SIM900D Hardware Design
3.5.2 Recommended battery pack
Following is the specification of recommended battery pack:
Table7: Specification of recommended battery pack
Items
Description
Battery type
Li-ion
Manufacturer
Jiade Energy Technology
Normal voltage
3.7V
Capacity
NORMAL 1100mAh
Charge Voltage
4.200±0.050V
Max Charge Current
1.0C
Charge Method
CC / CV (Constant Current / Constant Voltage)
Max Discharge Current
1.0C (for continuous discharging mode)
Discharge Cut-off Voltage
3.0V/ cell
Internal resistance
Initial
≤
130m
Ω
3.5.3 Implemented charging technique
SIM900D has battery charging function. There are three pins related with the battery charging function: VCHG,
VBAT and TEMP_BAT/ADC0 pins. The VCHG Pin is driven by an external voltage, system can use this Pin to
detect a charger supply and provide most charging current through SIM900D module to battery when charging is
in fast charge state. The VBAT give out charging current from SIM900D module to external battery. TEMP_BAT
Pin is for user to measure the battery temperature. Just let this Pin open if battery temperature measuring is not
your concern.
So it is very simple to implement charging technique, It needs connect the charger to the VCHG Pin and connect
the battery to the VBAT Pin.
The SIM900D detect charger supply and the battery is present, battery charging will happen. If there is no charger
supply or no battery present the charging will not be enabled.
Normally, there are three main states in whole charging procedure.
z
DDLO charge (Pull-up mode) and UVLO charge (Per-charge mode);
z
Fast charge;
z
Trickle charge;
DDLO charge and UVLO charge:
DDLO (deep discharge lock out) is the state of battery when its voltage is under 1.5V. And UVLO (under voltage
lock out) means the battery voltage less than 3.3V and more than 1.5V. The battery is not suitable for fast charge
when its condition is DDLO or UVLO. The SIM900D provides a small constant current to the battery when the
SIM900D_HD_V1.02
09.04.2010
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