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Texas Instruments Charging NiMH Battery using bq24105-Q1 In Automotive Applications Application notes
Application Report
SLUA843 – November 2017
Charging NiMH Battery Using bq24105-Q1 in Automotive
Applications
Jing Zou
ABSTRACT
NiMH batteries are often used in automotive applications due to its stable chemical characteristics. This
application report is mainly focused on charging topology for NiMH batteries in eCall application and using
bq24105-Q1 to charge the battery following the charge profile.
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2
3
4
Contents
Introduction ...................................................................................................................
Charging Procedure for in Vehicle Application...........................................................................
Application Example Using bq24105-Q1 .................................................................................
References ...................................................................................................................
2
2
3
6
List of Figures
1
Charge Temperature Characteristics of HR-AAULT (1000 mAh) (Provided by FDK) .............................. 2
2
Recommended Charging Procedure for backup batteries for In-vehicle Applications (Provided by FDK)
3
3
Example Block Diagram Using bq24105-Q1 with MCU forNiMH Battery Charge
4
4
5
......
..................................
bq24105 Charge Cycle with FDK HR-AAULT at Room Temperature ................................................
bq24105 Charge Cycle with FDK HR-AAULT VBAT at Different Temperatures ....................................
6
6
List of Tables
1
STAT1 and STAT2 Status Indication...................................................................................... 6
Trademarks
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1
Introduction
1
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Introduction
NiMH battery is often used in automotive application such as eCall/telematics system as backup battery
due to its stable chemistry characteristics. However, NiMH battery voltage changes with battery cell
temperature. With same amount of capacity, the NiMH battery cell voltage can be much higher at 0°C
comparing to at 25°C illustrated in Figure 1.
Figure 1. Charge Temperature Characteristics of HR-AAULT (1000 mAh) (Provided by FDK)
Thus environmental effect such as temperature plays an important role in automotive application. Thus
environmental effect and the characteristics of NiMH battery should be take into consideration when
designing eCall system. Using the termination method such as negative delta V, dv/dt, or dT/dt, has
potential risk. For example, when the environment temperature is cold, heat generation from the engine
can cause the temperature surround the battery raises fast and cell voltage would drop respectively. The
rising of the environment temperature can potentially cause false termination by dT/dt or –ΔV and lead to
under charge the battery. Therefore, it is recommended to use timer control for charging termination for invehicle application.
2
Charging Procedure for in Vehicle Application
Figure 2 is a charging procedure recommended by FDK for in-vehicle application. Basically, this charging
method is intended to maintain the back-up battery fully charged. When the back-up battery is not fully
charged, timer charge is performed; and when there is enough charge in the battery, maintenance charge
is performed. The charge current is recommended to set below 0.1C of the battery. The timer is set based
on 80% of the battery capacity. For 0.1C charge current, the safety timer can be set to 8 hours for
charging termination; while for 0.05C rate, the safety timer can be set to 16 hours for charging termination.
There are three different charging procedures based on cell voltage. The first case is when the battery
open circuit voltage is below 1.24 V/cell, charge is enabled along with safety timer. Once safety timer is
expired, the charging is terminated. Then battery open circuit voltage is checked again, if at this time the
voltage is below 1.34 V/cell, then the charge is re-enabled and timer reset. If the battery voltage rise
above 1.34 V/cell, charge current should be pulsed at rate of 1.2s off and 58.8s on. The second case is
when battery open circuit voltage is between 1.24 V/cell and 1.34 V/cell. Charge is enabled along with
safety timer. The only difference from the first case is that after timer expires, the pulse charging starts
right away. The third case is when the battery open circuit voltage is greater than 1.34 V/cell, the NiMH
battery should be pulse charging. Battery temperature should be monitored at any time. Once the battery
temperature falls below 0˚C or rises above 60˚C, charging should be suspend.
