Objective1

To learn the specific charge/discharge characteristics of a Lead Acid battery through experimental testing of a remote triggered Lead Acid Battery.

Objective 2

See the performance of the battery by analyzing the energy output and power output. Compare with other battery characteristics and see which battery will be more suitable for high current applications

Objective 3

Each type of battery chemistry, whether it be lead acid, lithium, nickel metal hydride, or others has specific characteristics that define its electrical operation, size, weight and other properties. This experiment introduces the student to some of the electrical characteristics of a lead acid battery. Specifically, we will investigate: 

• Charge and discharge curves - Lead-acid batteries have unique charge and discharge curves (voltage vs. time during charging and discharging). Amongst others, these curves can be used for: 
  • Quickly determining the State of Charge (SOC) of the battery based on its voltage, as used in lead acid powered electric vehicles[2]
  • Determing algorithms for safe charging and discharging since over-charging or over-discharging batteries can reduce the lifetime of batteries, damage them, or even lead to fire and explosion,
  • Understanding the float behavior of lead acid batteries, or how the voltage of a battery changes when a charge or discharge process is stopped.
• Energy capacity vs. discharge rate is an important design parameter for energy storage in lead-acid battery based solar photovoltaic systems and for 12V automotive batteries. The energy capacity vs. discharge rate affects the weight, size, and cost of a battery and device. Amongst others, this information is useful for:
  • Sizing a battery for an application, by understanding the usable capacity of the battery which changes as a function of the discharge rate,
  • Identifying the duration for which a device can operate off battery power

Theory

Lead acid (PbA) batteries are one of the most widely used types of batteries today. Every automobile has a lead acid battery for starting the engine and powering the electric system. Older electric vehicles used large numbers of lead acid batteries arranged together into a battery pack to form the traction battery to propel the vehicle. Most energy storage systems for solar photovoltaic  energy systems are also composed of lead acid batteries. 

Lead acid batteries were the mainstay of electrical energy storage for most of the 20th century. Lead acid batteries are relatively safe to use, exhibit no memory effect, and are simple to determine the state of charge (SOC) or depth of discharge (DOD). The details on calculating the DOD and SOC can be found from [2].

However, newer battery technologies are rapidly replacing lead-acid batteries, especially in portable applications, where the heavy weight of lead acid batteries put them at a disadvantage and in applications where space is at a premium. These newer technologies, such as Lithium, Nickel Metal Hydride and Nickel Cadmium are covered in other Virtual Labs in this series. 

 

Back ground and Theory:

 

A battery is an electrochemical device in which electrical energy is converted and stored in chemical form for storage. The chemical energy can then be easily reconverted into electrical energy.

Two primary types of chemical batteries exist: Primary and secondary. A primary battery is not normally rechargeable and is designed to only last one discharge cycle, after which it must be replaced. Secondary batteries are rechargeable. They can be discharged and recharged repeatedly.

Basic battery operation:

Two electrodes (positive and negative, made of two chemically different materials) are separated by an electrolyte - a solution that easily conducts ions (charged particles)

An Electrical Load is applied to the cell, causing the cell to discharge.

• Electrons are pulled from the positive terminal of the battery through a chemical reaction between the   positive terminal and the electrolyte

• Electrons flow through the electrical load

• Electrons return to the negative terminal

• Electrons are put back into the negative side of the battery through a chemical reaction between the     negative terminal and the electrolyte

• Battery becomes discharged when the chemical reactions are not possible any longer – the chemicals have all been transformed into other chemicals that do not support electron producing chemical reactions

Rechargable batteries:

In many batteries, the chemical reactions are reversible when voltage is applied to the battery (Charging). Rechargeable batteries are also called Secondary batteries, as opposed to Primary batteries, which are single use only.

Ratings:

The voltage of an individual cell is fixed by battery chemistry.

The current is a function of the rate of chemical reaction in the battery, which is characterized by the Equivalent Series Resistance (ESR). Then from Ohm’s law, we can see that for a fixed voltage, the current is controlled by the resistance.

Current = Voltage / Resistance = V / ESR

The capacity of the battery is defined as

Capacity = (Voltage) * (Amp-hours).

The duration for which a device can operate off a battery's power can be calculated by using the below formula:

Time = Capacity / Power = (Voltage * Amp-hours) / (Voltage * Current).

This can also be calculted as: 

Time = ((State of Charge of the battery in percentage) * (Total Full Energy of the battery)) / (Loaded Voltage * Current).

For example, if the battery has a 10 Ah (Amp hour) rating, it can provide: 1 Amp for 10 hours OR 10 Amps for 1 hour. The capacity is usually defined at a standard charge/discharge rate (C-rate), which is the the charge/discharge rate (in Amps) that the battery will provide for the specified # of hours. For example, under discharge, C/10 = 5.2 A implies that the battery will provide 5.2 Amps for 10 hours.

