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Battery Capacity Tests
BATTERY CAPACITY TESTS
The capacity tests specified in the ANSI IEEE 450 standard are categorized into four tests,
• Battery pack acceptance test • Battery pack performance test • Battery pack service test • Battery variable power test
The battery acceptance test is used to determine if the battery bank meets its purchase specification or the manufacturer’s specification. This test is performed at the manufacturing facility or upon installation of the battery.
The battery performance test is performed periodically to measure the battery pack capability, including operation, age, deterioration, and environment. This test can be performed at any time during the entire life of the battery.
The battery service capacity test determines whether or not the battery system, as per manufacturer specifications, will meet the battery pack requirements during load and duty cycling. This test is not designed to effectively predict battery life or replacement, rather it is performed as part of the preoperational or periodic DC system check.
Before a battery pack undergoes an acceptance or performance test, a complete preventative maintenance inspection should be scheduled. The inspection of the battery pack should include measuring and recording the battery terminal resistance of all the connections in the pack. Resistance tests should be performed on the battery pack. The battery pack should be placed on a float charge for three days and not more than seven days prior to performing a capacity test to ensure that the battery pack is at a 100% SOC.
On the contrary, service tests are performed on the battery in an “as is” condition and do not require pretest conditions. For service capacity tests, the rated discharge current and the testing period should ideally match the duty cycle of the system. This means that the user should be prepared to manually or preprogram the test equipment to change the current discharge levels at specified time periods as required by the duty cycle.
For the acceptance and the performance capacity tests, a rated discharge current and the testing period are selected from the battery manufacturer’s cell performance data sheet based on the battery model type, amp-hour rating, the load unit’s current rating, and the final end voltage per cell. Typically, the voltage for a VRLA cell as selected to be 1.75V
per cell. This voltage is consistent with good engineering practice to take in to account DC device and inverter systems operating ranges. The ranges allow for a 10% variation of operating voltage levels.
The battery rated discharge current selected is then corrected to a test discharge current based on the average of the temperature readings that are recorded previously. The test discharge current is equal to the rated discharge current divided by a discharge correction factor.
The resistive load unit and battery monitoring equipment is set up and connected to the battery pack. For battery acceptance and performance capacity tests, a constant discharge current is maintained either automatically or manually when the load is switched on. The current drops to only 10 to 15% over the entire test period. The individual cell voltages and the battery pack’s overall voltage are read and recorded at selected intervals during the test using data monitoring equipment. Time is kept, and the test is concluded when the terminal voltage decreases to a value equal to the final end voltage (typically 1.75V) times the number of cells in the battery pack.
In case an individual cell approaches reversal of its polarity (defined by ANSI/IEEE 450 plus 1V or less), and the final terminal voltage has not yet been reached, the test should be stopped long enough to remove the weak or reversed cell(s) from the string. When the weak or reversed cell(s) in the string are replaced, the discharge timing and the discharge test is continued. However, removal of the weak cell should be done as quickly as possible, within six minutes or 10% of the test time, whichever is shorter. This prevents the test results from being skewed by the weak or discharged cell. If the weak battery cannot be removed from the battery pack in time, the test must be aborted and rescheduled to allow the battery pack to be recharged and equalized. The possibility of the weak cells should be anticipated, and connectors with premade links should be made prior to when the discharge test is conducted. This eliminates delays that may result due to removal, replacement, and reconnection of the batteries in the pack. A new final terminal voltage should also be determined when continuing the test, based on the remaining number of batteries in the pack.
If a battery bank does not meet the requirements of the duty cycle, a complete maintenance inspection should be scheduled, and any necessary corrective actions should be taken. The battery pack’s rating should also be reviewed based on the duty cycle of the load it is being applied to. Since a service test is performed on the battery in the “as found” condition to reflect the quality of the maintenance and operating practices, it cannot effectively predict battery life or battery replacement time.
The battery acceptance or performance capacity is determined by the equation
% Capacity at 25°C (77°F) = Ta/Ts ¥ 100
where Ta is the actual time to the final end voltage and Ts is the rated time to the final end voltage.
As suggested earlier a battery pack that demonstrates a pack capacity of less than 80%, should be replaced. As a normal design practice a VRLA battery is sized to include a 10 to 15% growth factor and a 25% aging factor. Oversizing a VRLA battery by more than an aggregate growth factor of 40% is not possible. This is due to the rapid deterioration of the battery end-of-life deterioration. In addition, the new battery pack delivery pack ranges between 6 to 14 weeks, depending on the battery size. Spare batteries to populate an entire replacement pack are generally not stored. This is owing to the self-discharge of the battery pack that occurs during long duration of storage.
The variable power discharge is a simplified version of the urban driving time power test. This power discharge effectively simulates the dynamic discharging and can be implemented in the laboratory as the simplified urban driving test. The 100% power discharge rating on the graph is intended to be 80% of the battery consortium’s power goal.
The test may be performed based on the manufacturer’s battery ratings. As an example, if this profile is scaled to 80% of the battery consortium goal of 150W/kg the battery discharge will be at a peak power of 120W/kg and an average power of 15W/kg. A lower power version of this profile may be used for testing batteries that cannot be operated at the nominal peak power requirement.
The battery pack will be charged and the pack temperature is stabilized in accordance with the manufacturer’s recommended procedures or per the battery test plan. The 360-second discharge test profiles are repeated end-to-end with no time delay (rest period) between them. The maximum permissible transition time between power steps is one second, and these transition times are included in the overall length of the discharge profile (i.e., a discharge test is always 360 seconds long). This discharge regime is continued until the end-of-discharge point specified in the test plan (normally the rated capacity in Ahr) has been reached. Alternately, up to the battery voltage limit, whichever occurs first, has been reached. If the maximum power step cannot be performed within the voltage limit and the specified end-of-discharge has not been reached, power for this step is limited to the weakest battery in the battery pack. This may be because the battery cannot sustain 5/8 of this
power within the voltage limit (or the specified discharge is reached), at which point the discharge test will end. The end-of-discharge point is based on the net capacity removed (total Ahr–regeneration Ahr) from the battery pack. In this case, the test is terminated at the point that the first battery reaches the end-of-discharge point. The battery is fully recharged as soon as practical after the discharge. If the battery can be damaged as a result of regeneration, the power, current or voltage may be regulated during the discharge test. Table 3–3 shows a 20-step test profile, also known as the 360-second frame that simulates the driving conditions and is repeated until the weakest battery in the pack reaches 100% DOD.
Figure 3–11 illustrates the change in the resistance of an 85Ahr VRLA battery with respect to the %DOD for a discharge test. The variation is the worst-case source resistance calculated for the performance of the battery pack when the EV goes from a cruising speed of approximately 30mph (20A) to a hard acceleration 60mph (170A).
Step Number Duration (secs) Discharge Power (%) Description (W/kg) 1 16 0 Rest 2 28 -12.5 -15 3 12 -25 -30 4 8 12.5 15 5 16 0 Rest 6 24 -12.5 -15 7 12 -25 -30 8 8 12.5 15 9 16 0 Rest 10 24 -12.5 -15 11 12 -25 -30 12 8 12.5 15 13 16 0 Rest 14 36 -12.5 -15 15 8 -100 -120 16 24 -62.5 -75 17 8 25 30 18 32 -25 -30 19 8 50 60 20 44 0 Rest
