So I have read and researched and read and more. I see some people say that in a regulated mod dual battery setup has no impact on battery life vs a single battery mod. Just voltage. I see others say that you do get better battery life with a dual battery mod. I have a Topside and Topside Dual. I use a single 18650 battery with the Topside as I don't have any 2x700 batteries. I run both at 48 watts with the same exact build. The Topside Dual goes way longer before needing a battery swap than the single. Also run a Famovape Magma dual battery with an Innokin iSub B tank. Battery life is much longer than any of my other single battery mods running my other iSub B tanks. So what's the skinny? How do my results go against what most people say that a dual battery mod does not get any better battery life? Tap'd from my iPhone XR

Whoever says dual battery mods don’t get more battery life than single cell mods is wrong. Assuming we are talking single & dual cell mods that use the same battery at the same wattage. The dual should get twice the life as the single. Now a single 21700 and a dual 18650 will be closer but the dual would still win. Basically look at the mah numbers assuming they are reputable cells with accurate numbers. Whatever combination has a higher number will last longer. So 2-18650’s with 3000mah is 6000mah and a single 21700 with 4000mah. 6000 is bigger than 4000 so the dual cell 18650 will last longer.

They're full of beans. All else being equal a dual-18650 mod will give you approximately twice the time between charges as a single-18650 mod. It's quite straight-forward, really. Let us say a particular cell is rated for 2500mAh. Nominal voltage of an 18650 is 3.7VDC. This means the cell is capable of supplying 9.25 watt-hours of energy. (Not really, but close enough for this explanation.) If you put two of these cells together, whether in series or parallel, you now have available 18.5WHR of energy. Now, it is true that if you put the cells in series the result is 7.4VDC. But the mod is designed to expect this. It doesn't really matter whether you have 2500mAh at 7.4VDC or 5000mAh at 3.7VDC, the result is still 18.5WHR of energy in storage.

Look at it this way. 2 beers will last longer than 1 beer. Will 2 beers last twice as long as 1 beer? Depends on how fast you drink.