and although types may differ, the principles remain the same.

    Each of these three types is available as either eight-hour or twelve-hour chargers, depending on the user’s requirements.

  16. When does a traction battery charging process terminate?

    Modern constant current chargers may monitor the battery voltage to determine when the voltage stabilises or starts to go down. At this point the battery is deemed to be fully charged and the charge is terminated.

  17. What is an equalizing charge and when should I use it?

    All cells in a battery have slightly different characteristics. This means, in effect, that the amount of charge required is different for each cell. This is accentuated after very deep discharges and may be observed by small variations in the value of voltage and specific gravity readings between each cell after charging has been completed.

    If the variation is allowed to continue, the differences may be further accentuated and the effective working life of the battery may be reduced. To avoid this, an equalizing charge is recommended. This is an extended charge which should be given at least once a week. Most chargers have an automatic switch which selects the special rate for an equalizing charge. Such a charge is complete when the voltage and specific gravities of the cells have remained constant over three successive hourly readings.

  18. What are the implications of high temperatures on battery operation and how can I avoid them?

    While the capacity of a traction battery is increased somewhat at higher temperatures, heat also has other adverse effects and all practical means should be employed to keep the battery temperature normal. Higher temperatures also increase the charging current and may result in a considerable overcharge. This means increased water usage, greater ‘formation’ (corrosion) of the positive plate spines and a shorter battery life. Any tendency for battery temperatures to rise above 45°C due to operating conditions, should be minimised by:

    • Avoiding over-discharge
    • Charging in a cool location
    • Supplying ample ventilation during charge by opening the battery compartment and circulating air by fans if necessary

    Note: For every 10 °C rise in average battery temperature, the service life of lead acid batteries is reduced by approximately 50%.

  19. How does a modified constant current/taper work?

    This charger uses a first rate of constant current of:

    Battery Ah / 5 = Amperes until the battery reaches a voltage of 2.40 volts per cell. After this stage the charger switches to a second rate taper charge mode, with the battery voltage rising to approximately 2.44 volts per cell, allowing the current to taper down. This taper rate depends on battery discharge and age. The taper rate is fixed by the charger timer for a 5 hour period after which the control unit switches to the equalizing mode at 30 Amperes. This rate will continue until the overall charging time has reached 12 hours. On a new battery under equalizing charge the voltage will rise to approximately 2.6 to 2.7 volts per cell.

  20. What are the implications of overcharging?

    Overcharging is another form of abuse which can have a serious effect on battery life, its effects are not as immediately obvious as those of undercharging. It is, therefore, important to check battery voltage and charge current from the charger’s ammeter, and compare it with the recommended rate.

    Overcharging due to excessively high charging rates or charging for too long a period produces excess gassing, high temperatures and corrosion of the positive spine. These all result in shedding of active materials and greater water loss.

  21. When is a battery over-discharged and what can an over-discharging cause?

    If undercharging is combined with over-discharging the effects are intensified. Strictly speaking, a battery is not overdischarged at any rate unless more than its capacity at that rate has been taken out. Nevertheless, it is highly undesirable to take out anything approaching 100% of its rated capacity on a regular basis.

  22. What are the implications of undercharging?

    Undercharging over a period of time is one of the most destructive forms of abuse to which a battery can be subjected. The formation of excessive amounts of lead sulphate cause the positive plates to expand and break up while the negative active material hardens and loses capacity. The best protection against under-charging is a regular check on the specific gravity readings of the battery after the normal recharge has been completed. If readings show consistent undercharging, remedial action must be taken.

  23. What are the best storage conditions for traction batteries?

    Batteries should be stored in a clean, cool, dry and well ventilated location away from radiators, heating ducts or other sources of heat and protected from exposure to direct sunlight. Before storing, it is necessary that the battery be fully charged and the electrolyte at the proper level. Disconnect leads or cable connections to prevent use or possible discharge through tracking during prolonged storage. Do not remove the electrolyte or strip out the cells. If the storage temperature is 25°C or higher, check the specific gravity at least once a month. If it is below 25°C, every second month. Whenever the specific gravity falls to about 1.230 or lower, give a freshening charge. Always give a freshening charge prior to returning to service.

  24. What are typer chargers?

    These are normally used on surface motive power batteries and can be either air or oil cooled. As illustrated below the current in a taper charger falls (or tapers) as the charge proceeds and the battery voltage rises.

    The charge starts at the ‘first rate’ for an undefined period until 2.35 volts per cell is reached. For this period the starting current should be:

    Battery Ampere hours (Ah)

    7.2 = Amperes

    The charger control senses the cell voltage and at 2.35 volts per cell activates the ‘second rate’ usually for a period of 3-4 hours after which the unit switches off. Battery manufacturers normally specify the maximum charge current at 2.5 volts per cell as follows:

    Battery Ah

    12 = Amperes

    Many taper chargers have an automatic equalize facility, but sometimes the equalize charge is set manually by a switch.

    Equalize charging current should be:

    Battery Ah

    30 = Amperes

    It usually takes about 12 hours for a taper charger to charge a fully discharged battery.


The taper charger is highly susceptible to mains voltage fluctuation. A 10% increase in voltage input can result in up to 45% increase in output current. Where the voltage supply is not stable the taper charger is not recommended.