30-second summary
Laptop Battery
Nowadays, the vast majority of laptops contain lithium-ion batteries.
A lithium-ion battery, also known as the Li-ion battery, is a type of secondary (rechargeable) battery composed of cells in which lithium ions move from the anode through an electrolyte to the cathode during discharge and back when charging.
There are several specific advantages to lithium-ion batteries. The most important advantages are their high cell voltage, high energy density, and no memory effect.
Lithium-ion batteries are used in many laptop computer batteries, cordless power tools, certain electric cars, electric kick scooters, most e-bikes, portable power banks, and LED flashlights.
Cylindrical vs. Prismatic vs. Pouch Cells
Li-ion cells (as distinct from entire batteries) are available in various shapes, which can generally be divided into three groups:
- Cylindrical cells with solid bodies without terminals, such as those used in older laptop batteries. Cells with a cylindrical shape are made in a characteristic “jelly roll” manner, which means it is a single long “sandwich” of the positive electrode, separator, negative electrode, and separator rolled into a single spool. One advantage of cylindrical cells compared to cells with stacked electrodes is the faster production speed. One disadvantage of cylindrical cells can be a large radial temperature gradient inside the cells developing at high discharge currents.
- Flat or pouch cells with a soft, flat body, such as those used in cell phones and newer laptops; are lithium-ion polymer batteries. Pouch cells do not have a rigid enclosure and use a sealed flexible foil as the cell container. This is a somewhat minimalistic approach to packaging; it reduces weight and leads to flexible cells that can easily fit the available space of a given product. For pouch batteries, the absence of a case gives pouch cells the highest gravimetric energy density. These batteries are increasingly popular with smartphone manufacturers. Their soft, lightweight design also offers more safety measures than hard metal casings. When a critical issue with a pouch cell occurs – often due to internal pressure overheating or shortening the batteries – the pack will noticeably expand with gas.
- Prismatic cells with rigid plastic case. Lithium-ion (Li-ion) battery prismatic cells are thinner and lighter than cylindrical cells. These cells, coming in rectangular aluminum or steel casing, have fairly long lifespans but aren’t as easy to keep cool compared to their cylindrical counterparts.
Characteristics of Lithium-ion Batteries
To compare and understand the capability of each battery, some important parameters are characteristic of each battery, also within a type of battery. These parameters are a reference when a battery is needed, and specific qualities are required since batteries are used in all types of devices and for infinite purposes.
Cell Voltage
The voltage of electric batteries is created by the potential difference of the materials that compose the positive and negative electrodes in the electrochemical reaction.
Almost all lithium-ion cells work at 3.8 volts. In order to make current flow from the charger to the battery, there must be a potential difference. Therefore battery chargers or USBs for almost all smartphones provide a voltage of 5V. The typical voltage for a battery is 11.1V or 14.8V depending upon the numbers of cells.
Cut-off Voltage
The cut-off voltage is the minimum allowable voltage. It is this voltage that generally defines the “empty” state of the battery.
Li-ion battery has a higher cut-off voltage of around 3.2 V. Its nominal voltage is between 3.6 to 3.8 V; its maximum charging voltage can go to 4– 4.2 V max. The Li‑ion can be discharged to 3V and lower; however, with a discharge to 3.3V (at room temperature), about 92–98% of the capacity is used. Importantly, particularly in the case of lithium-ion batteries used in the vast majority of portable electronics today, a voltage cut-off below 3.2V can lead to chemical instability in the cell, resulting in a reduced battery lifetime.
Capacity
The coulometric capacity is the total Amp-hours available when the battery is discharged at a certain discharge current from 100% SOC to the cut-off voltage.
Laptop battery capacity typically ranges from 2,000mAh (milliamp hours) to 7,000 mAh. This means that laptops usually hold between 2 and 7 amp hours of battery life.
C-rate of Battery
C-rate is used to express how fast a battery is discharged or charged relative to its maximum capacity. It has units h−1. A 1C rate means that the discharge current will discharge the entire battery in 1 hour.
Most li-ion batteries can only withstand a maximum temperature of 60°C and are recommended to be charged at a maximum of 45°C under a 0.5C charge rate. C rating for a 18650 battery is usually 1C, meaning we can consume a maximum of 2.85A from the battery.
Self-discharge
Batteries gradually self-discharge even if not connected and delivering current. This is due to non-current-producing “side” chemical reactions that occur within the cell even when no load is applied.
Li-ion rechargeable batteries have a self-discharge rate typically stated by manufacturers to be 1.5–2% per month. The rate increases with temperature and state of charge.
Degradation
Some degradation of rechargeable batteries occurs on each charge-discharge cycle. Degradation usually occurs because electrolyte migrates away from the electrodes or because active material detaches from the electrodes.
Most modern 18650 lithium-ion batteries, which are common for laptop batteries, have a typical cycle life of 300 – 500 (charge, discharge cycles).
Depth of Discharge
Depth of discharge is a measure of how much energy has been withdrawn from a battery and is expressed as a percentage of full capacity. For example, a 100 Ah battery from which 40 Ah has been withdrawn has undergone a 40% depth of discharge (DOD).
For lithium-ion batteries, the cycle life of a cell strongly depends on the DOD. The loss of lithium ions and active electrode material is higher for larger DOD cycles. At high DODs, additional degradation mechanisms can occur, resulting in the decomposition and dissolution of cathode material and capacity fading.
