Hello van1
With 2x18650 you get 16,28 wh, with 6x2,5ah nimhs = 18 wh, I use in mine 7 cells with 2,7 ah = 22,68 wh.
So if you use 7x 2,5 ah you get 21 wh. You can also use 8 nimh's, then will the led's driven in DD until the voltage drops below 12 volts.
I hope this helps
Best regards
____
Tom