New Battery

Hey I've got a Cool Fire IV and an Aspire Cleito tank at the moment, and I am running at 40W, but I've found the battery life is shit compared to when I was running it at 20W with a 1.6 ohm tank.

Time for a new set up or can I replace the battery with something more durable?

I'm in the UK. Thanks for the help in advance, its appreciated.