Why not just install and use a battery conditioner ?
For the past 2 years I've been using a CTEK XS 3600, the best £40 odd I've ever spent, and very easy to semi-permanently wire in too.
CTEK MULTI XS 3600 - Features
Edit : just clicked on the link in the first post and the page that shows the battery even shows the CTEK as an optional extra, now £49.95.