I know this topic has been discussed many times, but...
I started to get occasional electrical hiccups in my '06 DB9 Volante. I had a roof movement error, speedometer malfunction, airbag warning, and transmission error all within a period of four weeks.
I keep my car on a CTEK tender whenever it's in the garage, but I noticed an airbag warning after I'd had it out in the driveway for a while one Sunday. I took it to my local auto parts store and had them test the battery. It tested ok, but I was unconvinced and decided to invest in a new Autocraft Platinum AGM.
Since then... no errors, no glitches, no warnings, no problems.
And to think I nearly bought a new roof control module!

 

I wonder what level is considered "okay" when testing a battery compared to the level required for the battery to fully support our cars without gremlins.