The BMS

Teknomadix BMS has higher functions such as monitoring battery internal and ambient temperature, controlling the charge rate of a connected battery charger, monitoring and recording usage data, and remote diagnostics. The BMS works as a guard, in a cooperation with other in vehicle systems that provide battery protection, generate status signals, warnings, or disconnect the battery from the load or charge source should any of the parameters the BMS is monitoring become out of the set bounds.

Battery Management Systems (BMS)

Teknomadix Micro BMS Master Board V1.0

 

Designing a BMS

It is necessary to understand what needs to be controlled by the BMS and why it needs controlling. This requires a deeper understanding of the fundamental cell chemistries, performance characteristics and battery failure modes particularly Lithium battery failures. The battery can not simply be treated as a black box.

 

Automotive BMS

Automotive batteries require the most demanding management from a BMS. It has to interface with a number of other on board systems, and it has to work in real time in rapidly changing charging and discharging conditions.

As a vehicle accelerates and brakes frequently, it operates through harsh and uncontrolled environments. This is in stark contrast to a home solar battery bank or an emergency backup battery system such as in a hospital or computer data center.

 

A proper BMS in a battery powered electric vehicle has a lot of responsibility:

  • Monitoring the conditions of individual cells which make up the traction battery
  • Maintaining all te cells within their operating limits (Voltage and Amperage)
  • Protecting the cells from out of tolerance conditions
  • Providing a "Fail Safe" mechanism in case of uncontrolled conditions, loss of communications or abuse
  • Isolating the battery in cases of emergency
  • Compensating for any imbalances in cell parameters within the battery chain (Active Balancing)
  • Setting the battery operating point to allow regenerative braking charges to be absorbed without overcharging the battery.
  • Providing information on the State of Charge (SOC) of the battery. This reports to the vehicle "Fuel Gauge" over the vehicle network (CANBUS)
  • Providing information on the State of Health (SOH) of the battery, or Cycle count.
  • Interfacing with control instructions from related vehicle systems, such as the onboard charger, and providing the optimum charging algorithm for charging the cells
  • Telemetry & Logging to enable recording of battery usage and abuses
  • Emergency "Shut Down" in case of cell failures

Teknomadix BMS design incorporates a lot more functions than simply managing the battery. We can determine the vehicle's location, desired operating mode, whether it is accelerating, braking, idling or stopped, and implement remote control over the power management system.

Automated Cell Protection

The prime function of our BMS is to provide the necessary monitoring and controls to protect the battery cells from out of tolerance ambient or operating conditions. This is of particular importance in automotive applications because of the harsh working environment. As well as individual cell protection the automotive system must be designed to respond to external fault conditions by isolating the battery as well as addressing the cause of the fault. For example cooling systems can be turned on if the battery approaches an overheat state. If the overheating becomes excessive then the battery can be remotely disabled.

Multi Level System

The BMS is part of a multi level system with the following safety objectives and safeguards

BMS Software

  • Monitoring of all key indicators coupled to control actions. (Cooling, Power disconnect, Load management)
  • Monitoring of all vehicle operational actions. (Acceleration, Stop, Idle, Charge, Driving)
  • Remotely & Autonomously controlled safeguard actions in case of out of limits conditions

BMS Hardware - Fail safe back-up

  • Hardware disconnect in case of software failure.
  • Battery disconnect in case low voltage BMS power supply fails

Battery Containment

  • Battery traction packs are designed in a robust outer container, engineered to IEC standard IP-68 to withstand harsh dirty and moist environments.

BMS Implementation

Battery Monitoring Unit (BMU), the Battery Control Unit (BCU) and the CAN bus vehicle communications network interface with the rest of the vehicle energy management systems.

The BMS is also coupled to other vehicle systems which communicate with the BMS via the CAN bus vehicle network such as the Thermal Management System and anti theft devices which disable the battery.

Battery Monitoring Unit

The Battery Monitoring Unit is a microprocessor based unit incorporating three functions or sub-modules. These sub-modules are not necessarily separate physical units but are described separately for clarity.

Battery Model

The Battery Model characterizes the behavior of the battery in response to various external and internal conditions in a software algorithm. The model can utilize these inputs to estimate with a high accuracy level (98%) the status of the battery at any instant in time.

