I use aBatteryMINDer http://www.thebatteryminder.com/12v1...29.htmlcharger . the kind use to keep battery,s fresh over the winter on my motorcycles. Been using it to charge the same battery for the last 4 years.
Or this one http://www.thebatteryminder.com/12v1...nder-p-29.html but the one I had did not work as well as the Battery MINDer