# ELECTRIC VEHICLE BATTERY CHARGING

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```					4 ELECTRIC VEHICLE
BATTERY CHARGING
• 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
charge.
4.1 CHARGING A SINGLE VRLA
BATTERY
• 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
life.
• 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.
4.2 CHARGE COMPLETION OF A
SINGLE VRLA BATTERY
• 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
Battery
• 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
period.
• The duration of this equalization phase
should not exceed six hours.
Recharging a Series String of VRLA
Batteries
• 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.
4.3 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 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
BATTERIES
• 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
cells.
• 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
temperature.
• 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
25°C.
• 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
Methods
• 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
batteries.
• 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
fluctuates.
• 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
profile.
• 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.
4.6 ENVIRONMENTAL
INFLUENCES ON CHARGING
• 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.
~ END ~

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 views: 14 posted: 11/13/2011 language: English pages: 40