James May shows why Tesla engineers are so smart, they come full circle to being fantastically stupid.

[1000MPH]

My Dad has a Ford C-Max hybrid that killed the first three 12v batteries in less than a year. You could wake it up simply by hooking up a trickle charger for a few seconds to bring the voltage up, then the main battery would kick in. I really wanted to try a 9v battery but he’s not as adventurous.
Anyway, he said he was once driving an early Prius, a company car, and ran out of gas coming down the 101 towards that truck-stop just outside Paso Robles. Everything turned off, lights, and all, and he coasted into the truck-stop. Can anyone confirm if this was a known “feature“ of the early cars, or did I fall for a tall tale?

 

[Kiwi_ME]

Kona Electric owners have reported a few dead 12V batteries which seem to originate from leaving the hatch cover slightly ajar for more than a few hours. Compounding that, the automatic timed 12V charging does not trigger while the hatch is open or ajar. And, once a 12V battery has been discharged below a certain point, it can be difficult to restore it to full capacity.

The 12V battery is however fully accessible and there is a mechanical key to open the door to operate the mechanical hood latch.

In the Kona, the 12V battery is normally charged off the main battery for 20 minutes at 14.6 V every 4 hours when parked, reverting to daily after around 60 hours. It's also charged for 30 minutes at the start of every charge of the main (traction) battery. While driving the 12V system is maintained between 13.1 and 14.6 V so there's no drain, but also notably driving the car is not a productive way to charge it, if that was needed. There is a Utility mode for that which is also used for camping or running the HVAC while parked.

Like some other cars today there is a sensor (IBS) on the 12V battery negative terminal (see photo) that logs current (by each direction) and voltage, 24/7. But the car's electronics has no way of knowing that the 12V battery is in trouble until an event occurs that wakes up the car. It will then download the IBS data and extend or shorten the next planned 12V charge period as required.

Tesla's design mistakes may be a result of compartmentalism in engineering groups or simply an over-estimated trust in the EV's ability to keep itself alive. I've seen it happen many times in my design career and certainly facepalm moments can occur. But the need for a 12V battery in EVs is going to be with us for a few more years as that's the pragmatic approach to the design engineering. Tesla will almost certainly be the first to eliminate it and I think the Model 3/Y already has some features that will push it in that direction.