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Comment by bryanlarsen | Hacker News Reader

That's not reassuring. This recall is the exception. The low voltage 12V battery has been far more unreliable in EV's from all brands than the high voltage battery has been.

Is an EV like an ICE in that the 12V bus has power while the car is running even if the 12V battery is dead? In an ICE the alternator puts 13.5 volts onto the 12V bus so a dead battery will prevent a car from starting but it will stay running on a dead battery if boosted to start. I imagine an EV does something similar but I don't know.

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If the car is on (high voltage battery pack energized) then there is 12V supplied from the high voltage pack through a DC to DC converter.

The 12V battery dying is only an issue if the car is parked and the high voltage battery is disconnected. Then there may not be enough power to 'wake' the car up again.

In a lot of EVs, when the car is "on" there is a DC/DC converter powering the 12V system from the HV battery. So if the car was "on" and experienced a loss of the 12V battery it could continue operating for some period of time.

I think the implication is that people could be driving with a dead 12V as if nothing is wrong. Then when they experience the HV failure in the recall, they would have no power whatsoever for safety systems.

That wasn't intended to be my implication. The replies to the original comment answered my question. Generally if you have a dead 12V you know it because you need a boost to get started, and you don't drive like that for long. I'm not worried about a cascade failure, I was worried about the 12V dying while somebody was driving and then having no brakes. That concern has been addressed.

Yes, this drives a requirement for latent fault detection of LV battery faults. In general when a safety function is decomposed to provide statistical safety, there is a requirement to time-bound a single failure, since without that the decomposition doesn’t buy you anything. Latent fault detection is the standard option for time bounding for automotive — for aviation, you have a second escape hatch that issues that are reliably found during annual / 100 hour inspection and can be safely missed for that long can be caught by inspection instead.

I don't know about other EVs, but in a Tesla, a failing 12V battery will be detected and the car presents a warning on your screen about it.

Mine failed after ~5 years. Replacement was inexpensive ($128) and Tesla service drove to my house to install it.

When the 12v battery is dying, at least in a Tesla, it warns you, and starts disabling certain features, more likely to help preserve the battery. For example, heated seats run off 12v. It makes sense that a lot of commodity auto industry parts would run off 12v because the supply chain is there, and because you want low voltage in the cabin anyway.

Like another poster in this thread, my original model 3 battery went ~5 years (typical 12v failure age in a car), and I bought it for $89(!!) at Tesla. Autozone wanted $125 for the same group battery. I did a DIY replacement. For some reason, that one failed after a year and a half. Just bad luck I guess.

> "The low voltage 12V battery has been far more unreliable in EV's from all brands than the high voltage battery has been."

Some EV makers, including Tesla, have switched to Li-ion (often LFP) low-voltage batteries. These tend to be better suited to EV duty cycles than lead-acid, and improve reliability and longevity, as well as saving space and weight.

I can't speak for all EVs but my Ford with a 400v hybrid system (DC-to-DC, no alternator) was able to keep operating perfectly with no 12v battery whatsoever. There was an assembly defect where the positive terminal connecting the battery to the fuse box eventually came partially loose and would disconnect as the engine bay warmed up. It would start up fine and drive with zero issues but then completely black out as soon as the car was turned off.

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