Just like lipos are at 50% at 3.85 volts life can be checked for capacity as well. What is required is a much better meter. Life do not have a flat discharge curve, yes they drop off very slowly but they do drop off and with a good meter they can be checked for capacity that way.
I have no experience with the above meter but I have used a voltmagic for the past 4 years on A123 and it has been bang on regarding capacity left. It works just like a voltwatch on a NIXX battery and when I hit the yellow light I put back in the same amount each and every time so I know it works.
http://www.voltmagic.com/