The 2015 15" had a 100Wh battery, the 2018 has an 83Wh battery. The 2015 gets better battery life because it has 20% more battery capacity.
I'd guesstimate I get at most 3 hours on the 2017 before the battery level gets low enough I have to start planning where I'll next plug in. Not ideal.
I suspect that Apple will be seeing a lot of 2015 MBPs brought in for battery replacements...
physics might have a word with you :)