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Li-ion Battery
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Objective
To learn the specific charge/discharge characteristics of a Lithium- ion (Li- ion) battery through experimental testing of a remote triggered Li- ion Battery. 

Each type of battery chemistry, whether it be Lithium-ion, lead acid, 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 Lithium-ion battery. Specifically, we will cover: 

  • Charge and discharge curves - Lithium-ion batteries have unique charge and discharge curves (voltage vs. time during charging and discharging). Amongst others, these curves can be used for: 
  1. Quickly determining the State of Charge (SOC) of the battery based on its voltage, as used daily by billions of people all over the world to see how much battery is left on your laptop or mobile phone,
  2. Determining the low-voltage cutoff at which a battery voltage will fall below the value required for operating the electronics of portable devices,
  3. 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,
  4. Understanding the float behavior of 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 electric and hybrid vehicles with Lithium batteries, electric power tools, and portable electronics devices. The energy capacity vs. discharge rate affects the weight, size, and cost of a battery and device. Amongst others, this information is useful for:
    1. Sizing a battery for an application, by understanding the usable capacity of the battery which changes as a function of the discharge rate,
    2. Identifying the duration for which a device can operate off battery power by using the formula: Time = Energy / Power = ((State of Charge of the battery in percentage) * (Total Full Energy of the battery)) / (Loaded Voltage * Current)

Introduction

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.

As we are all aware, a significant number of the modern electronic equipment we take for granted every day, such as mobile phones, laptop computers, music players, cameras and countless others are powered from rechargeable batteries.

 

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

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

  • Electrons return to the negative terminal

  • 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.

 

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

 Current = Voltage / Resistance = V / ESR

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

For example, if the battery has a 10 Ah (Amp hour) rating, it can provide:1 Amp for 10 hoursOR10 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.

 Series and Parallel Connection

 

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

 

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

     

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

 

For Example,

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

 

Parallel connction  increases current rating but the voltage ramains same.

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

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

 

For Example,

 

 

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

 Li-Ion Battery

The Li-ion Battery was first proposed by Exxon with lithium metal in 1970 and then the rechargeable battery was developed with lithium cobalt oxide in 1979. Due to safety issue, Lithium metal electrode is replaced with lithium ion even though the energy density is less.  In 1991, Sony commercialized the Li-ion Battery. Now, more than 60% of portable rechargeable batteries used are Li-ion Battery. The lithium is the lightest metal of all metals.

Li-ion works on the principle of the movement of ions from electrodes.

 

Positive electrode



 

Overall reaction

 

 

Li-ion has a nominal single cell voltage of 3.6 V, which is fixed by the battery chemistry. In order to obtain higher voltages, cells are put together in series. Li-ion replaces Ni-MH batteries in portable electronics. 

 

 

The Specific Energy refers to the amount of energy that can be stored per unit weight. This value is very important for portable equipment as heavy batteries will be difficult and energy consuming to move around. The Specific Energy of  Li-ion batteries is much higher than all the batteries. The cost of  Li-ion is higher than the Ni-MH Batteries.

 

Energy Density describes how much energy can be stored per unit volume. Again, for portable electronic equipment, the space required for a given storage capacity is an important figure. The Energy Density is twice than that of Ni-Cad, approximately equal to the NiMH Lead-acid batteries.

 

Specific Power refers to the maximum amount of power can that can be delivered. In electrical terms, this is the maximum Discharge Rate of the battery. The lifetime of Lithium ion  is less compared to Ni-Cad but higher than the lead acid , NiMH and lithium polymer.

 

 Li-ion is lighter compared to all other equivalent energy density rechargeable batteries.  It has no memory effect. Self discharge is very less and environmental friendly after replacing the lithium metal by lithium ion. It can be recycled. The nominal cell voltage is higher than other batteries.

 

Deep Discharging of a Lithium-ion battery reduces the life cycle. Therfore, a Lithium-ion battery should be stored in a cool place with at least 40% charge. They should be also maintained within safe voltage and current limits with a protection circuit. It should be often fully charged to prevent crystalline aging. There are restrictions for transport of large quantities of Lithium batteries due to safety issues.

    

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