Capacitor Drainage Vs Plug In Appliances In Cars

[MFDSSIM]

I need clarification on an issue that came up at our training academy.

The issue at hand was a question brought up during a discussion about battery (capacitor) drainage time following a car crash.

Can it be true that if an automobile were to have a plugged in appliance, such as a cell phone, laptop computer, portable DVD player, etc... plugged into the cigarette lighter, that this appliance can cause accidental deployment of airbags due to the fact that these appliances have their own capacitors & that they could (in some way) reverse the flow of current, causing the accidental airbag deployment? I'd be very surprised if this were actually true, but wanted input from the field & some Experts to come forth & clarify this issue & put it to rest...if it were unfounded!

[kd7fds]

I am not an expert on airbags, but from an electrical standpoint, I might give it a maybe.

The only reason I will say that is there are devices to jumpstart your car that you plug into the cigarette lighter. If you can charge a battery enough to start your car that way, it might be conceivable that another device plugged in the same way might be able to provide enough voltage to discharge an airbag.

More likely for devices like laptop computers, which have large batteries, than something like a cell phone with a small low voltage battery

[Guest]

On this Firehouse.com go to Training Videos and you will be able to watch a short video on this feedback question presented by Ron Moore. How much power is needed to keep the capacitor in the derm powered up, I don't have that answer. I agree kd7fds that if you can charge/jump start a car thru the cigarette lighter then why wouldn't the power from even cellphones keep the power up on the derm capacitors.

[bkmcse]

We just watched a training video with State Farm and Ron Moore and this was one of the lessons. They stated that YES this is possible and to unplug anything plugged into the cigarette lighters. This could be a cell phone, laptop, portable dvd player, video games, etc. Also with todays vehicles make sure you get them all. My car has one in the traditional front seat location and one in the center console for the back seat, a lot of vans/suv's have them in the back of the vehicle. Be careful and stay safe!

[DeputyMarshal]

I agree kd7fds that if you can charge/jump start a car thru the cigarette lighter then why wouldn't the power from even cellphones keep the power up on the derm capacitors.

Because most electronics -- cellphones, etc. -- have simple protective charging circuits designed to allow current to flow in only one direction. Otherwise they would discharge their batteries whenever they were left plugged in and the power source was turned off. (Note that the light present on many cellphone cigarette lighter plugs turns off when the car is turned off. IOW, it doesn't stay on from current backfeeding from the cellphone's battery. If you really want to test the theory, put a multimeter on the end of a cigarette lighter plug attached to a cell phone or any other appliance you wish to test. Does it show any current flow? Most likely it won't, IMHO.)

Just because it's possible to backfeed a trickle charge through a cigarette lighter socket intentionally doesn't mean that every appliance the plugs into one will do the same thing by accident. It's not inconceivable that there are cheap appliances that can back feed power but it's a good bet they're the exceptions rather than the rule.

[fdsq10]

MFDSSIM,

Why would you be surprised if this was true, do you have expierence or knowledge to dispute your instructors information as being false. You state in your post you wanted input from the field and some experts, your INSTRUCTOR was your EXPERT. I find it disrspectful when you question your instructors KSA's, what you should of done was ask your instructor to explain to you why this is possible and you might have learned something that day. Take time and view the video the others have posted put on by State Farm. STAY SAFE

[DeputyMarshal]

your INSTRUCTOR was your EXPERT

Not always.

Show you instructors the respect they deserve but don't be deluded into thinking that they don't make mistakes.

Never stop questioning "experts". Get a second opinion and then a thrid. Gather your own data and make your own judgements. Sometimes an "expert" is just somebody from 20 miles away carrying a briefcase...

(A good instructor will never find it "disrespectful" when a student asks honest questions. When you find an instructor who does it's time to find a better instructor.)

[nmfire]

You're both right. YES, most modern device will only allow current to flow in one direction. HOWEVER as far as I'm concerned, if I didn't build it, then I don't trust it. I know of devices that CAN and DO backfeed. Nobody here can point to a device and tell me "Its ok, that thing won't cause a problem". So the simple and safest way to handle this, since we don't know for sure, is to unplug the things or plan around them.

