Battery Pack Corrective Actions

ventilation is included in the system, the ventilation interlock will allow any vehicle to charge. However, if ventilation is not included in the system, the mechanical ventilation interlock will allow vehicles equipped with nongassing batteries to charge, but not vehicles equipped with gassing batteries.

Ventilation

Part II, Uniform Building Code, of California Title 24 Code of Regulations addresses location and ventilation issues associated with EV charging. Specifically, these codes address where EV charging equipment can be installed. If a ventilated charging system is to be installed, the codes specify how much mechanical ventilation must be provided to ensure that any hydrogen off-gassed during charging is maintained at a safe level in the charging area.

The ventilation rates specified in the building codes are calculated to comply with the NFPA requirements published in Standard NFPA 69, Explosion Prevention Systems. This standard establishes requirements to ensure safety with flammable mixtures. Section 3–3, Design and Operating Requirements, requires that combustible gas concentrations be restricted to 25% of the Lower Flammability Limits. This design criterion provides a safety margin in atmospheres containing hydrogen. Hydrogen is combustible in air at levels as low as 4% by volume of air. Therefore, in order for the charging station to not be classified as hazardous, the hydrogen concentration must not exceed 10,000ppm, which equates to 1% hydrogen by volume of air.

BATTERY PACK CORRECTIVE ACTIONS Connection Resistance

The intercell and terminal connection resistance is set as a baseline during the installation of the traction battery pack. This recording process ensures that an increase in the resistance values can be detected at an early stage, especially those caused by loose connections and corrosion.

Normal installation resistance varies greatly as a function of the size of the installation. The variation of the installation resistance is typically from 10 to 100 mW. The installation resistance of a battery or series of batteries can be measured using an ohmmeter or a conductance meter, or a measurement of a voltage drop during capacity testing. It is

a good practice to refer to the manufacturer specified guidelines for expected values.

It is a common practice to use either a 20% change in the previously established baseline value or a value exceeding the manufacturer’s recommended limit. The corrective actions required are determined by the analyses of the effects of the increased resistance.

Thermal Runaway

When a traction battery is operating on float or overcharge in a fully recombinant mode, the net chemical reaction is minimum. Virtually all the energy (V ¥ I) results in heat generation. During the design of the system, the heat generated during the charge-discharge process should be dissipated without raising the battery temperature. In case the temperature of the battery pack rises during the charge-discharge process, more current will be required to maintain the float voltage. The additional current results in the production of excessive oxygen, which in turn generates more heat. The heat is produced as a result of the reconversion of water at the negative plate of the battery.

The net result is that the battery undergoes a meltdown due to thermal runaway. The possibility of a thermal runaway can be minimized by the use of battery pack ventilation using forced cooling. The ventilation maintains the battery temperature between and around cells. In addition, the charger output (voltage and current) is regulated by using a temperature compensated charge.

Cell/Unit Internal Impedance/Conductance

AC impedance and AC conductance tests are performed to determine battery pack internal impedance or internal conductance of the traction battery. The impedance and conductance tests provide baseline information from similar cells in the battery.

The internal cell impedance of the traction battery cell consists of the physical connection resistances exhibited by battery terminal to battery plate welds and similar plate-to-plate connections. The ionic conductivity of the electrolyte and the activity of the battery during the electrochemical processes occurring at the plate surfaces contribute to the total cell impedance. With the multicell units, there are additional contributions due to intercell connections. The resultant lumped impedance element can be qualified using either AC impedance or AC conductance test methods.

The AC impedance test is performed by passing an AC current of known frequency and amplitude through the battery pack under test. The test accurately measures the resultant AC voltage drop across each cell/unit. The AC voltage measurement is taken between the positive and negative terminals of the individual cells or the smallest group of cells possible. Compute the resultant AC impedance using the Ohm’s Law. The ambient temperature, cell and battery life, and discharge history are factors that impact the AC impedance.

The AC conductance test is performed by the user applying an AC voltage of known frequency and amplitude across the battery. In addition, observe the AC current flowing in response to the voltage applied. The AC conductance is the ratio of the AC producing it. Only the in-phase current component is considered for the measurement of AC conductance. The effects of spurious capacitance and inductance impact the out-of-phase component of the battery. The out-of-phase component is not considered for the AC conductance.

AC impedance and AC conductance are inversely related. Cell conductance is directly proportional to the active plate area and decreases as the active area is lost. The loss of active area results through processes of plate aging and deterioration. The decrease in the cell’s capacity with time, the cell’s AC impedance increases, and its conductance decreases.

When making field measurements of either AC impedance or AC conductance, the observed values should be compared with baseline values appropriate to the specific installation. If such values are unknown, comparisons can be made to values obtained by averaging measurements over the entire string of the battery pack. When the cell impedance increases by more than 30%, it is recommended that the battery manufacturer instructions should be followed. If the cell impedance increases by more than 50%, it is recommended that load analyses of the battery pack should be performed. If the cell impedance increases by more than 80% of its reference value, it is recommended that the battery manufacturer should be contacted for additional actions. Load testing of the battery pack is recommended as soon as the battery cell impedance increases by more than 70%.

Equalizing Charge

During periodic equalization charges, it is not necessary to correct cell/unit imbalances in the battery pack as long as the individual batteries themselves gain the necessary charge. Equalizing the charge should be performed within manufacturer recommended guidelines and

limits. This includes the duration of the equalization charge and the magnitude of the current applied during the equalization process.

Ripple Current

In order to limit the ripple current, a battery charger with low electrical noise levels must be used for charging the traction batteries. An acceptable charger is one that does not raise the average fully charged battery pack operating temperature measured at the negative terminal by more than 5°F above ambient temperature in free-standing condition.

Battery Charging Parameters

The charging efficiencies, hdaily and hoverall of the EV can be calculated as follows:

• Determine the miles traveled since the previous battery pack charge • Determine the kWhr consumed during the recent charge

The daily battery pack charge efficiency

hdaily = Miles Traveled since Last Charge/kWhr Consumed

The overall charging efficiency hoverall of the EV can be calculated by:

• Determining the miles traveled during the entire test program • Determining the kWhr consumed during the test program

hoverall = Miles Traveled during the entire Test Program/kWhr used