Yes - 0.5a is considered a slow charge on an rx batt.

If this is a wall type charger, you need to leave it plugged in about 3hrs. If its a peak type charger. the charger will shut off when it detects the peak.

You can charge rx batts up to 1amp and still get decent life out of them (fast charge). You can charge it at higher rates (1.5 - 1.6a) but yu won't get much service life from it./ Thes batts don't like heat and seems to do best with a slow charge.