SIM900D Hardware Design
M odule
B attery Pack
R
T E M P
T herm istor
T E M P_B A T
V
B A T
V
C H G
5V
Figure11: Battery charger and pack
The function of detecting the temperature of battery should be supported by the software in the module. It’s a
customization function. The R
TEMP
is a NTC thermistor.
It's recommended to use NCP15XH103F03RC from
MURATA. The impedance of the NTC thermistor is 10Kohm at 25
℃
. Please refer to the above figure for the
reference circuit.
3.5.1 Battery pack characteristics
SIM900D has optimized the charging algorithm for the Li-Ion battery that meets the characteristics listed below.
To use SIM900D’s charging algorithm properly, it is recommended that the battery pack you integrated into your
application is compliant with these specifications. The battery pack compliant with these specifications is also
important for the AT command “AT+CBC” to monitor the voltage of battery, or the “AT+CBC” may return
incorrect battery capacity values.
z
The maximum charging voltage of the Li-Ion battery pack is 4.2V and the recommended capacity is
1100mAh. If the Battery packs with a capacity more than 1100 mAh, it will cost more time for charging.
z
The pack should have a protection circuit to avoid overcharging, deep discharging and over-current. This
circuit should be insensitive to pulsed current.
z
On the SIM900D, the build-in circuit of SIM900D’s power management chipset monitors the supply voltage
constantly. Once the Under-voltage is detected, the SIM900D will be power down automatically.
Under-voltage thresholds are specific to the battery pack.
z
The internal resistance of the battery and the protection circuit should be as low as possible. It is
recommended not to exceed
200m
Ω
.
z
The battery pack must be protected from reverse pole connection.
SIM900D_HD_V1.02
09.04.2010
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