I've no idea what the so-called "C-rate" is...

You need to know this!

C is the cell's capacity in mAh. This is usually right there on the cell.
For example, I have some decade-old Panasonic nicads marked 500mAh. (That's
not very high capacity, compared to recent cells.)

If you charged the cell at 50mA, that would be a 0.1C charge rate.