Rainbow Electronics GE863-PY User Manual

Page 25

GE863 Hardware User Guide

1vv0300783 Rev.0 - 10/06/08

page 25 of 80

4.5.1.4 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. GE863 uses constant
voltage.
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.
When the charging current falls below a certain fraction of the fast charge current value, then the
battery is considered fully charged, the final charge stops and eventually starts the maintenance.
The pulsed charge process has no voltage regulation, instead the charge continues with pulses.
Usually the pulse charge works in the following manner: the charge is stopped for some time, let's say
few hundreds of ms, then the battery voltage will be measured and when it drops below its maximum
value a fixed time length charging pulse is issued. As the battery approaches its full charge the off
time will become longer, hence the duty-cycle of the pulses will decrease. The battery is considered
fully charged when the pulse duty-cycle is less than a threshold value, typically 10%, the pulse charge
stops and eventually the maintenance starts.
The last phase is not properly a charging phase, since the battery at this point is fully charged and the
process may stop after the final charge. The maintenance charge provides an additional charging
process to compensate for the charge leak typical of a Li-Ion battery. It is done by issuing pulses with
a fixed time length, again few hundreds of ms, and a duty-cycle around 5% or less.
This last phase is not implemented in the GE863 internal charging algorithm, so that the battery once
charged is left discharging down to a certain threshold so that it is cycled from full charge to slight
discharge even if the battery charger is always inserted. This guarantees that anyway the remaining
charge in the battery is a good percentage and that the battery is not damaged by keeping it always
fully charged (Li-Ion rechargeable batteries usually deteriorate when kept fully charged).