U.S. Department of Transportation National Highway Traffic Safety Administration
Reseach Note  
April 1997  

Disconnect Vehicle Batteries Safely

Have you ever arrived at a crash scene and had something unexpected happen to you while securing a vehicle? Does your training lead you to believe that disconnecting the negative cable on a battery removes all power from a vehicle? A simple check may prove you wrong and could protect you from injury!

An early result from fire safety research being conducted under a settlement agreement between General Motors Corporation (GM) and the U.S. Department of Transportation (DOT) has identified important information that the National Highway Traffic Safety Administration (NHTSA) believes may be extremely valuable to initial responders, but may easily be overlooked.

During a recent crash test conducted by GM, simulating an impact by one vehicle to the left front corner of another vehicle, with a closing speed of 65 mph, an interesting and unexpected result occurred. After the vehicle came to rest, the fire department approached the vehicle to inspect for post-collision fire. Fire officials noted that the battery had broken open and smoke was rising from the battery. The fire officials disconnected the negative battery cable, and thought that the vehicle was secured - that the vehicle's electrical system was no longer receiving power from the battery.

Several minutes later, however, an observer noticed that the vehicle's tail lights appeared to be "on." Since the test was being conducted outdoors, it was first assumed that this was merely a reflection from the sunlight. It turned out, however, that the taillights indeed were still "on." The fire department was asked how this could be possible when the vehicle's battery had been disconnected.

A careful post-collision "tear-down" revealed that a sheet-metal screw in the vehicle's power distribution box had penetrated the side of the battery during the collision, creating a circuit for the electricity to flow. Even though the battery was severely damaged during the crash and three of its six internal cells had lost battery acid, the battery still generated approximately 7.5 volts. The power and heat generated by the electrical current flowing through this unfused circuit were sufficient to light the vehicle's tail lights and to start a fire in the vehicle.

The message to initial responders is that a simple inspection of a vehicle during the "securing" procedure should include a check for anything out of the ordinary. Is the vehicle's electrical system still receiving power even though the battery has been disconnected? (Radio on? Dome light? Wipers? Outside lights? Dash Lights? Blower motor?). All of these should be "off" if the battery is no longer connected.

If there is any evidence that power from the battery is still running any part of the electrical system, check that the negative cable has been disconnected from the battery. Then, carefully disconnect the positive cable at the battery. Use care not to touch any metal part of the chassis with the cable or your tools, to minimize the risk of creating a spark. Check the electrical system again to make sure it is not receiving power.

Taking these precautions is especially important for initial responders during extrication of victims in air bag equipped vehicles when the air bags have not deployed. Although most frontal crashes that are severe enough to break open a battery (such as occurred in this crash test) are also severe enough to deploy an air bag, all precautions should be taken to ensure that an air bag does not deploy during extrication.

Because the results of this crash test were unexpected, NHTSA would like to receive information from initial responders about similar situations they may have experienced. In particular, the agency would be interested in receiving information about fires, shock hazards or similar occurrences involving vehicles that were believed to be safely secured, as well as information about experiences of initial responders with hard-to-reach batteries. If you have information, please contact Mr. Carl Ragland at NHTSA on (202) 366-4728 or through the Internet on CARL.RAGLAND@NHTSA.DOT.GOV. This information could be very helpful for future research and education efforts.