SLUA425 – June 2007
Mao Ye .............................................................................................................
PMP - Portable Power
ABSTRACT
This application report gives an example of using the bq24105 charge integrated circuit to provide a high-efficiency, switching-mode charging solution for a lead-acid battery.
The bq24105 was designed to charge single-, two- or three-cell Li-ion and Li-polymer battery packs. It does not have the features to properly terminate a charger on lead-acid battery packs. This application report gives an example of using the bq24105 to provide a high-efficiency, switching-mode charging solution for a lead-acid battery.
Lead-acid battery charging system design specification:
•
Input power source Vin: 17
±
1 Vdc
•
Battery voltage Vbat: 12-V lead-acid battery
•
Battery bulk voltage regulation: 14.8 V
•
Battery refresh voltage: 13.6 V
•
Fast-charge current: 0.5 A for Vbat
≤
13.5 V, 1 A for Vbat > 13.5 V
•
Precharge current: 0.45 A
•
Termination current: 0.9 A
Figure 1
shows the modified schematic from the bq24105 EVM schematic. To focus on the changes made for this application, several pin connections are not shown such as STAT1, PG, TTC, CE, and TS. Those pin connections are the same as shown in the EVM schematic.
The design changes are straightforward. In constant current phase, the STAT2 pin is off with a high-impedance output. As a result, input voltage turns on Q3 through R1 and D1. So, R17 is parallel with
R7 to set the bulk voltage regulation to 14.8 V. When termination current is detected, the STAT2 pin goes on and pulls down the Q3 gate to ground to turn off Q3. R7 and R5 set the refresh voltage to 13.6 V. The internal reference voltage of bq24105 is set to 2.1 V for voltage regulation and 2.05 V for recharge threshold. Thus, the resistor value for R5 and R7 can be easily calculated by the following:
R
7
+
V
V
REFRECHARGE
RECHARGE
R
5
* V
REFRECHARGE
+
2.05
R
5
13.6
* 2.05
+ 0.1775R
5
,
R7=48.7k standard value.
select R5=274k, R7=48.6k, so select
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J1
DC+
DC-
C1 C2
D1
L1
R4
0.1
W
C9 m
BAT+
BAT -
J2
+
R8
20 k W
R15
20 k W
C5
Q1
BSN20
R1
1.5 k W
R6
0
D4
BZX84C11
R16
2.21 k W
Q2
BSN20
R9
2.21 k W
10
7
8
9
3
4
1
2
5
6
R18
20 k W
C10 m
20
19
18
17
16
15
14
13
12
11
C7
C4
C8
Figure 1. Lead-Acid Battery Charger Using bq24105
R5
274 k W
R17
649 k W
R7
48.7 k W
Q3
BSN20
R
7equ
+
V
REFCHARGE
R
5
V
CHARGE
* V
REFCHARGE
+
2.1
R
5
14.8
* 2.1
+ 0.1654R
5
, R7equ + 45.3k, where R7equ is the equivalent resistance of R7 parallel with R17.
R
17
+
R
7
R
7
*
R
7equ
R
7equ
+
48.7k
45.3k
48.7k
* 45.3k
+ 648.9k, select R17 + 649k standard value.
When the battery voltage is low, the gate pulldown resistor R18 turns off Q1 and Q2 . It also provides a bias current path for D4 when the battery voltage is high enough to turn on D4. A 20-k
Ω resistor is selected.
R8 sets the fast-charge current to 0.5 A and R9 sets the precharge current to 0.45 A. D4 is an 11-V zener diode. Assume the Q1 and Q2 gate turnon threshold is 2.5 V, when battery voltage is above 13.5 V; then
D4 turns on and Q1 and Q2 are on to change ISET1 and ISET2 pin resistance to change the fast and termination current values. R15 is used to change the fast-charge current from 0.5 A to 1 A, and R16 is used to change the termination current from 0.45 A to 0.9 A. Due to the simple double relationship of the current values, R15 and R16 can be easily chosen as same as R8 and R9, respectively. R8 and R9 can be easily calculated as following:
R
8
V
ISET1
K
ISET1
+
R
SNS
I
CHARGE
+
1 1000
0.1
0.5
+ 20k, select R8 + 20k standard value.
R
9
V
ISET2
K
ISET2
+
R
SNS
I
PRECHARGE
+
0.1
1000
0.1
0.45
+ 2.22k, select R9 + 2.21k standard value.
C9 and C10 are used to eliminate the possible current oscillation. Otherwise, the small change on battery voltage is amplified by Q1 on the ISET1 pin; therefore, the charge current has a large change. As a result, output current change affects Q1 again. Current oscillation happens when this closed-loop positive feedback satisfies the unstable condition. C9 and C10 give voltage perturbation a low- impedance loop to ground to prevent this kind of current oscillation.
The measured result of charging voltage and current profile is shown in
Figure 2
. The fast-charge current change from 0.5 A to 1 A happens when battery voltage is in the 12.5-V to 13.1-V range. The precharge to fast charge happens at a 10.6-V to 10.7-V range, which is set by the charger IC (71.4% of the battery regulation voltage which corresponds to 3 V/cell threshold for an Li-Ion battery with 4.2 V/cell regulation voltage). When battery voltage is below 7.4 V, which is set by the charger IC (corresponding to 2 V/cell threshold for Li-Ion battery with 4.2 V/cell regulation voltage), a 50-mA, short-circuit current is applied to battery.
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The whole charging process was shown in
Figure 3 . At the beginning, the fast-charge current is 0.494 A. It
increases to 0.976 A when the battery voltage increases to 13.1 V. Finally, the battery voltage increases to
14.8 V and termination occurs at 0.86 A as shown in
Figure 4 . After that, the pulse charging takes over.
The bulk voltage regulation is 14.8 V, and the refresh voltage is 13.6 V as shown in
Figure 5
. In the pulse charging period, charging current is turned off after battery voltage achieves a 14.8-V bulk voltage regulation and charging current tapers down to 0.9A termination current threshold until battery voltage falls to 13.6V refresh voltage threshold. Full, fast-charging current of 1 A is then re-enabled to the battery until the battery voltage rises to 14.8 V. This cycle repeats indefinitely.
The open-drain status pin STAT1 and STAT2 output indicate various charger operations as shown in
Table 1
. Compared with the data sheet, the difference is that the charge complete is indicated by the blinking of LEDs which is caused by the pulse charging. Note that OFF indicates that the open-drain transistor is turned off. The complete schematic is shown in
Figure 6
Table 1. Status Pins Summary
Charge-in-progress (Short circuit, precharge, fast charge)
Charge complete (Pulse charge)
Charge suspend, timer fault, overvoltage, sleep mode, battery absent
STAT1
ON
Blinking
OFF
STAT2
OFF
Blinking
OFF
Charging Profile
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
5 6 7 8 9 10 11 12 13 14 15
Battery Voltage - V
Figure 2. Measured Charging Voltage and
Current Profile
Figure 3. Measured Charging Lead-Acid
Battery
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Figure 4. Termination of Lead-Acid Charger Figure 5. Pulse Charge of Lead-Acid Charger
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Figure 6. Complete Schematic
Using bq24105 as Lead-Acid Charger 5

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