2017 Miata RF Battery Issue

I have had an ongoing problem with my 2017 MX-5 RF. whereby it loses its charge after only a week or two (max) of non use. This is my 4th Miata (having owned one of each previous generation) and never experienced this problem before. Have taken it twice to-date to Mazda dealers and, after checking, they indicate nothing is wrong. They suggest a trickle charger at all times. Has anyone else had this problem--and/or suggestions? Very frustrating.