Bluetooth headphones lose battery charge rapidly but only when connected to 945

Batteries typically drain faster when the device has to work harder for connectivity. Take your cell phone to a place with bad reception and it'll burn its battery way faster. Maybe the same goes for your headphones. When the bluetooth unit is on the left ear unit and your watch on your right wrist (usually facing outward), it could be that it needs to work harder. You could experiment with wearing the watch on your other wrist, see whether that makes a difference.

Thanks for the reply and I'll certainly give that a go. Here's another part of the mystery that I've just discovered since my original post. 
Went for a run last week and the headphones died after about 50 minutes (as usual). I then put the headphones in their portable charging unit and they immediately showed a 3/4 charge - indicating they weren't dead at all!
Have ordered some more expensive headphones and hoping that the issue resolves itself that way.

In that case, temperature can also play a role. If you use the headphones normally indoors or in warm environments, taking them for a run near freezing point can shock the battery. I don't know your environments, but just wanted to hand that to you.

Thanks again - much appreciated. Went out for a run yesterday on a very cold day (about 4 degrees C) for 75 minutes and they lasted for the whole run - maybe mine prefer the cold! :-) :-)