Battery Management System Design

Overview

Battery management system (BMS) is the brain of the power flow in the pod. It manages the rechargeable battery pack that will be used in the Hyperloop pod. It is responsible for protecting the battery, monitoring the state of charge (SoC) and state of health (SoH), calculating the secondary data, reporting the data, controlling and balancing the environment. All in all, the basic task of a Battery Management System (BMS) is to ensure that optimum use is made of the energy inside the battery powering the portable product and that the risk of damage inflicted upon the battery is minimized. This is achieved by monitoring and controlling the battery’s charging and discharging process.

Primary functions of BMS

Control charging of the battery, with practically no overcharging, to ensure a long cycle life of the battery.
Monitor the discharge of the battery to prevent damage inflicted on the battery by interrupting the discharge current when the battery is empty.
Keep track of the battery’s SoC and use the determined value to control charging and discharging of the battery and signal the value to the user of the portable device.
Power the load with a minimum supply voltage, irrespective of the battery voltage, using DC/DC conversion to achieve a longer run time of the portable device.

BMS Parts

Designed general BMS block diagram can be seen below:
Power Module
The basic task of the power module is to charge the battery by converting electrical energy from the mains into electrical energy suitable for the use in the battery. Here, the mains mean "energy source".
The efficiency of the energy conversion process has to be sufficiently high, because the poorer the efficiency, the higher the internal temperature.
The Battery
The basic task of a battery is to store energy obtained from the mains or some other external power source and to release it to the load when needed.
In the case of Li-ion batteries, an electronic safety switch has to be integrated with the battery.
The battery voltage, current and temperature have to be monitored and the safety switch has to be controlled to ensure that the battery is never operated in an unsafe region.
A voltage range, a maximum current and a maximum temperature, specified by the manufacturer, determine the region within which it is considered safe to use a battery.
The maximum voltage is dictated by two factors, the maximum battery capacity and its cycle life.
The cycle life denotes the number of cycles the battery can be charged and discharged before it is considered to be at the end of its life.
A battery’s life ends when the capacity drops below a certain level, usually 80% of its nominal capacity.
DC-DC Converter
The basic task of a DC/DC converter is to connect a battery to the various system parts when the battery voltage does not match the voltage needed.
The Load
The basic task of the load is to convert the electrical energy supplied by a battery into an energy form that will fulfil the load’s function.
The Communication Channel
The basic task of a communication channel in a BMS is to transfer monitor and control signals from one BMS part to another.

Specifications

BMS topology that will be used by the team is the distributed BMS. There will be one BMS board per three cells. The designed battery system structure can be seen below.

BMS Circuit Structure:

BMS Slave Block Diagram:

The module diagram of the BMS board that we aim to design can be seen below. BMS Module Diagram

As for cell balancing, we are aiming to use active cell balancing although active cell balancing is also more sophisticated than passive cell balancing. Why? In passive cell balancing, energy is drawn from the most charged cell and dissipated as heat, mostly through resistors. In active cell balancing, energy is drawn from the most charged cell and transferred to the least charged cells, generally through capacitor-based, inductor-based or DC-DC converters. In passive cell balancing, BMS discharges the cells which have a higher voltage than the lowest voltage cell. Passive cell balancing does not improve the system runtime, wastes the energy by dissipating the heat even though it provides a fairly low-cost cell balancing method.

Modeling and Simulation

For modelling and simulation, MATLAB will be used by implementing the models in Octave language.
Simply, models consist of equations.
Modelling and simulating the equivalent circuit models help to give a feeling for how cells respond to different usage scenarios and create the basis for the BMS algorithms.

How to Model OCV and SoC

Open-circuit voltage

"Ideal voltage source"
v(t) = Open circuit voltage. This means that the voltage is not a function of the current or past usage. Voltage is constant in this case.
Open circuit model is not very useful for BMS application but provides a good starting point.
Facts:
Batteries supply a voltage to a load
When the cell is unloaded and in complete equilibrium (i.e. open circuit), the voltage is mostly predictible.
An ideal voltage source will be a part of our equivalent circuit model.

OCV vs. SoC plotted below.
Since OCV is also a function of temperature, we can include it in the model as OCV(z(t), T(t)), where z(t) is SoC and T(t) is temperature functions.
Depth of discharge (DoD) is the inverse of SoC (i.e. DoD = 1 -SoC).

Modelling State of Charge (SoC) and Definition of Total Capacity Brief Intro:

When a cell is fully charged, its open-circuit voltage is higher than when it is discharged.
So, we can improve the model by including the dependence on the cell's charge status.
Let's take SoC as z(t) as mentioned previously:
- z = 100% when the cell is fully charged.
- z = 0% when the cell is fully discharged.
The total capacity, Q, is the total amount of charge removed when discharging from z = 100% to z = 0%. Q is usually in Ah or mAh.

Modelling SoC:

The circuit diagram for the modelling is shown below.
We can model SoC as following: where the polarity of i(t) is positive on discharge.
In discrete-time, if we assume that the current is constant over sampling interval dt:

Coulombic Efficiency

Cells are not perfectly efficient.
Coulombic efficiency η[k] <= 1 on charge since some charge is typically lost due to unwanted side reactions in the cell.
Usually, modelled as η[k] = 1 on discharge.

How to Model Voltage Polarization

Polarization implies to any movement of the cell's terminal voltage away from OCV. For instance, the cell's voltage drops when it is under load.
The polarization can be modelled as a resistance in series with the ideal voltage source, i.e. OCV source. This is called the Rint model.
The circuit diagram of the Rint model:
The Rint model can be formulated as shown below:
v(t) > OCV(z(t)) on charge and v(t) < OCV(z(t)) on discharge.
Power is dissipated by R0 as heat to the environment which means the energy efficiency imperfect.
Rint model is sufficient for simple electronics but not for advanced consumer electronics.
i(t) x R0 models the instantaneous response to a change in input current. Thevenin Model Cell Voltage
Cell voltage in the Thevenin model is:
Process to identify parameter values from test data if we write the voltage in terms of element currents as shown below:
To find the expression for the current through R1, it is clear that current through R1 plus current through C1 equals to i(t).
Then, since voltage across C1 equals voltage through R1 as they are connected in parallel:
All in all, if we re-arrange those formulae into standard ODE:

Tasks

[x] Decide which BMS topology will be used
[x] Decide which kind of cell balancing will be implemented
[x] Create a battery system diagram
[x] Create the BMS module diagram including each unit planned to be included in the BMS
[x] Create a block diagram for the BMS
[x] Create the wiring diagram between the BMS module and the peripheral elements in the pod
[ ] Simulate and model SoC and OCV
[ ] Design the circuit models for SoC
[ ] Design the circuit models for SoH
[ ] Design the circuit models for Limit Estimation
[ ] Design the circuit models for Thermal Management
[ ] Design the circuit models for Cell Balancing (Active Cell Balancing Model)
[ ] Design the circuit models for Discharger
[ ] Design the BMS PCB on Altium Designer
[ ] Order 22 of the designed BMS PCBs

HyperloopManchester / Hyper_Hardware