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AC75 Hardware Interface Description
Strictly confidential / Preliminary
3.5.3 Battery Pack Requirements
The charging algorithm has been optimized for rechargeable Lithium batteries that meet the
characteristics listed below and in Table 8. It is recommended that the battery pack you want
to integrate into your AC75 application is compliant with these specifications. This ensures
reliable operation, proper charging and, particularly, allows you to monitor the battery
capacity using the AT^SBC command. Failure to comply with these specifications might
cause AT^SBC to deliver incorrect battery capacity values.
•
Li-Ion or Lithium Polymer battery pack specified for a maximum charging voltage of 4.2V
and a recommended capacity of 1000 to 1200mAh.
•
Since charging and discharging largely depend on the battery temperature, the battery
pack should include an NTC resistor. If the NTC is not inside the battery it must be in
thermal contact with the battery. The NTC resistor must be connected between
BATT_TEMP and GND.
The B value of the NTC should be in the range: 10kΩ +5% @ 25°C, B
=3435K ± 3% (alternatively acceptable: 10kΩ +2% @ 25°C, B
note that the NTC is indispensable for proper charging, i.e. the charging process will not
start if no NTC is present.
•
Ensure that the pack incorporates a protection circuit capable of detecting overvoltage
(protection against overcharging), undervoltage (protection against deep discharging)
and overcurrent. Due to the discharge current profile typical of GSM applications, the
circuit must be insensitive to pulsed current.
•
On the AC75 module, a built-in measuring circuit constantly monitors the supply voltage.
In the event of undervoltage, it causes AC75 to power down. Undervoltage thresholds are
specific to the battery pack and must be evaluated for the intended model. When you
evaluate undervoltage thresholds, consider both the current consumption of AC75 and of
the application circuit.
•
The internal resistance of the battery and the protection should be as low as possible. It
is recommended not to exceed 150mΩ, even in extreme conditions at low temperature.
The battery cell must be insensitive to rupture, fire and gassing under extreme conditions
of temperature and charging (voltage, current).
•
The battery pack must be protected from reverse pole connection. For example, the
casing should be designed to prevent the user from mounting the battery in reverse
orientation.
•
It is recommended that the battery pack be approved to satisfy the requirements of CE
conformity.
Figure 9 shows the circuit diagram of a typical
battery pack design that includes the protection
elements described above.
Figure 9: Battery pack circuit diagram
AC75_HD_V00.202
to BATT+
Page 42 of 120
s
= 3423K to B
25/85
= 3370K +3%). Please
25/50
to BATT_TEMP
ϑ
NTC
Protection Circuit
+ -
Battery cell
Polyfuse
27.04.2006
to GND
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