2
Charging NiMH Battery Using bq24105-Q1 in Automotive Applications
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Application Example Using bq24105-Q1
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FDK Recommend Charging Procedure for Back-Up Batteries In-Vehicle Applications
*:Tentative voltage
**:Timer : 80% Charging
Engine ON
C/20, Timer 16h
No
C/10, Timer 8h
No
Battery Temp.(T)Check
0deg.C<T<50deg.C
Wait 5min
Engine OFF
***:OCV : Open circuit
voltage
Yes
Yes
No
1
Start Standard Charge
(C/20** constant current Charge)
Start timer
2
Battery Voltage (OCV***)
>1.24*V/cell
Yes
Battery Voltage (OCV***)
>1.34*V/cell
No
Yes
Start Standard Charge
(C/20** constant current Charge)
Start timer
Health check
(If customer need)
3
Wait 10min
No
Battery Temp.(T)Check
0deg.C<T<60deg.C
Maintenance charge using Pulse Charging
(FDK recommend Pulse charging)
Wait 10min
No
No
Yes
Battery Temp.(T)Check
0deg.C<T<60deg.C
No
Timer
>=16h**
Yes
Yes
Timer
>=16h**
Battery Temp.(T)Check
0deg.C<T<60deg.C
No
Yes
Yes
No
Engine OFF
No
Engine OFF
Yes
Yes
Stop charging
Figure 2. Recommended Charging Procedure for backup batteries for In-vehicle Applications (Provided
by FDK)
3
Application Example Using bq24105-Q1
Even though the bq200x series can support this charging method, the operating ambient temperature
rating for the bq200x is from –20°C to 70°C. Automotive applications typically require operating
temperature rating from –40°C to 125°C. The bq24105-Q1 is an automotive qualified switch mode charger
which has adjustable battery regulation voltage and safety timer. The device was original design for Li-ion
based battery charging. However, with external MCU, the charging circuit can be modified such that the
charging profile meets the FDK recommended procedure above.
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Application Example Using bq24105-Q1
BQ24105
VIN
CIN
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10 …F
1.5 k
Adapter
Present
1.5 k
Done
1.5 k
Charge
GPIO1
LOUT
3 IN
OUT 1
4 IN
OUT 20
6 VCC
PGND 17
2 STAT1
PGND 18
19 STAT2
SNS 15
RSNS
10 µH
COUT
MMBZ18VALT1
3
Battery
Pack
+
-
10 …F
Pack +
Pack -
103AT
GPIO2
5 PG
7 TTC
CTTC
BAT 14
6.7 k
ISET1 8
RISET1
ADC
VTSB
6.7 k
GPIO3
0.37 …F
16 CE
ISET2 9
RT1
RISET2
10 VSS
0.1 …F
13 NC
STAT1
GPIO1
STAT2
GPIO2
/CE
GPIO3
TS 12
VTSB 11
VSS
0.1 …F
RT2
0.1 …F
MCU
GND
143 k for 3-cell
221 NŸ IRU 4-cell
100 NŸ
Copyright © 2017, Texas Instruments Incorporated
Figure 3. Example Block Diagram Using bq24105-Q1 with MCU forNiMH Battery Charge
The block diagram above shows an example of the configuration to use bq24105-Q1 to charge NiMH
batteries. Since the cell voltage of NiMH batteries varies with temperature with the same capacity, the
regulation voltage should be set to higher than 1.34 V/cell. The battery regulation voltage can be set by
resistor divider from BAT to FB and FB to VSS. The VOREG can be set to 5.1 V and 6.8 V for a 3-cell pack
and 4-cell pack respectively using Equation 1. VIBAT is 2.1 V (typical), which can be found from the
datasheet specification table.
VOREG =
(R1 + R2)
R2
x VIBAT
(1)
In order to set the current, first choose RSNS based on the regulation threshold VIREG across this resistor.
The best accuracy is achieved when the VIREG is between 100 mV and 200 mV. If the result is not a
standard sense resistor value, choose the next larger value. Using the selected standard value, solve for
VIREG. Once the sense resistor is selected, the ISET1 resistor can be calculated using Equation 3 where
KISET1 is 1000 V/A (typical), VISET1 is 1 V (typical), and RSNS is the charge current sense resistor.
R(SNS) =
VIREG
IOCHARGE
KISET1 T VISET1
RISET1 =
RSNS T ICHARGE
(2)
(3)
The charge time is programmed by the value of a capacitor connected between the TTC pin and GND
using Equation 4 where K(TTC) is 2.6 min/nF (typical). It is verified on the bench that the timer can be up to
20 hours. Note that the precharge timer is fixed at 30 minutes.
tCHARGE = C(TTC) T -(TTC)
(4)
Temperature is monitored through TS function in the charger at all time. To initiate a charge cycle, the
battery temperature must be within the V(LTF)-to-V(HTF) thresholds. If battery temperature is outside of this
range, the bqSWITCHER™ suspends charge and waits until the battery temperature is within the V(LTF)-toV(HTF) range. During the charge cycle (both precharge and fast charge), the battery temperature must be
within the V(LTF)-to-V(TCO) thresholds. If battery temperature is outside of this range, the bqSWITCHER™
suspends charge and waits until the battery temperature is within the V(LTF)-to-V(HTF) range. The
bqSWITCHER™ suspends charge by turning off the PWM and holding the timer value (that is, timers are
not reset during a suspend condition). Note that the bias for the external resistor divider is provided from
the VTSB output. Applying a constant voltage between the V(LTF)-to-V(HTF) thresholds to the TS pin disables
the temperature-sensing feature.
