AriesMu said:
OMG what's "the 12 volts" ?
By sheer quantity, most of the LEAF's systems run on plain old 12V automotive power: lights, steering, brakes, infotainment, wipers, windows, ventilation fans, etc. This power is taken from a very ordinary 12V lead-acid automotive battery that gets recharged from the main high-voltage traction battery by a DC-DC converter. I'm not fully conversant with the details, but from many horrific anecdotes on this forum over the years, it appears that Nissan heavily weighted the design of this 12V recharging system to minimize its drain from the high voltage battery, much to the detriment of the 12V battery's service life. As the 12V battery degrades, or even just temporarily becomes deeply discharged, its output voltage drops, and this seems to confuse the LEAF's computers, to the point that virtually ANY conceivable system misbehavior can happen. Radio flipping back and forth between easy listening and demagogue talk stations on its own while windshield wipers twitch spasmodically? Probably the 12V power has browned out. Car claiming to have three hundred miles of range remaining on 1/4 charge; is it a miracle? No, probably not; check your 12V battery. Passenger window rolling up and down while the headlights cycle on and off? Yep; prolly yer 12V battery, buddy...
{Joe Isuzu} You see, a huge portion of the car's functionality is implemented in software, executed by various (don't know how many) on-board microcomputer systems. Each of those requires a steady, unwavering supply of DC voltage, probably in the range of three to five volts, that has to be derived, somehow, from the "12V" battery voltage, which anywhere from nearly fourteen volts down to ten, or maybe even only nine volts! If the computer's power browns out, it can go off into unspecified firmware la-la land. Preventing this is obviously a job for some sort of regulator, but I'm sure you can appreciate how impossible it would be to construct something that could guarantee to deliver THREE volts from a power input that might only be no more than three times as much! And who's to say that severe battery degradation/depletion wouldn't lead to an even deeper cut in battery voltage? I mean, it's not as if the onboard diagnostics would be in a position to note such an event in the non-volatile record of "trouble codes", because in order to do
that, the monitoring computer(s) would have to be able to remain in operation for several milliseconds AFTER the battery voltage dropped below some level of concern. What with the onboard computers requiring many hundreds of watts (remember, they have very nearly as much computing power as a cell phone) just to keep their electronic thumbs twiddling, you'd need a storage capacitor of several thousand microfarads in size to give them that much "supply hold up". Far too large and costly to justify. {/Joe Isuzu}
