Battery Management Systems (BMS)

Ensures that the battery pack operates safely and reliably.
Prevents battery from operating outside of the safe operating area, calculate/reporting secondary data
Within the BMS, algorithms are run to generate accurate estimations of outputs, based on inputs
Redirects the recovered energy into battery pack
In EV’s, some noticeable amount of energy stored in the batteryis not shown at the dashboard, since it is reserved for hybrid operations
- Ex: for the Mitsubishi Outlander PHEV (all versions/years of production), 0% of the state of charge presented to the driver is a real 20-22% of charge level

BMS Thermal

Colling is either passive (relies only on convection current of the surrounding air) or active (using fans), and liquid, air, or through phase change
Adds to weight of BMS, and air cooling require large amounts of power than liquid cooling

BMS Inputs (High Level)

cell/pack voltages
temperature sensors for cells
- Get a sense of the temperature distribution of cells
current flowing into/out of battery pack)
- Determine if charging/discharging and at what magnitude

BMS Outputs (High Level)

State of Charge (SOC)
- Ex: battery percentage in phones/EVs
State of Health (SOH)
- Current battery capacity compared to the beginning of life
- Ex: SOH of a phone battery depletes over time
Safe Operating Envelope
- Indicates how much current can be charged/discharged at any given time
Faults and Status Signal
- May be required for other triggers
Maximum charge current limit (CCL)
Maximum discharge current limit (DCL)

Provides Protection Aganist:

Means Of Protection

State of Charge (SOC)

SOC - Capacity

SOC = (Capacity Remaining) ÷ (Total Capacity)

The voltage level of the pack will reach termination voltage as mAh is discharged (SOC also depletes)
Calculated using Coulomb Counting, where the charging/discharging current of the battery is measured and integrated over time to determine the SOC
In a Current vs. Time graph, the curve indicates the current output at a given time, and the area under the curve indicates the total current drawn (capacity removed). To calaculate this integral, it’s split into time intervals, then mulltipied by the current at the time, then summsed all together for an acurate estimation.

Inverse is Depth of Discharge (DOD)
- EX: 70% SOC = 30% DOD
Coulomb counting gives precise estimation of battery SoC, but they are protracted, costly, and interrupt main battery performance.

SOC - Energy

More accurate for fuel gauges in EV’s, as it indicates expected run time, usage, and distance remaining
In the graph, an SOC(E) of 50% is achieved when the area underneath the curves (amount of energy supplied) is equal on both sides, which occurs at around 42% SOC(C)

SOC in BMS

consists of measurement sensors, controllers, serial communication, and computation hardware with software algorithms such as Proportion Integral Derivative (PID) and adaptive control laws, Kalman filters estimator (RESEARCH ALL THIS MORE) to calculate SOC and SOH
- Estimation Techniques for State of Charge in Battery Management Systems on Board of Hybrid Electric Vehicles Implemented in a Real-Time MATLAB/SIMULINK Environment | IntechOpen
  - Great research on Li-ion BMS, read this later when you get more time

Other Ways to Calculate SOC

Measure the temperature of the electrolyte
- If the battery has not been charged/discharged within the last four hours, the ambient or surrounding air temperature can be used
Measure the open-circuit voltage (RESEARCH THIS MORE), then use a reference to determine SOC
- Car and Deep Cycle Battery Frequently Asked Questions (FAQ) Section 4 (jgdarden.com)
- Car and Deep Cycle Battery Frequently Asked Questions (FAQ) Section 4 (jgdarden.com)
  - Check this for distinction: Car and Deep Cycle Battery Frequently Asked Questions (FAQ) Section 4 (jgdarden.com)
- Interpolation may be required.

Cell Configurations to Chargers

Series

Cells are connected in series, two-wire connect the positive and negative terminals to the respective ends of the series.
- Example: Each cell in a 4s1p must be charged to 4.2 V (total 16.8 V is set for charger across the config). Ideally, each cell must be a charge to 4.2 V. However, it is possible that each cell is charged to different levels and at different rates (avoid this).

Consist of a balance connector that connects to each cell of the pack
Each cell is also connected to:
- Two transistors
- DW01A (Microsoft Word - DW01A-DS-10_EN.doc (escooter.org.ua)), which protects each cell from overcharge/discharge/current (below is a typical application circuit from DS)
- Note the parasitic diode (RESEARCH THIS MORE) at M1. Even when M1 is off, charging is still possible due to this, since current flows from BATT- to BATT+ while charging. Conversely, if M2 is off discharging is possible due to M2’s diode, since current flows from BATT+ to BATT-

Each cell is also connected to a BB3A (HY2213 Datasheet 範本 (hycontek.com)), for charge balance control (to ensure that each cell is charged to the same maximum voltage). Below is an example circuit of a cell (N-MOSFET)
When the battery voltage exceeds the overcharge detection voltage, the OUT pin outputs the logic HIGH to discharge the battery through the resistor. When the battery voltage dips below the overcharge release voltage (VCR), the OUT pin outputs logic LOW to turn off the N-MOSFET.
Near the battery pack are 6 Power N-MOSFET’s (N-channel 75V - 0.0095 - 80A - TO-220 - TO-220FP - D2PAK STripFET™ II Power MOSFET (digikey.com))
(Diagram at https://youtu.be/rT-1gvkFj60?t=313 )
- When input current exceeds BMS lmit (calculated using voltage and shunt resistor), the certain voltage level activates a passive component network (RESEARCH THIS MORE), and turn off the MOSFET

NOTE: Most BMS include these 3 components, although some may omit the balance charging.