4
Charging NiMH Battery Using bq24105-Q1 in Automotive Applications
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Application Example Using bq24105-Q1
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1
1
F
C
V
V
HTF
LTF
462 =
VO(VTSB)
VO(VTSB)
RTHHOT T @ V
F 1A F RTHCOLD T @ V
F 1A
HTF
LTF
VO(VTSB)
F 1
VLTF
461 =
1
1
+
RT2 RTHCOLD
VO(VTSB) T RTHCOLD T RTHHOT T B
(5)
(6)
Where:
• V(LTF) = VO(VTSB) x % LTF-100 / 100
• V(HTF) = VO(VTSB) x % LTF-100 / 100
3.1
Charging Flow Control Logic
Open circuit battery voltage should be measured after pulling CE pin high. The battery voltage is
measured by the ADC in the MCU. the following is an example of the charging flow for MCU
programming. Timing parameter can be adjusted accordingly.
• Case 1. Battery voltage < 1.24 V/cell
pull /CE pin low to initiate charge, let it charge until STAT1 and STAT2 turns high, high
then pull /CE high to measure the battery voltage
if VBAT < 1.24V/cell, go back to case 1
if 1.24V/cell < VBAT < 1.34V/cell, go to case 2
if VBAT > 1.34V/cell, then go to case 3
•
Case 2. 1.24V/cell < battery voltage < 1.34 V/cell
pull /CE pin low to initiate charge, let it charge until STAT1 and STAT2 turns high,
then pulse /CE pin at a rate of 2s LOW and 58s HIGH
check the battery voltage at the end of every 58s when /CE is high
if VBAT < 1.24V/cell, go back to case 1
if 1.24V/cell < VBAT < 1.34V/cell, go to case 2
if VBAT > 1.34V/cell, then go to case 3
•
Case 3. Battery voltage higher than > 1.34V/cell
pulse /CE pin at a rate of 2s LOW and 58s HIGH
check the battery voltage at the end of every 58s when /CE is high
if VBAT < 1.24V/cell, go back to case 1
if 1.24V/cell < VBAT < 1.34V/cell, go to case 2
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Application Example Using bq24105-Q1
3.2
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Test Result
The following is the result tested using bq24105EVM and MSP430 Launchpad (MSP-EXP430F5529LP)
and FDK HR-AAULT battery in 3s configuration. The output regulation voltage on the bq24105-Q1 is set
to 4.5 V, charge current is set to 100 mA, the charging timer is set to 8 hours. The limitation on using the
bq24105 is the STAT1 and STAT2 pins only has three statuses. It does not have a unique status for timer
fault. Once the STAT1 and STAT2 is OFF and OFF, the CE pin is toggled by the MCU according to the
above flow. However, for overvoltage, sleep mode, and battery absent, charge will not resume until those
faults are clear regardless of the status of CE pin
Table 1. STAT1 and STAT2 Status Indication
Charge State
STAT1
STAT2
Charge-in-progress
ON
OFF
Charge complete
OFF
ON
Charge suspend, timer fault, overvoltage, sleep mode, battery absent
OFF
OFF
4300
70
4300
4200
60
4200
4100
50
4000
40
3900
30
3800
20
3700
10
VBAT (mV)
4100
ICHG (mA)
VBAT (mV)
SPACER
4000
3900
3600
0
0
20000
40000
Time (s)
60000
3700
0
80000
D002
Figure 4. bq24105 Charge Cycle with FDK HR-AAULT at
Room Temperature
4
10000 20000 30000 40000 50000 60000 70000 80000 90000
Time (s)
D001
Figure 5. bq24105 Charge Cycle with FDK HR-AAULT
VBAT at Different Temperatures
References
•
6
5qC
45qC
Room Temperature
3800
Datasheet: Synchronous Switch-Mode, Li-Ion, and Li-Pol Charge Management IC (bqSwitcher),
SLUS953
Charging NiMH Battery Using bq24105-Q1 in Automotive Applications
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