The capacity usually increases for lower charge/discharge currents and decreases for higher charge/discharge currents.

Series and Parallel Connection

• When connected in series, the battery voltages add

                - Positive terminals of one battery connected to the negative of another, and so on

• When connected in parallel, the battery currents add

                - Positive terminals of all the batteries connected together, negatives all connected together

• Multiple cells are connected in series to obtain higher voltages

 

Connecting in Series (Double voltages, Same capacity (Ah) )

Series connection adds the voltage of two batteries, keeps the capacity as same (Ah).

For Example,

 

Double Voltage (12V), Same Capacity (10Ah)

Two 6V Batteries joined in series produces 12V, But the total capacity is still 10A.

Connecting in Parallel ( Same voltage, double capacity (Ah) )

Parallel connction  increases current rating but the voltage ramains same.

 

Same Voltage (6V), Double Capacity (20Ah)

Two 6V Batteries connected in parallel have the same 6V, But the current increases to 20A

 

Connecting in series/parallel(Double Voltage, Double Capacity(Ah) )

Batteries connected in series/parallel increases both the voltage output and current rating.

 

 

 

 

Double Voltage (12V), Double Capacity (20Ah)

Two sets of batteries already connected in parallel are joined them together to form a series produces 12 V and 20 Ah.

 

Lead Acid Battery:

There are two types of Lead Acid Batteries available. They are,

· Automotive Battery

- Traction Battery

Automotive batteries are used for the conditions where there is high current drain for infrequent short duration. They recharge when the engine reaches the operating speed.

Traction batteries are used when there is moderate current drain at continuous stretch. They do not recharge during the operating conditions.

Construction:

A Lead acid battery can have many cells each with positive and negative plate and an electrolyte which is a mixture of Sulphuric Acid and water.

There are two types of constructions:

Flat plate

Tubular plate

In flat plate structure, the positive electrodes are metallic with lead dioxide as the active material. The negative plate is a spongy lead plate which is in a grid structure.

 

 

In the negative rod, the sulphate ions react with the spongy lead forming the lead sulphate and 2 free electrons. This will reduce the negative electrical property of the negative plate. Similarly the positive lead oxide plate also reacts to form lead sulphate there by reducing its strength. When the free electrons are used up then the reaction is slowed down until they are supplied.

The open circuit voltage or the emf of the cell is the difference between the two potential of the plates. It is about 2.1V as long as no excess path is provided. Here in this stage very little or no reaction takes place so that the charged battery can remain as such for a while.  The open circuit voltage can decrease by 1 mV per day, during storage. If there is no loss of electrolyte in this time then we can call it as ‘self discharge’. 

 

Producing current:

The excess of electrons in the negative plate and the absent of it in the positive plate will decrease the rate of reaction unless taken care of. An external wire can start the movement of electrons between them. The electric current between them can be high because the wire is short circuited between them. If a resistance is passed in the way then we can have the terminal voltage between the ends.

With the reaction the electrolyte reduces its activity due to the reduced the concentration. The electrodes are converted to lead sulphate too. Till the recharging procedure the lead sulphate remains as such. While recharging the plate is reconverted and the acid concentration is restored.

 

Discharge Curve of a Lead acid battery:

There are typically three signifcant regions to a battery discharge curve. Assuming you start with a fully charged battery, when the discharge starts, the "surface charge" of the battery is taken off, and the voltage drops rapidly, albeit for a very short amount of time.

The next phase of discharging is in the bulk or main part of the discharge. During this phase, most of the energy of the battery is discharged. For a lead acid battery, this happens in a relatively linear manner, with the voltage dropping in proportion to the Depth of Discharge, or inversely proportional to the State of Charge. Most of the discharge process occurs during this middle portion.

During the final stage of discharge, the voltage again drops rapidly. This is referred to as the "knee". Discharging a battery through this portion significantly reduces the number of cycles that the battery can endure and can result in permanent damage to the battery.

The discharge curve characteristics of the particular battery, a Panasonic LC-R061R3P, used in this experiment is given below. This image is taken from the datasheet for the battery (link to the full datasheet given in the References section for this experiment). In this plot, you can clearly see the three regions of the discharge curve. 

Lifetime of a Lead acid battery: 

The lifetime of a Lead acid battery is generally determined by the number of cycles that it can go through before serious capacity degradation occurs. The number of cycles that the battery can go through is also dependent upon the Depth of Discharge (DOD). Repeatedly discharging a battery more deeply (to a greater DOD) results in the battery having a diminished lifetime. Discharging a battery less deeply (lower DOD) results in extending the lifetime of the battery. 

General best practice to maximize the lifetime of lead acid batteries is to only discharge them to 50% capacity. Repeatedly discharging to a lower SOC (larger DOD) will reduce the battery lifetime.