Does anyone know why Tesla hasn't made their own 12V brick out of Li ion 18650s to serve as the auxiliaries battery instead of using the crappy old Lead Acid 12V batteries?
+1 I have been wondering the same thing.
Issue is in the charging logic of the 12v, not the chemistry.
Actually, the issue is the amount of amp-hours that can be derived from the 12V source at any given time. The li-ion batteries can't provide enough "umph" in such a small package. Your standard 12V battery has anywhere from 275 to over 600 cranking amps, depending on the need.
You just need plenty of them in parallel. Three normal 3.6 V 2.9Ah 1C batteries you would need four in series to get 14.4V and 200 in parallel for 580 amps. Total 800 batteries. That takes some space though. Some RC LiPo batteries can deliver well over 10C and some of them in short pulses over 100C with constant discharge somewhere in 50C range.
5C batteries you would need 40 in parallel which is much more manageable number of batteries. Tesla batteries deliver something like that (85kWh giving out 400+kW in short bursts).
I think Iowa92x knows why, batteries themselves are not the problem.
Could you or Timo or someone explain if/how NiMH would fare as the 12V source, and the constraints. The NiMH as used, for example, in Prius have much lower capacity than Li-ion, but amazingly long cycle lives, with Vancouver BC taxis having packs that last in taxi use for 400,000- 650,000 km. (250,000-400,000 miles)