There are potential dangers both to you and to your vehicle during the process of jumpstarting (using jumper cables from one vehicle's battery to provide power to start another vehicle with a dead battery.). The dangers are present on both sides of the jumper cables.

The first danger I would like to mention is a danger to the electronics in the vehicle itself.

As a two way radio technician there was an interesting problem I saw many times over. I won't go into detail as to what the problem was except to say that the radio would consistently blow the fuse even if the radio was turned off. I was able to trace the problem down to a voltage spike caused by jumpstarting the vehicle or using the vehicle to jumpstart another vehicle.

Modern cars are full of electronics that can fail for the same reason those radios did.  Voltage spikes as high as 300 volts have been recorded and that is much higher than a 12 volt system is designed to handle. Fortunately it does not happen often but voltage spikes can and do damage and destroy vehicle electronics.

The next danger is the possibility of an exploding battery. I know this can happen because I have personally experienced it when trying to jump start a church bus to pick up kids for vacation Bible school. It turned out that it was not the battery that was bad but the starter motor. Fragments can fly off the battery like shrapnel from a bomb and battery acid can burn skin and even blind if it gets into your eyes. I was fortunate to only have some clothes destroyed by the acid.

The third danger is sparking caused by the hook up. These sparks can be dangerous to your eyes, so when jumpstarting a vehicle the person connecting the cables should always wear eye protection. Another danger presented by sparks is the possibility of fire if there are any gas fumes inside the engine compartment.

Dead batteries happen to all of us. Maybe the lights get left on or the battery is just getting old and we need a new one.

Try to avoid jump starting your vehicle. If possible, it is best to put a battery charger on the battery and let the charger charge the battery. The next best thing is to call a professional tow company to start your car.

Jump starting should never be attempted if there is the smell of gasoline in the engine compartment of either vehicle.

If jump starting is the only solution then use the following methods.

First remove any accessories such as radar detector, CB radio, and cell phone are unplugged or inline fuse removed. Turning off is not sufficient you must remove any potential of damage by unplugging them or removing inline fuse. Check and confirm that lights, radio, A/C, blinkers, and any other systems that can be switched off is turned off. This includes the dome light, so shut the door.

Carefully check the battery polarity. Do not trust the red wire from a battery is positive and the black wire is negative; look at the markings for the terminal. With the engine on the vehicle with the good battery connect the jumper in the following order: one end of the red lead to the positive terminal of the dead battery, the other end of the red lead to the positive terminal of the good battery, the black wire to the negative terminal of the good battery and finally connect the other end of the black wire to a clean paint free spot on the engine or frame of the dead vehicle. Negative terminal should not be directly connected to negative terminal of the dead battery; while making the connections use caution not to let the ends of the cables touch each other.

After establishing contact start the engine on the working vehicle and allow the dead battery to be charged before attempting to start. It is best to place enough charge into the dead battery to start the vehicle with the jumpers removed but is this is not possible shut the engine off on the vehicle with the good battery before trying to start the other. Both vehicle alternators should not be operating simultaneously while connected.

When the vehicle starts immediately disconnect the cables in the reverse order they were connected.

Being a nice guy could cost you a lot of money. It is not a wise decision to try to help a stranger in a parking lot who has a dead battery by giving him a "jump." Modern cars are just too prone to damage to either vehicle to take the chance.