Charging and Discharging of Lead Acid batteries

The Charging and Discharging Characteristics of Lead Acid Batteries

The voltage of a lead acid battery when at rest (not supplying current or being charged) will vary according to how fully charged the battery is.

The graph shown to the right represents a typical 24 volt lead acid battery which has not been charged or had current drawn from it for a couple of hours.

Battery Discharge Characteristics

A full charged battery will have a voltage of around 25.5 volts. As current is drawn off and the level of charge is reduced, the voltage will fall quite quickly at first (again it would be necessary to stop drawing current for a couple of hours to be able to measure the true voltage of the battery).
With further drawing of current, the rate of voltage drop slows down and will reach around 24.0 volts when the bettery is at half capacity.

As the battery approaches the fully discharged state, the voltage starts to fall more quickly again.

It is important for a battery to never be fully discharged, so your inverter will normally disconnect the supply when the voltage is around 22 volts.

An interesting point to note here is that when an inverter or other power load is drawing a high current from the battery, the voltage will drop. This may mean that the battery needs to be somewhere over 50% charged to avoid the inverter cutting out due to low voltage.

The larger the battery, the smaller this voltage drop will be, and the greater the % of the charge will be useable when drawing high currents.

Battery Charging

If a voltage is applied to the battery which is greater than the battery's voltage, a current will flow through the battery in the reverse direction to when it is supplying current, and the battery will charge.
The rate of charge or current that will flow will depend on the difference between the battery voltage and the voltage that is applied to it (from solar panels etc). Solar panels intended for a 24 volt system are likely to be capable of producing over 30 volts. This voltage ensures that the panels are capable of charging the battery fully.

While it is beneficial to a battery's performance and life to be fully charged on regular occasions, however once a battery has been charged to it's full capacity, it is important not to continue charging as this will damage the battery. A Charge Controller is necessary to ensure that the battery is not over charged. BR>

Battery Efficiency

The Lead Acid battery is not 100% efficient at storing electricity - you will never get out as much as you put in when charging. Overall, an efficiency level of 85% is often assumed.
The efficiency will depend on a number of factors including the rate of charging or discharging. The higher the rate of charge or discharege, the lower the efficiency.
The state of charge of the battery will also affect charge efficiency. With the battery at half charge or less, the charge efficiency may be over 90%, dropping to nearer 60% when the battery is above 80% charged.
However it has been found that if a battery is only partially charged, efficency may be reduced with each charge. If thissituation persists (the batteries never reaching full charge), the life of the battery may be reduced.

Lead Acid Battery Basics

                                                     

Lead Acid Battery

Rechargeable  Lead Acid  Battery Basics

                                            Rechargeable  Lead Acid  Battery Basics

A Lead Acid Battery cell is formed by inserting two parallel lead plates ( electrodes), one coated
with  lead dioxide (PbO2), in a container with Sulfuric acid (H2So4).
The cell has 2 Volts nominal voltage. Installing six such, series connected, cells in a container
forms the popular 12V Lead Acid Battery.
Chemistry Lead Acid Battery :
Connecting resistive load between the lead electrodes, causes current to flow from the
positive (Pbo2) electrode to the negative (Pb) electrode, thus discharging the battery.
Battery charging is accomplished by connecting the electrodes to a voltage source (charger) with
the proper polarity.

The following chemical reactions take place during the charge - discharge cycle:

During the discharge process, the reactions cause both electrodes to be coated with Lead Sulfate
(PbSO4) crystals. Charging restores the situation.
              

Electrochemical formula: PbO2 + Pb + 2H2SO4                             PbSO4 + 2H2O + 2e¯
              

More insight is deduced by considering each electrode separately:

                   

At the negative electrode:              Pb + SO4                                   PbSO4 + 2e¯
                        
                         
                      

At positive electrode:     PbO2 + SO4 + 4H   + 2e¯                         PbSO4 + 2H2O
                            

The electrolyte is a mixture of Sulfuric Acid and water. Specific gravity of Electrolyte measures the
content of Sulfuric Acid in the solution. It is typically in the range of 1.200 to 1.300. Whereas
water has Specific Gravity of 1, and pure Sulfuric Acid has Specific Gravity of 1.835. State of
charge and other major battery characteristics are strongly related to the specific gravity of sulfuric
acid.


At charging, electrolysis of water produced oxygen and hydrogen as byproducts . In such process,
the external voltage source induces electrical current in the cell according to Ohms law. The
electrolyte due to its low resistance increases dramatically the current in the cell.
This current dissociates water molecules into (OH) ¯ and H   ions.

The reaction at the Positive Electrode:      2H2O                               O2 + 4H    +  4e¯

The reaction at the Negative Electrode:    2H2O + 2e¯                       H2 + 2(OH) ¯

Thus, during charging, water is decomposed into its constituents. The oxygen gas,
generated at the positive plate and the hydrogen gas, generated at the negative plate.

Lead Acid Battery Technology:
Flooded batteries and Sealed Lead Acid  batteries form the two basic lead acid battery types, they
differ mainly by the  way water is restored to the battery.

In Flooded batteries the oxygen and hydrogen gases generated at the plates, are vented to the air
through the top of the battery. Water should therefore be added during maintenance in order to
replace the missing quantity, and sustain the charge storage ability.

In Sealed Lead Acid (SLA) Batteries, generally Valve Regulated Lead Acid (VRLA) Batteries,
pressure relief valves do not allow gases to leave the battery. The oxygen generated at the positive
plate diffuses to the negative plate, where it is recombined with hydrogen to form water. Thus,
water refilling is not required and no maintenance is required.
The valves will however allow gas relief, when internal pressure exceeds allowed value, as may be
the case at battery overcharging.

Lead Acid Battery Applications
Flooded batteries, also called wet batteries, are used for energy storage purpose mainly for
stationery, long duration (several hours) long life systems, for Utilities, Telecom and
Industrial applications.
Sealed lead acid batteries (SLA), often called Maintenance Free Batteries,  are favored for standby
application and UPS applications. Their unique features in normal operation, such as maintenance
free, hermetically sealed, non spillable, non gassing and vibration resistance characteristics, makes
them an ideal choice to serve as standard UPS Batteries, as well as in numerous other applications
in additional fields, such as communication, security, transportation, emergency lightning, aviation
and more.