The Good the BAD and UGLY experience Of Cross Country Trip

[DrChaos]

i really appreciate this information. i really do. once we reached home i did get the 12v battery checked and no issue.

I'm surprised then, as a sudden shut off when the display shows the HV battery still has significant charge left is a symptom of a 12V failure. There is some other rare failure I guess. The older model S may be different, but all current cars have a buffer below zero so that 0% displayed state of charge is not actually the bottom, you can go to -4.5% when new maybe less when aged.

 

[ewoodrick]

Unfortunately the drain patterns of an EV computer & charger on a 12V lead acid seem to result in more battery wear and failures than a typical ICE car. On an ICE car the use pattern is a strong discharge for the starter motor, and then immediate recharge back up to maximum. Apparently that helps preserve lead acid life--both the high current discharge desulfates the electrodes and recharge backup helps.

EVs have different computer-like drain patterns. Notice how few laptops ever used lead-acid batteries? Because even back in the 1980s they found it would kill the batteries quickly and be heavy. The first practical laptops were NiCd or NiMH. The Mac first Powerbook 100 had a lead acid but they immediately replaced it with a different battery type---discontinued after less than one year.

When I had a BMW i3 the most common response in the forum (they even made a meme about it with a wizard and a crystal ball) for almost every inexplicable failure was "it's the 12V". Other EVs with 12V lead acid also report similar problems. Tesla attempts to give some warnings for imminent failure but there is no reliable way to do so because external voltage measurements don't give 100% reliable insight as to what is physically happening chemically on the electrodes.

If someone has a long trip with family again and the 12V battery is old I would get it checked ahead of time or take a replacement along ready to go.

After 2022 I think all Teslas have a lithium ion NMC accessory battery---and failures are negligible so far. Unfortunately not a drop in replacement--the electrical system is different.

I know only of the Porsche Taycan which also follows, other EV makers will discover this same problem on their own or not be willing to address it yet.

I think that you are simplifying and leaving out some cases, like prolong lights left on or doors open listening to music.
A Tesla will not cause the deep drain as the ICE does.

The rated life of both is effectively 3 years. Go beyond that and you are living on borrowed time. Sure, you can maybe get a lot longer. And you can also wake up to a dead battery in the morning!

 

[ewoodrick]

I'm surprised then, as a sudden shut off when the display shows the HV battery still has significant charge left is a symptom of a 12V failure. There is some other rare failure I guess. The older model S may be different, but all current cars have a buffer below zero so that 0% displayed state of charge is not actually the bottom, you can go to -4.5% when new maybe less when aged.

It doesn't take much to cause some failures.

A way to look at it is that the HV battery is worthless without the 12V battery. It's the 12V battery that drives all the circuits to engage even the contactors on the HV battery.
If the 12V battery even burps, it's going to shut the computers down. Once the computers go down the main contactors disengage, but most importantly, the intelligence to charge the 12V battery can't tell the HV battery to charge it.

A number of years ago I had a straight shift Ford Probe. The battery died, So I thought that just roll it down the hill and use the clutch to start it.
It didn't work. It took a few minutes to dawn on me.
Electronic ignition and other things!!!
Without the battery, the stuff needed for the engine to run wouldn't turn on.