I attached a Lascar EL-USB-3 Voltage Data Logger to my Sep 2019 TM3. Over a two week period, with average temperatures around 75F, voltage was logged at one minute intervals with the car in regular use. Minimum voltage seen was 12.50 volts, and maximum was 14.50 Volts, though those appear to be transients. Typically the minimum voltage was 12.75V before a 14.40 Volt top up charge cycle was initiated. Most of this time the vehicle was either in use or plugged into a wall charger. If this is truly a deep cycle battery, and I can't imagine that it isn't, these cycles don't appear to be something that places a great deal of stress on the battery, and I would expect to see a minimum of several years life unless the battery was defective or damaged. This based on my experience with battery banks on liveaboard boats.