QSystemBatteryInfo::currentFlow() returns -1 alwasy

currentFlow() always return -1 (http://doc.qt.nokia.com/qtmobility-1.2/qsystembatteryinfo.html#currentFlow-prop this doc says it will be -ve when charging but it return -1 even when it not charging)

QSystemBatteryInfo *binfo = new QSystemBatteryInfo();
int cur = binfo->currentFlow(); // cur everytime is -1

Environment:

Windows 7,
Qt SDK 1.1.3

C7 device SW : Qt 4.7.4 + Qt Mobility 1.2.0 + Symbian Belle 111.020.0305