You use 18650s or have big tanks?
Yep, single 18650. Most of the tanks I use are KFL's at (supposedly) 4.5 mls. At my power levels, a new battery won't be below half when the tank's empty. Fill tank and pop in a fresh battery, so that lighten's the load on the battery, theoretically extending the service life. And gives me peace of mind with both tank and battery "full."