My i3 from 2014 with 112.000km/70.000miles is still on it's first 12v battery. I never experienced a lock-up due to a failed 12v battery, but after reading this thread, I might do well by changing the battery. If the 12v battery is kept alive by the DC-DC converter, then it might be so that I gain more range by changing the 12v battery, because then, the DC converter does not have to top-up the 12v battery that much. Sometimes I notice that the range in the morning is less than the previous evening when I park the car. This may be due to temperature differences, but also because the DC-converter is charging the 12v battery overnight. Food for thought.. can anyone relate to this?