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Comment by tsomctl | Hacker News Reader

that an unsuspecting driver could bump the ignition key and cause the car to cut off engine power and deactivate its air bags.

I didn't know that turning off the ignition would deactivate the air bags. I've welded on vehicles before, and have talked to body work guys about it. Everyone says to disconnect the battery so you don't accidentally set off the air bags. It can happen by stabbing the mig wire into the air bag harness, or just by letting the welding lead lay next to it and inductively causing current to flow.

Seems like if you're parked in a parking spot, and someone smashes into you hard enough that the acceleration can trigger the airbags, you would want the airbags to go off. I don't know why you would ever want to disable the airbags (except baby in car seat).

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The airbag sensors require power, and would, perhaps only eventually, drain the battery. I use a bluetooth adapter on my OBDII port and I suspect this very small device drained my battery after a couple weeks of not using the car. It would be unacceptable for cars to come off the production line that way.

I don't think any car lasts more than a couple of weeks without being turned on, OBDII bluetooth dongle or not

For the airbag I think the sensors are passive, but even if they aren't, a grace period could be added (like, it will shut down after someone shuts off the car, then opens and closes the door)

You have a problem if yours does not.

Often, the problem can be summed up as "Canada is below zero for months at a time."

The ADXL78 series [1] which is recommended for vehicle collision sensing draws 1.3 mA. If it were the only load, it would take on the order of ten years to discharge a car battery, which is well below the self-discharge limitations. I don't know if lower power sensors are available, or if some rule like "enabled for 24 hours after the car is run" would be appropriate, but lack of power for the sensor does not seem like a good reason for the airbags to be disabled.

Bluetooth devices actively searching for devices to pair with could plausibly draw significantly more power than that.

[1] http://www.analog.com/static/imported-files/data_sheets/ADXL...

> if some rule like "enabled for 24 hours after the car is run" would be appropriate

There are pressure sensors, such as those used to determine if the passenger airbag should be enabled/disabled. That should be the reference for both driver and passenger; if seat = occupied, trigger airbag if collision detected, regardless of engine status, ignition switch position, etc

If only automakers could update this sort of logic over the air...

Electric arc welding can fry the electrics (think ECU etc) on a vehicle if you don't disconnect the battery. Airbags would be at the lower end of your concerns.

It's actually better to disconnect the ECU entirely if you can. There is a large wiring harness that brings 40+ wires together into one or a couple of plugs which you can undo in a few seconds.

Plenty of ECUs have been ruined by folks getting their exhaust welded when the people doing the welding don't really know what they're doing. Especially if they normally work on older cars which tend not to have sensitive electronics.

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