# CHAPTER 5: Battery Management System

## 5.1 Introduction to Battery Management System (BMS)
A battery management system is an electronic device that manages a rechargeable battery, such as preventing the battery from operating beyond its safe operating area, tracking its status, measuring secondary data , reporting that data, managing, authenticating and regulating its environment.

BMS is a part of the battery pack's intelligence. It monitors battery charging and discharging parameters by deciding how much electricity is expected to pass through the device. It prevents the battery from operating beyond its safe range of operation. Each model will have an exclusive BMS that is suited for that particular application based on number of series cells, current rating and other parameters.

## 5.2 Working Principle

When the battery pack is connected to load such as a motor or electronic circuitry electrical current is drawn from the battery pack. The current from each series link, however, may not be of the same magnitude resulting in an unbalanced state.

High quality cells should have similar characteristics and therefore remain in a healthy state. Nevertheless, even these cells can become unbalanced after several charge cycles with currents of higher magnitude. The situation is even worse for poor quality cells, because they hit the unbalanced state even faster.

This unbalanced state can create major problems for battery pack charging. The standard chargers available in the market do not have the capability of balancing, they only supply the measured voltage and current and charge up to the battery pack to reach that voltage level. If the cells in the battery pack are unbalanced, then few cells can be overcharged while other cells are under charged. This further affects the balanced battery pack state. We opt for the Battery Management System ( BMS) to prevent these instances. The BMS functions as a balance charger and cell monitoring device which typically resides inside the battery in the form of a circuit board rather than residing in the converter.

## 5.3 Functions

* Monitoring individual cells which form the battery pack, maintaining its operating conditions.
* Shielding the cells from tolerance limits. Isolating the battery pack in case of emergency or any changes occurs in the operating mode.
* Monitoring the conditions of individual cells which form the battery pack.
* State of Charge (SOC) and State of Health (SOH) information of the battery are measured which indicates the condition of the used battery comparative to the new battery.
* Some BMSs have thermal protection feature, which monitors the instantaneous temperature and this data can be used to stop the charging and discharging functionality when the battery pack becomes too hot and is unsafe to use.
* A few expensive BMSs include features such as Bluetooth connectivity, this allows the user to monitor the status of the battery in real time remotely.

## 5.4 Architecture

A battery-management system (BMS) typically consists of several functional blocks, including cutoff field-effect transmitters (FETs), fuel-gauge monitor, cell-voltage monitor, cell-voltage balance, real-time clock, temperature monitors, and a state machine.

Credits: http://erbug.boapu.hapolo.mohammedshrine.org/lithium-battery-management-system-wiring-diagram.html

**Cutoff FETs and FET Driver**

The FET-driver block is responsible for the connection and isolation of the battery pack between the load and the charger. The action of the FET driver is predicated on battery-cell voltage measurements, current measurements and real-time detection circuitry.
The FET drivers can be designed to connect to a battery pack's high or low side. To activate the MOSFETs a high-side connection requires a charge-pump driver. By using a high-side driver it allows the rest of the circuitry to have a stable ground reference. In some integrated solutions, low-side FET driver connections are found to minimize costs, as they don't need a charge pump. Also, they don't need high-voltage devices which consume a larger die area. Using the cutoff FETs on the low side floats the ground contact of the battery pack which makes it more sensitive to the noise introduced into the measurement.

**Fuel-Gauge/Current Measurements**

The functional fuel-gauge block keeps track of the charge that enters and exits the battery pack. Charge is the product of time and current. When designing a fuel gauge several different techniques can be used.

A current-sense amplifier and an MCU with an embedded low-resolution analog-to-digital converter (ADC) is one current-measurement method.The current-sense amplifier, which works in high common-mode settings, amplifies the signal, allowing measurements with higher resolution. This design technique sacrifices dynamic range, though. Other methods involve using a high-resolution ADC, or costly fuel gage IC.

