User's Manual

AC65/AC75 Hardware Interface Description
Confidential / Preliminary
s
AC65/AC75_hd_v00.372 Page 44 of 118 2006-08-03
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 AC65/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
25/85
= 3423K to B
=3435K ± 3% (alternatively acceptable: 10kΩ +
2% @ 25°C, B
25/50
= 3370K +3%). Please
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 AC65/AC75 module, a built-in measuring circuit constantly monitors the supply
voltage. In the event of undervoltage, it causes AC65/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
AC65/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 150m, 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
to BATT_TEMP to GND
NTC
Polyfuse
ϑ
Protection Circuit
+
-
Battery cell
to BATT+