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Temperature Compensation During Battery Charging
coefficient built into the charge management system should allow the charge current or clamp voltage to be regulated with any change of battery temperature.
VRLA batteries, nominally rated at 12.0V are best charged using a multistep charging algorithm that follows these steps:
• Step 1 A constant current is applied to the series string of VRLA batteries. The charging current is applied until the first battery (nominally rated for 12V) in the string reaches a voltage of 15.5V or until the last battery in the series string reaches a voltage of 14.5V. At this point, the current being applied to the series string is reduced to approximately 50% of the initial start value to prevent loss of water due to gassing. • Step 2 The reduced constant current is applied until the first battery again reaches a voltage of 15.5V or until the last battery in the series string reaches a voltage of 14.5V. At this point, the current being applied to the series string should be reduced to approximately 50% of the current applied to start Step 2. • Step 3 The current is again reduced by half as in Steps 1 and 2 until the current being applied to the first battery in the battery pack is at 1% of the battery’s three-hour rated capacity. For example, a 1% current of 90Ahr battery is 0.90A (approximately 1A). • Final Step The constant current is applied until all battery voltages have risen less than 0.01V in a 15-minute time period. This equalization time period is important as it brings all the batteries in the pack within a 5 to 10% range of the first battery achieving the charge in Step 3. Furthermore, it is important that this step must not exceed six hours to prevent gassing of the batteries, resulting in the loss of water vapor.
TEMPERATURE COMPENSATION DURING BATTERY CHARGING
The on-charge voltage limits must be compensated for temperature to account for the variation of the useful battery capacity with temperature. Table 4–1 summarizes the compensated lower and the upper
Table 4–1 Temperature compensated voltage limits.
Ambient Lower Voltage Upper Voltage Temperature (°F) Limit (V) Limit (V) 40 14.73 15.98 50 14.61 15.86 60 14.49 15.74 70 14.37 15.62 80 14.25 15.50 90 14.13 15.38 100 14.01 15.26 110 13.89 15.14 120 13.77 15.02
voltage limits with respect to the ambient temperature. Charging under ambient temperature over 120°F is not recommended.
Observations of the multistep algorithm for charging a series string of VRLA batteries:
Observation 1 Some of the batteries in the pack remain undercharged if the charging current is reduced when the first battery in the series string achieves a temperature compensated clamp voltage of 14.5V. Observation 2 Batteries equalize to within one cycle even with an unbalanced pack with improved battery capacity when an equalizing charge is applied to finish the charging for each individual battery. The charge is terminated when the first battery in the series string achieves a temperature compensated clamp voltage of 14.5V. Observation 3 Over gassing of the highest state of charge battery occurs when the battery pack is charged such that the last battery reaches a temperature compensated clamp voltage of 14.5V but without an upper clamp of 15.5V. Observation 4 Battery equalization occurs before the equalization of the battery pack within three to four charge cycles with moderate gassing on some batteries. The battery pack is being charged using the last temperature compensated battery clamp voltage of 14.5V with an upper clamp of 15.5V. Higher gassing of high SOC batteries is avoided when lower capacity batteries are part of the series string. The 14.5V voltage clamp results in less gassing than 15.5V while maintaining good equalization of the battery pack.