[rfdlou]

Subwoofer amps

You're both right. YES, most modern device will only allow current to flow in one direction. HOWEVER as far as I'm concerned, if I didn't build it, then I don't trust it. I know of devices that CAN and DO backfeed. Nobody here can point to a device and tell me "Its ok, that thing won't cause a problem". So the simple and safest way to handle this, since we don't know for sure, is to unplug the things or plan around them.

Many vehicles have aftermarket high power subwoofer amps that will cause the headlights almost go out when the bass hits hard. To avoid this and to help keep a steady flow of power a capacitor is added to the battery power cable at the amp (in the trunk). I was helping a kid at the firehouse install a new battery & was surprised that the hood light remained on for several minutes after the battery was disconnected. If these caps can supply enough power to light a lamp a few minutes, how long will they power the air bag systems?

[joshball]

MFDSSIM,

Why would you be surprised if this was true, do you have expierence or knowledge to dispute your instructors information as being false. You state in your post you wanted input from the field and some experts, your INSTRUCTOR was your EXPERT. I find it disrspectful when you question your instructors KSA's, what you should of done was ask your instructor to explain to you why this is possible and you might have learned something that day. Take time and view the video the others have posted put on by State Farm. STAY SAFE

I disagree, are you stating that every person that has instructor numbers is an expert?
Last time I checked, not every person that has got in front of a group of people whether it is firefighter instructor or college professor, doesn't make them an expert.

Best wishes

[BigRig]

True enough, some devices are protected from 'backfeed' of current, however I have actually used a multimeter and measured voltage at the battery cables from 'backfeeding' appliances.

I suggest caution in this respect and urge rescuers be aware of such.

[MFDSSIM]

Thanks to MOST OF YOU for your replies. As I can see there are numerous "theories" on the issue, yet various interpretations of them! Thanks for taking the time to HELP ME. Fortunately there are people out there who understand the basic concept of "asking a question" & "Answering them"...while others use forums such as this one to "strut their stuff" & "show off" without even answering the question in the first place, which in my view only "discredits" the use of forums for information exchange without it turning into a show off "I can pee farther than you" forum.
To those who actually gave constructive information & answered the question & provided some input, I say thank you...to the other one I say: "thanks for nothing".

A wise senior fire INSTRUCTOR once gave me the best advice I ever received BEFORE BECOMING AN INSTRUCTOR: "Son, if you think you know you'd better shut up UNTIL YOU KNOW, 'cause you'll end up looking like an *** for something you didn't know in the first place".

Thanks to you all...almost!

P.S. When I referred to an "Expert" I was hoping Ron Moore would throw in his view on the issue, 'cause I don't think anyone doubts or questions HE IS THE EXPERT...

[LeeJunkins]

Click on the link and you can see for sure. This is a capasitor to a sterio system. you can read the volt meter for your self 14.9 v with the battery disconnected



http://midsouthrescue.tripod.com/photo/index.album?i=48

[LeeJunkins]

It is true that many appliances may not feed back such as a cell phone however the two small prongs on the bottom of the phone are open and we use them in classes to set of airbags.
Our big concern is there is potential for electrical currant in the vehicle and with systems that can actually be set off by a rescuer sliding across the seat, it is to our best interest to elemnate as many sources as possible.

Now to the reason why it is a must A capasitor stores currant as it passes through it, to move the currant through you must have a neg ground and a pos wire going to it to complete the curcit if the battery is disconnected the capasitor is still charged, if the pos wire going to it is shorted anywhere in the vehicle that charge will go to ground causing an arc.

[nmfire]

If these caps can supply enough power to light a lamp a few minutes, how long will they power the air bag systems?

A long time. Considering the airbag system is armed even with the ignition off and doesn't kill the battery, it probably draws less current than that light bulb. Logical conclusion, those big thumpin' stereo capacitors will effectively defeat the battery removal as far as securing the airbag goes.