Understanding the current consumption of the load activity over time determines the best form of design for a fuel gauge. The most accurate and cost-effective approach is to use a 16-bit or higher ADC with low offset and high common-mode rating to calculate the voltage through a sense resistor.

**Cell Voltage and Maximizing Battery Lifetime**

To evaluate the overall health of a battery pack it is important to monitor the cell voltage of every cell in a battery pack. All cells have an operating voltage window where there should be charging / discharging to ensure proper operation and longer battery life. When an application uses a lithium chemistry battery, the operating voltage usually varies from 2.5 to 4.2 V. Operating the battery outside the voltage range significantly reduces the lifetime of the cell and can render it useless.

One easy way to decide whether a battery pack is charged is to track the voltage of each cell to a specified voltage point. The first cell to exceed the voltage mark trips the maximum charge limit of the battery-pack. A cell in a battery pack weaker than average results in the weakest cell reaching the mark first, preventing the rest of the cells from completely charging.

As defined, a charging scheme does not optimize battery-pack ON time per charge. The charging scheme reduces the lifespan of the battery pack, as it requires more cycles of charge and discharge. A weaker cell is quicker to discharge. This also occurs on the discharge cycle; the weaker cell first trips the discharge cap, leaving the remainder of the cells with the remaining charge.

There are two ways to improve the ON time per battery pack charge.

* The first method is to slow down the charge throughout the charge cycle to the weakest cell. This is done by connecting a bypass FET to a cell-wide limiting current resistor. This takes current from the cell with the highest-current, resulting in a slowing cell charge. As a result the other cells in the battery pack will catch up.

* The second approach is to align the battery pack into a charging-displacement scheme on the discharge cycle. It is achieved by taking charge from the alpha cell via inductive coupling or capacitive storage, and by injecting the stored charge into the weakest cell. It slows down the time taken to exceed the discharge limit by the weakest cell, better known as active balancing.

**Temperature Monitoring**

Today's batteries deliver lots of current while keeping the voltage steady. It can result in a runaway state that causes the battery to catch fire. The chemicals used to construct a battery are extremely volatile — an impaled battery with the appropriate object may also cause the battery to catch fire. Temperature measurements are not only used for protection, they can also decide whether charging or discharging of a battery is desirable.

Temperature sensors track each cell for energy storage systems [ESS] applications or a grouping of cells for smaller and more compact applications. It is normal to use thermistors operated by an internal ADC voltage reference to control the temperature of each circuit.

**State Machines or Algorithms**

Many BMS systems require a microcontroller (MCU) or a field-programmable gate array ( FPGA) to handle sensing circuit information, and then make decisions with the information obtained. A digitally encoded algorithm allows for a stand-alone solution with one chip.

**Other BMS Building Blocks**

Other functional BMS blocks may include battery authentication, real-time clock (RTC) and memory. For black-box applications, the RTC and memory are used — the RTC is used as a time stamp, and memory is used to store data. This lets the user know the battery pack actions before a disastrous event. The authentication block of the battery prevents connection of the BMS electronics to a third party battery pack. The voltage reference / regulator is used for controlling peripheral circuitry across the BMS.

## 5.5 How to choose a BMS?

**Voltage rating**
The voltage rating is usually mentioned in the form of the number of series in the battery pack. For instance, 7 cells of 3.6V each in series produces 36V. Hence a 7S BMS circuit with appropriate current should be considered.

**Current rating**
BMS is rated with different values of current, ranging from low power BMS to high power BMS depending on the load connected. Thus, a BMS with suiable nominal current and peak current rating is selected based on the load parameters.

### Wiring the BMS

There are three main connections,

* C- [C minus]: This port is the charger connector. The negative of the charger wire is connected to this port. Charger’s positive terminal is connected to the positive of the battery pack.

* B- [Battery minus]: The first set of the battery pack’s negative terminal is connected to this port.

* P- [Pack minus]: Discharge Connector, which is internally connected to the Battery pack minus.

(Note: B- & P- are same ports)

Fig: 10S BMS connection