[sandworm101]

I know lots of people. The bulk of residential off-grid solar installations use lead-acid battery banks. Most all RVs and boats use lead-acid. And virtually every new car/truck/motorcycle/ATV/snowmobile/jetski, including the hybrid cars, ships with a 12v wet lead-acid battery.

[xxpor]

Even full EVs have old school 12v lead acids in them because they still have a full 12v system to run ~everything in the car that aren't actual traction motors (for compatibility with the existing automotive supply chain if nothing else).

There's a regulation at least in the US that says the HV system also has to be physically disconnected when the car isn't being used so if the 12v battery dies you're still stuck like a regular car, because you need the 12v battery to close the HV relay.

[hanche]

I have owned no less than three Kia Soul EVs. The first two would succumb to a flat 12V battery, but the latest (a 2020 model) will connect the HV battery to charge the 12V battery if needed, even when the car is parked and switched off. You can see it in action, as a blue light on the dash will flash while this goes on.

[xxpor]

Oh that's neat. I hope tesla adopts that, because one of the biggest issues has been the 12 v batteries dying quickly. I'm not sure if anyone's figured out the root cause, but IIRC people think that they abuse the battery in some way that an alternator wouldn't.

[pcdoodle]

I'm surprised this wasn't thought of from the very beginning. Must be the Big Battery Lobby.

[sandworm101]

That's interesting. I hadn't realized that full EVs still needed 12v batteries. I have jump-started a couple hybrids over the years (lights left on etc) but never a full EV. I would love to make a meme out of a Tesla needing to be jumped by my little honda.

[function_seven]

I remember my surprise when I learned EVs have regular car batteries in them.

The funny thing is, I can imagine a future where "jumper cables" isn't a hefty set of thick wires intended to supply the current needed for a starter. Instead, people will have a 20' long cable with cigarette lighter plugs on both ends. The HV relay can't require that much current to close, right?

[sandworm101]

Maybe, but the dead battery is still in the circuit. So a tiny wire won't provide enough current to raise the voltage enough across the entire circuit. (I assume that it requires something akin to a the solenoid of a IC car.) You don't need the cranking amps, but you will still need something thicker than a USB cable.

[function_seven]

Ah, good point. I guess it matters whether your battery is drained, or dead. Sometimes the battery just doesn't have the juice to turn the starter (you hear the clicking of the solenoid when trying to start, but no cranking). Other times it's so depleted that an external current supply will be sucked up by the thirsty battery.

[hanche]

It probably won’t take a large current, but I expect you need more than just enough to close a relay. Most likely, you’ll need enough to power some CPUs and data buses as well. That’s still tiny compared to the current required to run a starter motor, though.

[amluto]

There’s a different problem with your scheme. Tesla doors won’t unlock if the 12V system dies.

[8note]

I could see it being a push button switch instead, manually closing the relay.

But if the 12V is dead, the stuff running off of it will also be out

[mikestew]

Our Nissan Leaf has a 12 volt lead-acid battery. I guess that was easier than bleeding 12 VDC off the traction battery to run the radio.

[numpad0]

Actually it’s normal for heavy machineries to have separate low voltage power for computers and high voltage large current for actuators. Even open source 3D printers has provisions to do that if so desired.