User's Guide

UC864-E-AUTO / AWS-AUTO Hardware User Guide

1vv0300795 Rev.11 – 2010/11/18

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 of a battery voltage measure, indicating roughly its

charge status. If the battery is deeply discharged, meaning its voltage is lower than

the trickle charging threshold, then charging must start slowly, possibly with a

current limited to the pre-charging process. The current must be kept very low with

respect to the fast charge value.

During trickle charging the voltage across the battery terminals rises; when it

reaches the fast charge threshold level the charging process goes into a fast charge

phase.

During the fast charge phase the process proceeds with a current limited for

charging; this current limit depends on the required time for completing the charge

and on 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 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 constant final chargers, the voltage phase (final charge) must not start

before the battery voltage has reached its maximum value, otherwise the battery

capacity will be slightly reduced. The final charge can be of two different types:

constant voltage or pulsed. UC864-E-AUTO / AWS-AUTO uses constant voltage.

The constant voltage charge proceeds with a fixed voltage regulator (very accurately

set to the maximum battery voltage) and 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, the battery is considered fully charged, the final charge

stops and eventually starts the maintenance.

The pulsed charge process has no voltage regulation, instead charge continues with

pulses. Usually the pulse charge works in the following manner: the charge is

stopped for some time, let us 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 and 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,