Rainbow Electronics GM862-QUAD-PY - Manual (Page 26)

GM862-QUAD / PY Hardware User Guide
1vv0300748 Rev. 5 - 20/09/07
Reproduction forbidden without Telit Communications S.p.A. written authorization - All Rights Reserved page 26 of 68
6.2.1.3 Battery Source Power Supply Design Guidelines
• The desired nominal output for the power supply is 3.8V and the maximum voltage allowed is
4.2V, hence a single 3.7V Li-Ion cell battery type is suited for supplying the power to the Telit
GM862-QUAD / PY module.
The three cells Ni/Cd or Ni/MH 3,6 V Nom. battery types or 4V PB types MUST NOT BE USED
since their maximum voltage can rise over the absolute maximum voltage for the GM862-QUAD /
PY and damage it.
NOTE: DON'T USE any Ni-Cd, Ni-MH, and Pb battery types. Their use can lead to overvoltage on the GM862-QUAD /
PY and damage it. USE ONLY Rechargeable Li-Ion battery types.
• A Bypass low ESR capacitor of adequate capacity must be provided in order to cut the current
absorption peaks, a 100μF tantalum capacitor is usually suited.
• Make sure the low ESR capacitor (usually a tantalum one) is rated at least 10V.
• A protection diode should be inserted close to the power input, in order to save the GM862-QUAD
/ PY from power polarity inversion. Otherwise the battery connector should be done in a way to
avoid polarity inversions when connecting the battery.
• The battery capacity must be at least 500mAh in order to withstand the current peaks of 2A; the
suggested capacity is from 500mAh to 1000mAh.
6.2.1.4 Battery Charge control Circuitry Design Guidelines
The charging process for Li-Ion Batteries can be divided into 4 phases:
• Qualification and trickle charging
• Fast charge 1 - constant current
• Final charge - constant voltage or pulsed charging
• Maintenance charge
The qualification process consists in a battery voltage measure, indicating roughly its charge status. If
the battery is deeply discharged, that means its voltage is lower than the trickle charging threshold,
then the charge must start slowly possibly with a current limited pre-charging process where the
current is kept very low with respect to the fast charge value: the trickle charging.
During the trickle charging the voltage across the battery terminals rises; when it reaches the fast
charge threshold level the charging process goes into fast charge phase.
During the fast charge phase the process proceeds with a current limited charging; this current limit
depends on the required time for the complete charge and from the battery pack capacity. During this
phase the voltage across the battery terminals still raises but at a lower rate.
Once the battery voltage reaches its maximum voltage then the process goes into its third state: Final
charging. The voltage measure to change the process status into final charge is very important. It
must be ensured that the maximum battery voltage is never exceeded, otherwise the battery may be
damaged and even explode. Moreover for the constant voltage final chargers, the constant voltage
phase (final charge) must not start before the battery voltage has reached its maximum value,
otherwise the battery capacity will be highly reduced.
The final charge can be of two different types: constant voltage or pulsed.
The constant voltage charge proceeds with a fixed voltage regulator (very accurately set to the
maximum battery voltage) and hence the current will decrease while the battery is becoming charged.