I bought one of these to get a longer run time with my Fenix BT-20. I buy good quality 18650 batteries and have a good charger. I am sure you would get longer run times and longer battery life.

This charger: $25.61 Authentic Soshine SC-S1 Max 1-4pcs Li-Ion Battery Charger - black / US plug at FastTech - Worldwide Free Shipping
I'd get these: Panasonic NCR18650B Protected 3400mAh