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• Each VRLA quickly develops its own
  personality during its formation cycling to
  the extent that each battery behaves
  differently during recharge.
• Thus it is necessary to provide an
  additional charge during the recharging
  process of a series string of batteries.
• This charge is referred to as the equalizing
• The constant current-constant voltage (CI-
  CV) algorithm is the suggested recharge
  method for the 12 V VRLA battery.
• be applied to the battery once every 20 to
  30 battery charge-discharge cycles.
correct charging methods
• charge current (Ic) depends on
  - output power from the battery charger,
  - maximum and the minimum values of Ic
  related to the clamp voltage (Vc),
  - the time to recharge the battery from
  80% depth of discharge (DOD).
• Each VRLA battery must remain sealed for
• In order to achieve maximum battery cycle
  life it is very important to prevent
  excessive overcharge.
• Overcharge results is gas pressure build-
  up, which is vented as water vapor.
• The recharging of a VRLA battery using the CI-
  CV is completed when the current during the
  constant voltage phase has fallen to 0.80% of
  the battery's three-hour capacity (C/3).
• For example, a VRLA battery rated at 95Ahr
  (rated for a C/3 rate) shall have attained a full
  charge when the charge current has fallen to
  0.008 x 95 (approximately 1A).
  Temperature Compensation
• Charging a VRLA battery increases the battery
  temperature above ambient.
• The rise in temperature leads to gassing
• if the ambient temperature is above 77°F, Vc
  must be reduced by 0.01 V/°F above 77°F.
• If the ambient temperature is below 77°F, Vc
  must be increased by 0.01 V/°F below 77°F.
• The following example will illustrate the correct
  method of calculating Vc for a VRLA battery
  rated at 60Ahr with a charge current of 60 A and
  an ambient temperature of 60°F.
 Temperature Compensation
• The three-hour current delivering capacity (C/3)
  of a 60Ahr battery is 60A. Ic is 20A, thus the
  ratio of Ic/C/3 is 20/60 = 0.33.
• The corresponding Vc at 77°F for the ratio
  Ic/(C/3) = 0.33 is 14.53 V.
• temperature is 77°F, Vc must be increased by
  17 x 0.012V = .24V.
• The correct Vc for a 20A inrush current for a
  60Ahr battery at 60°F is then 14.53 + 0.24 =
  14.77 V.
     Overcharging of the VRLA
• A typical VRLA battery requires 3 to 5% of
  overcharge during the daily recharge using the
  CI-CV algorithm.
• If 50Ahr are taken out from the battery at a
  100% state of charge (SOC) condition, then
  approximately 50 x 1.03 = 51.5 Ahr must be
  returned to the battery during the next recharge
  to return the battery to a 100% SOC condition.
• near the end of the battery's life, the battery will
  require 7 to 8% overcharge.
• A good battery condition check is that the
  charge current at the point at which the
  battery has received 3% overcharge
  should be approxi-mately 0.8% of the
  battery's three-hour capacity.
   Equalization Charging of a
     Single VRLA Battery
• As the VRLA batteries undergo cycling, their
  individual cells tend to fall out of step with
  respect to one another.
• an extended constant current charge is required
  to balance the cells in the battery.
• It is recommended that an equalizing charge be
  applied once every 20 cycles.
• The values of Ic and Vc will be same during the
  equalization charge as the daily charge.
  Equalization Charging of a
    Single VRLA Battery
• A battery reaches its 100% SOC when the
  voltage during the final Ieq phase does not
  rise more than 0.01 V during a 15-minute
• The duration of this equalization phase
  should not exceed six hours.
    Recharging a Series String of VRLA
• individual cells occasionally tend to fall out of step.
• results in an unbalanced pack of batteries when the
  batteries are connected in series.
• It is important to monitor the voltage and temperature of
  individual batteries in the series string.
• charge management system should change the charge
  current or modify the applied voltage based on the
  preset charging algorithm.
• the charge current or clamp voltage should be regulated
  with any change of battery temperature.
Multistep Algorithm for Charging
a Series String of VRLA Batteries
• Step 1 A constant current is applied to the
  series string of VRLA batteries.
