The Battery Performance Management System

7 ELECTRIC VEHICLE BATTERY

PERFORMANCE

THE BATTERY PERFORMANCE MANAGEMENT SYSTEM

A typical electric vehicle (EV) traction battery system consists of a chain of batteries connected in a series, forming a battery pack with nominal voltages ranging from 72 to 324V and capable of discharge/charge rates of several hundred amperes.

Owing to the fact that no two batteries in a pack are alike, or even that no two cells in a battery are identical or manufactured exactly the same, their parameters—such as capacity—may vary by a few percent. In the case of a new battery, these factors may not be very noticeable, but as the battery undergoes charge-discharge cycles, later on in the battery life these factors determine the performance of the battery pack. In addition, some cells in the battery undergo a change in their parameters such as open-circuit voltage and internal resistance rather abruptly, due to internal dendritic shorts, corrosion, excessive thermal gradients, or loss of electrolyte due to gassing as in VRLA batteries. Such phenomena can lead to hydrogen gas build-ups and may pose a fire or explosion hazard if not detected and acted upon early. This problem may be easily detected in a battery of up to 6 to 12 cells. A faulty cell can be easily disguised in a large battery pack consisting of tens or hundreds of series-connected cells. A similar problem exists for an excessively overdischarged (reversed) cell. Thus for the safety of the EV, it is essential to monitor the batteries individually and detect faults early.

In an EV, the battery of marginally lower capacity than the rest of the pack is the first battery to acquire and indicate a fully charged status. On the discharge side of the cycle, this battery leads the pack and is the first to experience full discharge and reversal of plates. While this battery may not be weaker in any other sense than that it has a relatively lower capacity, it is now the weak link in the chain. This battery will be the first to undergo repeated overcharge and overdischarge, eventually resulting in the failure of the battery.

For a smart monitoring system capable of managing the batteries individually, detecting and isolating a weaker battery is recommended. The Battery Performance Management System (BPMS) quantifies the potential problems associated with an electric vehicle battery pack. BPMS may point to a simple action such as equalization of the charge for either the NiMH or VRLA battery, or suggest replacing a faulty battery to restore the battery pack’s full capacity.

The main components of a BPMS include:

• Precision fuel gauge • Battery charge balance or battery capacity balance for out-of-step batteries and if possible individual cells • A reference to a standardized data set as the voltage and temperature cut-off control parameter (particularly for a rapid battery charger) • A data logger for evaluation and processing of battery performance data • A supervisory data acquisition and control system for battery pack thermal management

In addition, BPMS also has the capability to govern the charge cycle to suit a weak battery. This results in lower utilization of the full battery system capacity, but extends the life of the weaker battery (batteries) and hence improves the life of the entire battery pack. It will also reduce the risk of sudden mode failure. Furthermore, the decline of the weak battery’s capacity may be measured and quantified—and when a certain predetermined point is reached, the deteriorating batteries may be replaced, and the battery pack will be returned to its full-rated capacity. This method of smart charging eliminates the possibility of damage of the batteries due to excessive overcharging during the normal battery recharge cycle and results in a very long cycle life.

A battery in a battery pack can be reduced to a weak state by excessive discharge rates. These conditions of abuse are characterized by short powerful bursts of charging current at excessive voltages during regenerative (regen) braking. Regen can exceed the absolute maximum charge acceptance ability of the battery if it is not properly managed. This condition exceeds the charge acceptance ability of the battery in the range of 80 to 100% SOC (the charge acceptance ability of the battery in 100% SOC is zero). Under these conditions, the battery becomes a large heat sink.

Thus another function of the BPMS is to monitor the discharge or utilization side of the battery to determine the safe operation of the

battery—preventing excessive overcharging and overdischarging to allow a more accurate SOC determination as a fuel gauge. Monitoring of the individual battery allows early diagnostics and the detection of a weak or deteriorating battery before its failure. BPMS allows for charge/discharge control matching to the weak battery, preventing its abuse and extending the life of the entire battery. Other characteristics, such as internal resistance readings and their trends, point to a deteriorating battery or even a problem such as poor or corroded contacts (battery interconnects). When predetermined limits are exceeded, warnings can be presented to the driver.

BPMS also extends the concept of a truly smart charging system by placing total control of the battery system on board the EV. BPMS has the capability to both manage the energy flow throughout the operation of the EV, including thermal management of the battery pack, and provide a real-time interface to the power utility infrastructure.

A Model of the BPMS

BPMS, including the defined components, requires a self-adjusting battery model. The algorithm for determining the residual useful battery capacity and the “miles-to-go” includes calculation of the actual battery pack capacity with respect to the nominal battery pack capacity. The Peukert constant is replaced by 10 representative points. Based on the average load current iav, a linear interpolation is applied from the beginning of the battery pack discharge current to the most recent discharge current. In addition, the nominal battery pack current inom is also monitored and noted.

A correction factor for the capacity at the actual battery temperature C(T) with respect to the capacity of the battery at the reference battery temperature C(T0) is based on the following equation

C(T) = C(T0) ¥ (1 + a + b ¥ ln(iav/inom))

Calculation of the SOC takes into account losses resulting from the inner resistance Rint from the battery voltage Vbatt under discharge. The no-load battery voltage Vno-load may be expressed as

Vno-load = Vbatt + I ¥ Rint

• A second comparison of Vno-load with SOC using a graphical calculation with at least 10 support points for interpolation

• Correction of the actual battery pack capacity as a percentage of the nominal battery pack voltage

The actual battery pack capacity is determined by comparison of the discharged Ahr and the present battery pack capacity with respect to the SOC whenever:

• The battery pack has been fully charged as per the manufacturer’s specifications. • The battery pack discharge capacity exceeds a specified value.

Typically 70% of the current rated battery pack capacity.

The Typical BPMS Configuration

From the energy management interface standpoint, the complete system may consist of a pair of main controllers (MC), one serving in active mode while the other is serving as a hot standby. In addition, up to 30 battery monitors (BMONs) interface to the individual batteries, depending upon the battery pack configuration. Each BMON can monitor up to five 12V batteries in the battery pack and up to four auxiliary temperature and vent pressure sensors. The BMONs must be galvanically isolated from the traction battery pack. The BMONs are connected in a daisy chain, allowing communication via a high-speed data bus (HSDB). A typical system consists of 2MCs, 7 to 8 BMONs that monitor a battery pack in 12V increments.

The MC incorporates dual-processor architecture to perform all its functions: communications, processing and control, data storage, and data retrieval. Historical data of the battery pack is stored throughout the life of the battery system, in a nonvolatile RAM. Recent data for the last 30 charge and discharge cycles in full detail forms the first data tier, and older data that is averaged forms the second data tier and is compressed on a biweekly basis or depending upon the usage of the EV. The MC is equipped with data communication interfaces for the charging station and the onboard charger. This allows BPMS to control the fast-mode DC charging, and low rate/overnight charging from a high-voltage AC power grid. In addition to the interfaces, an additional data port provides for system maintenance and remote battery diagnostics.

The individual battery currents are sensed using high resolution Hall effect sensors. These sensors provide for precise summation over time both for the charge and discharge capacity of the battery pack. The MC