Implemented Charging Technique - SimCom SIM900D Hardware Design

Table 6: Specification of recommended battery pack
Items
Battery type
Manufacturer
Normal voltage
Capacity
Charge Voltage
Max Charge Current
Charge Method
Max Discharge Current
Discharge Cut-off Voltage
Internal resistance
4.4.3

Implemented charging technique

SIM900D has battery charging function. There are three pins related to 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 gives 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
measurement is not your concern.
So it is very simple to implement charging technique, user just needs to connect the charger to the VCHG Pin and
connect the battery to the VBAT Pin.
SIM900D battery charging happens after detecting charger supply and the battery present. If there is no charger
supply or no battery present, charging will not be enabled.
Normally, there are three main states in whole charging procedure.
DDLO charge (Pull-up mode) and UVLO charge (Pre-charge mode);
Fast charge;
DDLO charge and UVLO charge:
DDLO is the state of battery when its voltage is under 1.5V. And UVLO means the battery voltage less than 3.3V
and more than 1.5V. The battery is not suitable for fast charging when its condition is DDLO or UVLO. The
SIM900D provides a small constant current to the battery when the battery is between DDLO and UVLO. In
DDLO charging, SIM900D gives out 1mA current to the battery. And in UVLO charging, SIM900D provides
about less than 200mA current to the battery.
DDLO charging terminates when the battery voltage reaches 1.5V. UVLO charging terminates when the battery
voltage is up to 3.3V. Both DDLO and UVLO charge are controlled by the SIM900D hardware only.
Fast charge:
If there is a charger supply and battery present and the battery is not in DDLO and UVLO, SIM900D will enter
fast charge state. Fast charge is controlled by the software to make the current/voltage regulation. The charging
scheme for the Li-Ion battery is constant current (about 550mA) first, followed by constant voltage charging once
SIM900D_Hardware Design_V2.02
Description
Li-ion
Jiade Energy Technology
3.7V
NORMAL 1100mAh
4.200±0.050V
1.0C
CC / CV (Constant Current / Constant Voltage)
1.0C (for continuous discharging mode)
3.0V/ cell
Initial≤130mΩ
26
Smart Machine Smart Decision
2012-05-07