If being left stood i'd invest in a battery conditioner, batterys dont like constant charge flat charge flat, i used to get through one a year when i was hardley using mine.
Could well be the battery is now nackered, take it off and run it down to local battery place, they normaly test em for free.
Just leave one of these connected and your battery charging trouble are sorted.
http://www.amazon.co.uk/CTEK-4-Step-Battery-Charger-Conditioner/dp/B00JMJHZ4E/ref=sr_1_2?ie=UTF8&qid=1442230736&sr=8-2&keywords=battery+conditioner