The SOC is determined by integrating the current flow over time, modified to take account of the many factors stored in the battery log which affect the performance of the cells, then subtracting the result from the known capacity of the fully charged battery. This is described in detail here SOC.

The battery model uses a log of past history for maintenance purposes or to predict how many miles the vehicle may run before the battery needs recharging. The remaining range, based on recent driving or usage patterns, is calculated from the current SOC and the energy consumed and the distance covered since the previous charge (or alternatively from a previous long term average). The distance travelled is derived from data provided by other sensors on the CAN bus

The accuracy of the range calculation is more important for EVs whose only source of power a single daily charge.

Multiplexing

To reduce costs, instead of monitoring each cell in parallel, the BMS incorporates a multiplexing architecture which switches the voltage from each cell (input pairs) in turn to a single digital output line for sampling and comparison. Major cost savings are realized by reducing the number of control and sampling circuits keeping the overall system component count to a minimum.

The BMU also provides the inputs for estimating the SOH (Cycle Count) of the battery.

Performance Module

The Performance Module, like the Battery Model contains a reference data set with all the tolerances and limits relevant to the various parameters monitored by the Battery Model. The Performance Module also takes instructions from the CAN bus such as commands from the Inverter Controller via the BMS to accept a regenerative braking charge or from other autonomous vehicle systems or directly from the vehicle operator. This unit is also used to set and to monitor the vehicle operating mode parameters.

The Performance Module allows the operator to program the system, with custom settings specific to the customer's application, and preferred driving style. The user may configure lower limits, to conserve energy and extend range. Raise limits to increase stopping torque from regenerative braking, or to increase acceleration and top speed.

The Performance module compares the status of the measured battery parameters from the Battery Model with the customized settings from the Performance Module. Logic circuits provide control to initiate cell protection actions or to be used in the various BMS feedback loops which drive the system to its desired operating point or isolate the battery in the case of unsafe conditions. These error messages provide the input signals for the Battery Control Unit.

System Communications

Teknomadix BMS uses the CAN bus which was designed for communications between various internal functional systems in a vehicle. It also interfaces with external IOT systems for monitoring and control functions.

The system also includes standard automotive On Board Diagnostics (OBD) with Diagnostic Trouble Codes (DTC) made available to the service engineer. This connection allows technicians to identify any external causes of battery failure.

Battery Control Unit

The Battery Control Unit contains all the BMS power electronics circuitry. It takes control signals from the Battery Monitoring Unit to control the battery charging process and to switch the power connections to individual cells.

  • Controlling the voltage and current profile of the on-board charger output during external charge events.
  • Providing top up charge to individual cells for equalization of charge on all cells in the battery array.
  • Isolating the battery during fault or alarm conditions
  • Switching the regenerative braking charge into the battery as required
  • Routing excessive regenerative braking charge to the override shunt when the battery is fully charged
  • Responding to changes in the vehicle Performance modes
  • Binary Control VS Progressive Control
  • In its simplest form, the BMS provides a "binary" ON/OFF response to a fault or an out of tolerance condition such as an overload, merely isolating the battery completely by opening the main contactor switch. Progressive control provides more finite systems controls in the case of an out of bounds condition by utilizing the communications bus to modulate the demand on the battery back to within acceptable tolerances.

Cell Balancing

Is an essential function of the Teknomadix BMS. As noted above it is required to compensate for weaknesses in individual cells which could eventually cause the failure of the complete battery.

Master and Slaves

The master and slaves, organizes the cells into series parallel arrays with one slave managing each module.

  • The Slaves - Each cell has a temperature sensor as well as connections to measure the voltage, all of which are connected to the slave which monitors the condition of the cell and implements the cell balancing.
  • The Master - Multiple slaves can be connected to the master which monitors the current and integrates it over time to calculate the net Coulomb flow and this is modified using voltage and temperature data from the slaves to calculate the battery SOC. The master controls the main battery isolation contactor switch(s) initiating battery protection in response to data from the main current sensor or voltage and temperature data from the slaves. The master also provides the system communications.

screen shot 2019 07 25 at 5 04 38 pm

ohl

©2019 Teknomadix
CERN OHL V 1.2