I charge using a current limited PSU. At C/10 we can charge the battery continuously without harm. With my PSU I would set the voltage up above (1.4*number_of_cells), short the output, then turn the current up to 500mA and connect the battery.
The battery is charged when the voltage of the battery stops increasing or even goes down. Measure this with the PSU off.
If this is an old battery the cells may be of uneven quality and charging it will be difficult.