+1 for a trickle charge if the bikes parked up for more than a couple of weeks.
But I do find the park light key position a pest at times when leaving the bike in the daylight. I've nearly left the lights on again at work. Luckily hasn't been drained to dead flat yet.
The Dealer where I got my Tenere from has just replaced an original Yuasa battery in his own bike after 9 years service.

It may be worth checking the fluid level in your battery to if possible Keithy2.