Iotalota
I could not figure out how much current your Helix 7 G2N draws from the manual but it looks like between 600 and 900 mA.  That is probably too much for AA batteries. Sense you will be able to recharge them every night I would suggest the Panasonic 3400 mAh cells covered in this thread. 

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You should get 3 to 5 hours out of a set.   You might consider building 2 battery holders for 4 batteries each and putting them in parallel.