I am just back from Hawaii where I used the Retras for all my underwater shots. I used the battery extension compartment with new black Eneloops for 100% of the shots. I attended the Ocean Sciences Meeting in Portland, OR the week before going to HI. I packed the strobes with fully charged batteries prior to leaving for Portland complete with extenders. I.e., they traveled with eight batteries per strobe inside. I topped off the batteries upon arrival in HI and indeed found that they needed re-charging.
The strobes worked well. I depleted the strobe batteries by only one LED (of four) worth of charge (when I remembered to check) over the two-week diving period (with a day off to switch islands) and doing multiple dives per charge. I did four blackwater dives and did not bother recharging before doing a dive the next morning (three of the four).
My only mistake was removing the extender compartment together with the door once. It was challenging to pull the door off with my less than dexterous fingers. It is best to hold the compartment against the strobe with one hand while unscrewing the knob with the other. The unscrewing action lifts the door off very nicely. The battery compartment design is much better than other recently used AA-battery powered underwater strobes IMHO - Seacam 60D and Inon Z220 (same battery compartment as 240 and 330).