Bluetooth headphones iphone battery life

HI

I have purchased some sennheiser Bluetooth headphones and am pleased with them

however connecting them via BT to my iphone 6+ seems to drain the battery quite quickly even with the screen off

4-5 hours listening anf its down to about 30%

is that nornal for bt headphones?

what are people getting battery life wise on long train journeys etc

thanks in advance