You know it as the threshold voltage. Basically when the charge begins to drop 4 mv on NiMh or 10 mV on NiCd. so if a cell is fully charged at 1.35 volts per cell and then begins to drop beyound the threshold voltage is done! Which is why the Triton pauses every now and then.
John

EDIT -> I believe that the charging voltage is 1.42. 1.35 is fully charged at rest. It's also dependent upon temperature, but that's as far as I care to stick my neck out.