Vehicle power feedback problem

[Guest]

Just watched the video from Ron Moore on firehouse.com about disabling car batteries at MVA's to drain the airbags capacitor. He made a point on how if any electronic devices are plugged into the cigarette lighter that the capacitor never drains and maintains its charge until these items are unplugged as well as the car battery disabled.

Anyone else heard of this?

[Guest]

Sorry, the video is located at the link below, halfway down titled university of extrication disable the battery and power feedback problem

http://www.firehouse.com/videocenter/

[Guest bibbles10504]

I just watched the video, It tells you a lot. I havent ever heard of that before but it does make sense. I am assuming thats how those easy jump starters work, where you just need to plug them into your cigarette lighter and it will charge your battery.

[Guest Photounit]

I have not seem the video but I think things may have been exaggerated a bit..... Anything that is plugged into the cigarette/accessory outlet will draw power and not back-feed power into the cars electrical system ie... Cell phone, radar detector, scanner, etc..... Most of these devices have internal voltage regulators or at least a reverse polarity protection diode that would prevent this and even so, any engey these devices could be capable of producing would be negligable.... If you happen to have one of the previously mentioned "easy jump starters" plugged in to the accessory outlet to recharge it and then you crash, then that's another story........These devices are designed to supply a significant amount of power into the vehicles electrical system and I would agree under this scenario, it could cause problems..... The one thing that is another possibility are these huge capacitors that are added to the high powere stereos you here while you are trying to sleep at 3 am........ With values in the farads, they store large amount of power and are tied directly across the battery and could also be an issue.... Anything else, I doubt it........... Once the battery is cut, there are so many parts of the car looking for power now that once the battery is removed, the power will be depleted quickly by these devices........ Maybe we should be metering the battery leads once they are cut? I guess I should watch the video but I have to say I'm sceptical.....

[IzzyEng4 427]

Yes I have, every electrical device does have capacitors which their purpose is to store energy and regulate it through the electrical system of the device. Because the jack is still pluged into the cig lighter, the capacitors still will retain power and even some time after they are unpluged. Also the hard time I have about this like everyone else says is the feedback into the car's system. It plausible but more than likely will not happen unless the plug / device's regulator fails.

I can see if the device is a 12v to 110v converter as a possibility or something larger like those little portable refrigerators. But as a person who cautions around electricity, regardless of the source, I wouldn't even take the chance and if I could try an remove the device. Though it may be low volts, the amps some of these devices can really wake you up when you get jolted.

The thing I would be most worried about though are the capacitors used in car to power stereo amplifiers, those suckers pack a punch and they would be a more likely cause for back feed if they are not disconnected from the battery and their positive and negative wires separated from the electrical system (aka cut them too).

[Guest Photounit]

If the regualtor failed or what ever, I still don't see it happening....Most of these devices caps self-discharge (or the circuit itself does it) once the power is removed most likely within a minute or less..... Long before we are dispatched.... Even the cap for the airbag does this too although I'm sure they are using a very high quality, high value cap that has a low self discharge rate and high energy density often called a "super cap" and I would bet this cap is pretty small.

I guess I'm skeptical because I repair hi-tech electronics for my employer (And myself) and I have had the oppoutunity to work on such a varied amount of equipment/devices and I have yet to see anything in the course of my repairs that would produce the phenomenon described except for super caps or the large farad caps that are used to pump up the bass response in the high power stereos and even these will self discharge most likely before we are notified........

I'm not going to say any more until I see the video.... I've said too much already

Here's a link to a simple explanation of a super cap which is what they use to supply the ignition current for the airbag.......

http://www.doc.ic.ac.uk/~matti/ise2grp/ene...port/node9.html

[Guest seanc]

i actually heard about this while going through firefighter 1. it seem somewhat plausable, but as someone said above i belive mmost electronic divices have regulators to keep from the plug draining power instead of taking in. so its an iffy subject, if you see a cell phone plugged into the ciggarett lighter in a car crash, it takes about 2 seconds to rip it out, so why dont you

[Monty 35]

Sounds like something for Myth Busters!

Must admit, though I'm inclined to agree that anything plugged in will help discharge a capacitor more quickly. If there is something plugged in that is being charged, like a cell phone - and even if it did backfeed, it would become discharged pretty quickly I'm sure, given your typical electrical load of lights, radio, heaters, fans etc.

[Guest Photounit]

Okay.... I watched the video..... And I'm still not buying it..... Ron has done a lot of good stuff but I have to question this one..... I would like to see some science to back up his claims so I guess I'll have to provide my own.....I am going to try all the cell phone/car charger combinations that I can get my hands on and measure the input to the charger while unplugged to see if any will provide any power at their inputs..... Anybody want to bet? And yes, Myth Busters would be a good one.....

Another thing to consider regarding cell phones is that almost all cell phones now use Lithium Ion batteries. We have all heard stories about the LI laptop and cell phone batteries that have exploded and this IS NOT a myth...... One of the biggest drawback to LI batteries other than they can be volatile is they need to be accurately and meticulously charged. And because of this, each battery itself has it's own charging circuit built within the battery , additional charging circuitry within the device it powers as well as Power Management Unit (PMU) chips in the devices that squeeze every bit of energy out of a battery to increase the devices run time..... With all 3 of these in place, it is highly unlikely IMO that the cell phones battery is going to provide any power to anything other than the phone...... I will find out.....

This reminds me of a cell phone causing the fuel tanks of cars to explode while people were refueling fiasco...... This has never been proven and the most likely cause of the few incident that have occurred was static electricity..... There was local incident that occurred several years ago where a cars fuel tank ignited while the driver was talking on his phone. In the local paper the next day, the Chief of the FD that handled the incident was quoted as saying the cell phone was the ignition source.... The next day he retracted that statement..... Read this -

http://www.snopes.com/autos/hazards/gasvapor.asp

I still see the warning stickers on gas pumps today telling you not to use your cell phone and it may even be a law...... The claim was that the RF from the cell phone is the ignition source.... A modern portable cell phone emits 600mw max (often lower) of rf power at 800 mhz or 1900 mhz...... I always wondered why we haven't heard of any emergency vehicles exploding from the use of 800mhz trunked radios while they're refueling..... These radios emit 5 WATTS so there is surely enough RF to ignite the gas vapors with these radios...... And yes these radios are intrinsically safe, but that has no effect on the RF properties of the radio....

If all this is true, I'm going to have to market a new device for safe extrication..... An intrinsically safe bleed-off resistor..... You would connect this across the battery cables you just cut and turn the switch on which would put the resistor across the vehicles power supply and dissipate any power that may be present without the risk of producing any sparks......Anybody want to provide some capital for me to get started?

And remember, you heard it here first!