Tesla is using a small Lead-Acid battery in the S. It appears to be the size of a small "garden tractor" battery. I have not examined it personally yet, but I would hope it is AGM.
In order to properly care for the 12v battery, especially an AGM type, There absolutely must be a 3-stage charge algorithm, and it should also be temperature-compensated. This means the first stage is bulk charging, which should charge the battery at a constant current. This means the Model-S computer will watch the current drawn by the 12V battery and command the duty cycle of the DC-DC converter to regulate this amperage. During this stage the voltage will likely be 12-13v. After the temp-comp voltage climbs to a certain "knee" indicating around 80% SoC (State-of-charge), then it should switch to stage 2, called the "absorption" phase. During Absorption, the voltage will be held at a constant 14-15v (again depending on temperature). Once the current falls to a certain threshold, this will indicate charging is complete and this will kick in the final 3rd stage, called "float". The battery should then be held around 13 volts (temperature dependent) to maintain it's charge and the DC-DC will then be powering all the 12V loads directly.
If the S isn't doing this, then this is why 12v batteries are dying left and right.
A couple of points for clarification and technical accuracy: The DC-DC converter is NOT a buck converter, as that topology is non-galvanically isolated, which is required in the Model-S, where the High-voltage is totally isolated from the chassis for safety.
In order to have an accurate test of the conditions of the 12v battery in the S, we would need to log 3 variables at minimum; Battery Temperature, Battery Voltage, and amperage. We also need to determine the type used.
If anyone in the SF Bay Area wants to allow me to run some tests on their S, I'd be happy to share the results.
-Phil
