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SUBJECT: Vehicle Electrical Systems
TOPIC: Power Backfeeding
OBJECTIVE: Upon the conclusion of this training, the participant will understand the potential for and possible causes of power backfeeding into a vehicle's airbag electrical system.
TASK: The rescue team will develop a new guideline or revise an existing department protocol or guideline for dealing with the possibility of power backfeeding at vehicle crash incidents.
This author recently came across some rather earth-shaking information regarding the significance of portable appliances (such as a laptop or DVD player) plugged into the cigarette lighter outlet of a vehicle involved in a crash. It has always been the mindset of fire-rescue personnel that once we either cut or disconnect the battery cables on a crash-damaged vehicle and the airbag capacitor drains down, the airbag electrical system is essentially "empty" of electricity. Well, this long-standing concept may have just been shot down by modern vehicle technology.
Here is the scenario: You arrive at a crash and cut battery cables or disconnect them as per your protocol. You think the airbag system is now draining down and that the airbag capacitor will be empty in a few moments. From that point on, you think you have a higher degree of safety. After all, the battery is shut down and the capacitor is draining. That's where our thinking may be wrong.
My engineering contacts tell me that a portable device within the crashed vehicle, something that has its own battery power supply, which remains plugged into the cigarette lighter port of an automobile, can or may undergo "power backfeeding." The device's own portable battery power supply can feed current in reverse, back into the cigarette lighter outlet. Automotive engineers tell me that this is a design of the electrical system of a modern-day automobile. Auto mechanics buy special power backfeeding devices for use in repair shops. Called an "Auto Computer Memory Unit," this author bought one for $9 at a retail auto parts supply store. The unit has a cigarette lighter plug, a flexible neck and attachment points for a small battery. The memory unit plugs into the cigarette lighter and connects to a nine-volt transistor radio battery. Those nine volts of electricity are enough electrical power to keep the car's onboard computer, sometimes referred to as the "black box," charged. With power supplied to the onboard computer, the service technician can disconnect the real battery to work on the car. When finished, they do not have to reprogram the onboard computer of the car. The auto computer memory unit provided power by backfeeding through the cigarette lighter power port. The vehicle's computer remained energized the entire time while the repair work was being done.
So what does this all have to do with vehicle rescue, you may ask. Unfortunately for us, one of the circuits that power backfeeding will also keep energized is the airbag electrical system. It takes as little as seven volts of power to deploy an airbag. In other words, with an accessory device plugged into the cigarette lighter port at the time of the crash, the airbag capacitor may never drain like we thought it would when we take away the battery power! We cut the battery out of the electrical system and, really, nothing gets better as far as the airbag electrical system is concerned. It could be just as energized with the car's battery completely connected as it is without the battery hooked up. Power backfeeding is the reason.
Researchers advise that all cigarette lighter accessory power ports on vehicles since approximately 1997 model year have the capability of power backfeeding. It gets even worse. With the advent of third-row seating and seven-, eight- or even nine-person capacity vehicles, the power ports at the second and third-row seats may also have the ability to power backfeed.