Lithium-ion batteries have permeated every aspect of modern life, powering numerous devices, such as mobile phones, watches, tablets, and portable devices. Here’s a brief overview of lithium battery basics.

1. Basic Terminology

1) C-rate

This refers to the ratio of the current used to charge or discharge a battery to its nominal capacity. This describes how quickly a battery can be charged or discharged. Charge and discharge rates are not necessarily the same.

Examples:

1C: The battery is fully discharged in 1 hour.

0.2C: The battery is fully discharged in 5 hours.

5C: The battery is fully discharged in 0.2 hours.

2) Capacity

The amount of energy stored in a battery. Measured in mAh or Ah.

Combined with the charge rate, for example, if the G40E battery is 4800mAh and the charge rate is 0.2C, this means it takes 5 hours to fully charge the battery from empty (ignoring the pre-charge phase at very low charge levels). The charging current is: 4800mA * 0.2C = 0.96A.

3) Battery Management System (BMS)

This system controls and manages battery charge/discharge, monitors battery temperature and voltage, communicates with the host system, balances battery voltage, and manages battery pack safety.

4) Cycle

A battery’s complete charge-discharge cycle is called a cycle. If only 80% of its total energy is used each time, a lithium-ion battery can have a cycle life of thousands of times.

2. Battery Types

Currently, commercial lithium-ion cells are mainly cylindrical, prismatic, and pouch cells.

The 18650 cylindrical cell is currently the most produced lithium-ion cell, and is used in our G series monitors.

3. Series and Parallel Connection of Cells

Cells are the core component of a battery. The number of cells varies depending on the application, but all batteries require different cell connections to achieve the required voltage and power.

Note: Parallel connection conditions are very demanding. Therefore, connecting in parallel first, then in series, can reduce the requirements for battery consistency.

Q: What is the difference between three batteries in series and four in parallel and four batteries in parallel and three batteries in series?

A: Both voltage and capacity are different. Series connection increases voltage, while parallel connection increases current (capacity).

1) Parallel Connection

Suppose the voltage of a battery cell is 3.7V and the capacity is 2.4Ah. After parallel connection, the terminal voltage of the system remains 3.7V, but the capacity increases to 7.2Ah.

2) Series Connection

Suppose the voltage of a battery cell is 3.7V and the capacity is 2.4Ah. After series connection, the terminal voltage of the system is 11.1V, and the capacity remains unchanged.

Our G series monitors use six 18650 cells in three series and two in parallel, resulting in a battery voltage of 11.1V and 4.8Ah.

The Tesla Model-S sedan uses Panasonic 18650 cells, and an 85kWh battery pack requires approximately 7,000 cells.

4. BMS (Battery Management System)

The battery management system is the central control unit of the battery pack.

It primarily prevents battery failure modes such as overcharging, over-discharging, overheating or underheating, and internal short circuits. It also provides detection capabilities.

1) Balancing Management

Battery cells may experience slight variations during the production process. These differences may seem minor, but they can cause significant problems when the battery system is initially operating.

In the figure below, cell 3 reaches the end of discharge earlier than the other two cells. At this point, the battery stops discharging because further discharge would damage cell 3. This means that the energy in cells 1 and 2 cannot be fully discharged, leaving the battery pack with residual, unusable energy. As cycles increase, the difference in charge and discharge capacity between cells increases. The weakest cell will be subjected to a greater workload than the others, ultimately leading to premature failure of the battery.

• Passive balancing management dissipates excess energy in cells with higher charge levels. Resistors are commonly used to convert this excess energy into heat.

• Active balancing management transfers excess energy from cells with higher charge levels to cells with lower charge levels.

2) Thermal Management

The optimal storage temperature for lithium-ion cells is around 23 degrees Celsius.

• Active thermal management involves flowing gas, liquid, or refrigerant through all cells within the battery to maintain a uniform temperature.

• Passive thermal management utilizes the battery packaging to conduct and dissipate heat.

3) Software Control System

BMS designers study operating cells in a controlled laboratory environment to understand how they perform under various conditions, then translate this information into code. Through repeated steps, software designers may eventually develop an algorithm that accurately predicts cell performance under a wide range of conditions.

Most manufacturers utilize software as their core technology. Control software uses a series of mathematical formulas and calculations to understand the various states of all cells at different times. These algorithms are often based on complex models and are specific to the cell system and structure.

5. Lithium Battery Usage Recommendations

• Always keep lithium batteries charged. Overdischarging significantly impacts battery life.

• If you plan to use a lithium battery for an extended period, charge it regularly.

• If the battery temperature is too high, stop using it or add heat sinks to cool it down.

• Lithium batteries have a limited lifespan and will degrade rapidly after two or three years, regardless of use. When purchasing a battery, check the production date.