I have a 2021 Model Y and have purchased a portable lithium battery (1500W) recently that can be charged w/a 12V vehicle battery. It's not clear to me though, as I deplete the 12V battery in the Tesla, does the vehicle's lithium battery pack serve as a back-up to the 12V? Is it the case that the 12V battery doesn't get depleted because it can draw on the lithium battery as a supplement? Maybe after I drive around for a bit? Appreciate any insight to the advantages/disadvantages of charging the portable battery w/the Tesla 12V battery. Thanks!