Magic, perhaps your power requirements on your detector are greater than my basic unit.  The two Ryobi batteries tested may be getting a bit tired as they are over three years old, but they have been sufficient for three 7-10 day trips so far. 
So, so far for me,
Rechargeable Ryobi Lithium Drill Kit Batteries
2 batteries per trip (7.94 oz. each)
Complete Ryobi Kit (drill, two batteries, charger) $70,
3 Trips (so far) = $23.33 per trip
16 oz per trip
As the batteries may be getting tired I may add another for the next trip, which will bring the weight up all the way up to 1.5 lbs.
Anyway, whatever works, works.