• until the first battery (nominally rated for 12V) in
  the string reaches a voltage of 15.5 V or until the
  last battery in the series string reaches a voltage
  of 14.5 V.
• 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.
Multistep Algorithm for Charging
a Series String of VRLA Batteries
• Step 2 The reduced constant current is
  applied until the first battery again reaches
  a voltage of 15.5 V or until the last battery
  in the series string reaches a voltage of
  14.5 V.
• At this point, the current should be
  reduced to approximately 50% of the
  current applied to start Step 2.
Multistep Algorithm for Charging
a Series String of VRLA Batteries
• 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 0.90 A (approximately 1 A).
Multistep Algorithm for Charging
a Series String of VRLA Batteries
• Final Step The constant current is applied until
  all battery voltages have risen less than 0.01 V
  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.
• this step must not exceed six hours to prevent
  gassing of the batteries, resulting in the loss of
  water vapor.
• 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 voltage limits with
  respect to the ambient temperature.
• Charging under ambient temperature over
  120°F is not recommended.
           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
        4.4 CHARGING NIMH
• At the beginning of the charging process,
  the NiMH cell is at room tem-perature
• In charging progresses, the internal
  temperature rises up very rapidly
Temperature Sensing of Traction
        Battery Packs
• Temperature of a traction battery can
  be measured using an NTC thermistor
• this mea-surement is often skewed by loss
  of heat from the cell surface due to
  convection and radiation
• When measured at the same charge rate, large
  size cells generate more heat than smaller size
• This is because heat capacity of a smaller cell is
  higher than that of the larger cell.
• a large 3,000mAh NiMH cell with a volume of
  22,500mm3 has a capacity of 7.5mm3/mAh
• a 350mAh NiMH cell with a volume of 3,150mm3
  has a capacity of 9.0mm3/mAh.
•    The thermistor
-    resistor divider circuit, low current flowing,
-    thermistor self-heating, current drain
•    power supply
    - Stable: ambient temperature range
    - avoid voltage change across thermistor
• Thermistors: inexpensive, rugged, sensitive to
• negative temperature coefficient (NTC)
  thermistors are preferred, resistance decreases
  as temperature increases.
• when thermistor open - high resistance - a large
  temperature change.
• battery sensor design: thermistor resistance,
  temperature coefficient, time constant
• Temperature ranges of thermistors for VRLA
  and NiMH batteries should run between -10°C to
• Using a 5V power supply, a 10kΩ thermistor at
  25°C is chosen.
• linear temperature range: must be compensated
  to operate in the relatively linear portion
• battery heating and cooling mechanisms can
  affect the temperature sensor readings and
  thereby lead to over- or undercharging the
  battery. .
Temperature-Based Termination
• During battery charging process, monitor
  temperature to prevent cell failure.
• Some batteries during the process of
  charging may lack the drop in voltage —
  used to detect full charge (F4-6).
• Three methods of charge termination are
  commonly used: maximum temperature
  cut-off, temperature change, and
  temperature slope (dT/dt).
• The maximum temperature cut-off method is the
  simplest method of charge termination.
• cheap, unreliable.
• terminate the charge current at the
  predetermined cut-off temperature.
• resulting in overcharge or undercharge of the
• For NiMH batteries, the recommended maximum
  charge temperature is 50°C
• ambient affect batterie temperature under their
  maximum temperature
• The second, temperature change method
  compares the difference between the
  ambient and the battery temperature.
• This method may prove unreliable in case
  the ambient temperature of the battery
• this method does provide an excellent
  back-up charge termination method.
• The third charge switching method is based on
  the change of slope of the battery temperature
• a very effective technique for the early detection
  of overcharge.
• The cell temperature rises rapidly, indicating
  overcharge is occurring.
• An assumption made with the dT/dt method is
  that changes in the ambient will have a limited
  effect on the sensor relative to the heating of the
  cell due to overcharge.
• In the high temperature range between 40°C and
  55°C, charging NiMH batteries require a careful
  selection of set points, for both temperature-
  based and voltage-sensing charging systems.
• The charge time increases at lower
  temperatures so charge durations must be
  considered to provide adequate low-temperature
  charging, while avoiding excessive charge at
  normal temperatures.
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