In order to get the most out of your rechargeable batteries and lights, it is best to match cells of similar capacity. This means that the batteries that you put in your light should all have similar capacities.
How do you do it?
Here's my technique:
I used Enloop 2500mAh NiMH AA batteries, and charge them with a Maha Powerex MH-C9000 smart charger.
Since the Enloops are white, I use a black permanent Sharpie marker to number each battery. As I recondition each battery on the smart charger, I get a reading of each battery's actual capacity. This information is recorded in my battery log.
Using a log I can monitor the performance of each battery over time, and I can tell when it is time to discard a particular battery. Batteries naturally lose some capacity with time.
In order to match batteries in my lights, I can refer to my battery log. But there is a better way.
I made a chart of battery capacities and assigned a "dot" for each category. For instance:
Actual capacity
>2000 mAh = no dots
1980-2000 mAh = 1 dot
1960-1980 mAh = 2 dots
1940-1960 mAh = 3 dots
1920-1940 mAh = 4 dots
The lower the the capacity, the more the dots. These dots are maked on each battery with a Sharpie marker to indicate that individual battery's capacity.
Using this method, it is easy to match cells.
Just grab 2 or 4 batteries with the same number of dots and load them into your light.