I’m on the fire department and know that vehicle airbags are powered by capacitors that aren’t discharged immediately by the battery being removed (by cutting the cables). We were told (in the academy (12 years ago eek, i’m old)) that they’ll discharge in 30 minutes so always treat airbags as ready to deploy.
Recently a certain member was told by an “Engineer at Ford” that “designs all the capacitors” the capacitors will discharge in all vehicles after 5 minutes and in most vehicles is actually closer to 1 minute.
Is there some online reference that has some hard numbers on how long it actually takes to discharge the airbag capacitors?
It would be interesting to know if they are prevented from discharging back through the same wires that power them. Do they have something like a diode to prevent this or a large value resistor limiting discharge back to the car system, or are they isolated and a large value resistor discharges the capacitor directly? Do the sensors remain active until the capacitors discharge or is it just the explosive device that is the only concern? This was directed at the readers of this thread in general that might know this.
The wires that power them are likely to be either +12 volts (when key is on), or open-circuit (when key is off) - unless there’s a deliberate relay in there that disconnects the 12 volts and then connects that line to ground (through a resistor) when the key is turned off (or the battery lead is cut). That would have to be a deliberate design feature for the purpose of “safing” the system, and it sounds like that’s something that’s lacking (or at least not universally mandated).
If they don’t discharge back through the feed wire then shorting the the cut power cables could discharge the system almost instantly. This is why I ask for somebody that knows their design to input. Something as cheap as a single diode can work in place of a relay to prevent the current from a discharging flow.
ghostman is a Fireman and probably doesn’t care about the technical aspects of the circuitry in so much as the hard data relating to how long he has to wait…
Any ways, I believe it will depend more on the car than anything. My old 1997 Acura CL had a warning statement to wait at least 20 minutes before attempting air bag servicing. My current car, a 2004 Altima, states to wait at least 3 minutes before servicing the airbag. So, there is some obvious variance there.
Unless there is some standard for air bags set by the NHTSA then there will be variance as each car is built different and not only that, from different countries, and therefore different manufacturers. Even if two different brands use the same air bag itself, who is to say that the control mechanism of the SRS is the same? It will probably be different as SRS not only controls air bags but anything else like seat belt tensioners, etc.
Plus, age of vehicle will be a big factor. They may have had longer times for older cars as the technology was still new and they were unsure of reliability or maybe the explosive charge to expel the airbag required was then larger. Therefore requiring a larger capacitor to deploy.
A current generation or maybe even a generation ago might fall into the 5 minute rule. An older vehicle is up in the air.
That’d be a good paper to author for a Fireman, call up each manufacture and inquire and then publish it. You’d be famous!
This whole discussion stemmed from the fact that we <don’t> wait for the airbag capacitors to be discharged, we just always assume they’re live and deal with it. We cut the battery, but it’s not our #1 concern cause we’re not gonna wait 20 minutes to start cutting the car. If it was 30 seconds for the caps to discharge, then we’d consider changing our policies, but I think that we need to keep em the same.
Does anyone have an online service manual that mentions waiting X minutes for airbags? I did a quick search and all the actual service manuals are pay only.
As someone mentioned upthread, there is an outside chance that shorting the battery cables together would discharge the capacitors. You could add that step to your policy. Couldn’t hurt.
is there a possibility that shorting out the cables would cause any other hazard? also, i thought the capacitors were wired in a way that prevented that from discharging them… in case of an accident where the battery cables got shorted you would still want the bags to deploy…
IANAF, but it seems to me that this is exactly the kind of question national organizations love to find the answer to. There are national fire safety organizations. Contact them and they should contact auto manufacturers and federal regulators to locate an answer. Of course as has been pointed out, the answer is almost surely going to be somewhere between a few seconds to 30 minutes depending on the car and and liability conciousness of the manufacturer.
I just checked the BMW service site (I have a paid account there) and it says:
*Work on components of the airbag system should only ever be carried out with the battery disconnected, the negative terminal post covered and the plug connection of the cable leading to the gas generator disconnected. If only the battery is disconnected, the following prescribed waiting period must be observed without fail:
30 minutes for vehicles up to 9/93;
1 minute for vehicles from 9/93 *
I don’t think it is practical to compile a list of all possibilities and refer to it in an emergency situation - you’re going to need to look at the VIN in many cases to make the determination. Plus, there may be aftermarket modifications (see below) that render the factory instructions irrelevant.
You will probably eventually encounter a car that has a second battery, probably because of a huge stereo system being installed. Those installations also tend to have very large capacitors.
You may also find airbag-type detonators in unexpected places on some cars. In addition to the standard 8 airbags on a BMW, the seat belt tensioners have pyrotechnic devices. There’s also an explosive disconnect on the main battery lead (and the battery is in the trunk where you wouldn’t expect it). Even after that battery disconnect fires, there is still a connection to the battery - if the BMW’s computer decides that the car is having a major accident, it blows that disconnect and then unlocks all the doors and activates the hazard flashers.
Another similar safety issue for rescuers is the increasing number of hybrid or pure electric vehicles on the road. These have batteries with much higher total voltage and current than conventional cars; the batteries may be spread out in the car to improve weight distribution; and cables may be run through areas that you’d need to cut through to extricate victims from the car.
National Regs: That would be the National Fire Protection Agency (NFPA) and they don’t specify anything that specific.
I’m going to mention these specific cases and try to make a Suggested Operating Guideline to treat all air bags as armed and don’t count on cutting the battery cables to disarm the airbags in any reasonable time.
I recall the Chilton Guide for 1991-1994 Toyota Tercels mandate a wait period of 90 seconds between disconnection of the battery and doing any work which might trigger an airbag deployment.
I used to teach this stuff for a car maker (Volvo) and have done seminars for Fire agencies.
On a Volvo there is no time specified in the repair literature between battery disconnect and commencing work on the SRS systems. The capacitor is designed to supply power for up to one second after battery power is lost in the event of an accident.
Furthermore in all of these systems that I am aware of, turning the ignition key to the off position safes the system. While cutting the battery cable has the same effect (cutting power to the system) turning off the key is a bit of a belt and suspenders action. Jason Cravat Shorting the battery cables will not detonate the system. We do this on a regular basis, to reset the cars computers back to a zero state (erase all the adaption that has taken place. Joe Frickin Friday the bags are not supplied with either power or ground until there is an event that causes the system to trigger then both are supplied at the same time by power transistors in the control unit.
I hope this answers some of the OP’s questions. If you want you can contact me directly about this (email is in my profile)