User guide

UC864-E/G/WD/WDU Hardware User Guide

1vv0300766a Rev.4

- 03/02/09

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/G/WD/WDU 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, typically 10%.

When this happens, 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 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 UC864-E/G/WD/WDU internal charging

algorithm so once-charged battery is left discharging down to a certain threshold. It is

cycled from full charge to slight discharge even if the battery charger is inserted. This

guarantees that 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).

Last but not least, in some applications, it is highly desired that the charging process

restarts when the battery is discharged and its voltage drops below a certain

threshold. This is typical for the UC864-E/G/WD/WDU internal charger.

As you can see, the charging process is not a trivial task to do; moreover all these

operations must start only if battery temperature is inside charging range, usually 5°C

- 45°C.