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12-Volt Battery - Different Answers From Different Tesla Service Reps

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I recently had the 12-volt battery on my '18 Model S go dead. As I try to find out how this battery is charged, one Tesla service rep told me it's only charged by the heavy-duty battery system while another Tesla service rep told me it is charged by the wall charger. Does anyone know the truth as apparently the Tesla service reps do not know. Thank you.

Related to this issue are uncertainties coming from the Tesla folks about the "best" way of charging the batteries - how often and how much. One rep told me to plug the car in every night. The other told me to charge it only when needed.

Thoughts and explanation will be appreciated.
 
I recently had the 12-volt battery on my '18 Model S go dead. As I try to find out how this battery is charged, one Tesla service rep told me it's only charged by the heavy-duty battery system while another Tesla service rep told me it is charged by the wall charger. Does anyone know the truth as apparently the Tesla service reps do not know. Thank you.

Related to this issue are uncertainties coming from the Tesla folks about the "best" way of charging the batteries - how often and how much. One rep told me to plug the car in every night. The other told me to charge it only when needed.

Thoughts and explanation will be appreciated.
1.In your gasoline car, it's done by the alternator.

In Tesla, it's done via the DC-to-DC converter that converts the main battery 400V into the low 12V.

It does not matter whether your car is plugged in or not or running on the road or not, the 12V battery is charged when needed around the clock until the DC-to-DC converter stops doing that to prevent the low State of Charge of the main battery. It would sacrifice the cheaper 12V battery than saving it at the expense of the main battery.

2.There are many verbal instructions on how to charge your car but if they are unwilling to write them down then they are not official (just like a sales person who promises something but would to write it down on the receipt). The official way is written by Tesla. It's written down very clearly on your owner's manual:

Plug in as soon as possible
No advantages to delay plug in
100% for LFP
Non-LFP: 50-90% for Daily use and >90% for road trips. Service Center wrote on the receipt to set at 90% Daily for better range calculation.
 
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