I've been using Absorbed Glass Mat (gel) batteries for years now. The one on the K bike is over 6 years old and still starts the big beemer first time on the button. I don't keep it on charge all the time either, just top it up every month or so over winter but I'm not convinced even that's necessary as it never requires any significant charge. This ones made by Westco and I bought it at Tanya.co.uk - they have a fitment guide.
Just replaced the nearly 5 year old AGM on the GSA as it was starting to show signs of weakening. Have just replaced that with an Odyssey battery which is a well known AGM gel battery as well.
