To learn the specific charge/discharge characteristics of a Lithium- Polymer(Li- Po) battery through experimental testing of a remote triggered Li- Po Battery.
Background and Theory
Lithium polymer batteries are rechargeable battery of lithium-ion technology in a pouch format. Unlike cylindrical and prismatic cells, LiPos come in a soft package or pouch, which makes them lighter but also less rigid.
Each type of battery chemistry, whether it be Lithium-polymer, Lithium ion, 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-polymer 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 daily by billions of people all over the world to see how much battery is left on a laptop or mobile phone,
Determining the low-voltage cutoff at which a battery voltage will fall below the value required for operating the electronics of portable devices,
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 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:
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 by using the formula: Time = Energy / Power = ((State of Charge of the battery in percentage) * (Total Full Energy of the battery)) / (Loaded Voltage * Current)
The life of LiPo battery vs DOD and discharge rate is shown in the below graph:
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.
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.
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 Ã¢â‚¬â€œ the chemicals have all been transformed into other chemicals that do not support electron producing chemical reactions
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.
More Battery Basics
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 OhmÃ¢â‚¬â„¢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 Amp Hours is the number of Amps that a battery can produce for an hour OR the number of hours a battery can produce one Amp.
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.
The capacity usually increases for lower charge/discharge currents and decreases for higher charge/discharge currents.
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) )
Series connection adds the voltage of two batteries, keeps the capacity as same (Ah).
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.
The voltage of a Li-poly cell varies from about 2.7 V (discharged) to about 4.23 V (fully charged), and Li-poly cells have to be protected from overcharge by limiting the applied voltage to no more than 4.235 V per cell used in a series combination.
The lithium-polymer differentiates itself from conventional battery systems in the type of electrolyte used. This electrolyte resembles a plastic like film that does not conduct the electricity but allows the ion exchange. Since the metal casing is absent, it is much lighter than the Lithium ion battery.
The lithium-polymer electrochemistry currently covers a wide range of active materials such as LiCoO2, LiNiO2, and its Co doped derivatives. Harding uses LiCoO2 chemistry.
Charging & Discharging Chemical Reaction
When Lithium Polymer cells are first charged, lithium ions are transferred from the layers of the lithium cobaltite to the carbon material that forms the anode.
LiCoO2 + 6C --> Li1-x CoO2 + LixC6
Subsequent discharge and charge reactions are based on the motion of lithium ions betweenanode and cathode.
Li1-x CoO2 + LixC <----> Li 1-x +dx CoO 2 + Lix-dx C
During charge/discharge Li+ ions are transported back and forth between two insertion electrodes.
A strict charging regime is necessary to properly and safely charge Lithium Polymer batteries. Most batteries contain a protective circuit to prevent overcharge and over discharge. This circuit limits the charge voltage to a maximum 4.2 Volts. The circuit also contains a thermal sensor, which disconnects charge if the temperature reaches 90 Ã‚Â°C (194Ã‚Â°F). If a cell is inadvertently overcharged, the cell may heat up and vent with a flame.