In the days of vacuum tube amateur radio mobile installations we had to keep the vehicle running if the radio was turned on or we would have a dead battery very quickly. After many arguments with customers trying to explain that it was their battery that was bad and not the radio I still have not seen a solid state transceiver that would drain the battery on a vehicle so bad the vehicle will not start after being left on for a few hours. Left on for a week for more without starting the vehicle may drain the battery but not just a few hours.
Jump starting a vehicle can be very dangerous as I learned, almost the hard way, during the summer of 1977. We were trying to start a church buss to pick up kids for Vacation Bible School. It appeared the battery was dead and it would not turn over. We pulled up another vehicle and tried to see if we could get it started. We played with it for quite a while trying to get enough power in the battery to turn over the engine.
It turned out the problem was not the battery but the starter was frozen but we didn’t find that out until after the battery blew up while I was holding the jumper cable clamps and moving them to try to make a good contact. I think God was watching out for one of his idiots when the battery exploded because I was not injured at all though my shirt was destroyed by battery acid.
If you don’t know what you are doing jump starting a vehicle can prove dangerous for people, vehicles, and electronics.
When I was working on commercial two way radios it was not unusual for me to go out on a call and find fried components inside a radio because the vehicle was jump started. After repairing the radio I would explain to the customer that he should remove the radio fuse next to the battery before jump starting the vehicle. I would also tell them that they might consider replacing the vehicle battery. It was amazing to me how many times I would have to go back out in less then two weeks and find the radio fried because the vehicle was jump started with the radio fuse still inline. Usually when I gave them the over $100 bill for travel and repair I would tell them, in the most polite manner I could, “I told you so.”
Usually the damage done by a jump start was a protection diode would short out and I could just replace the diode and have them going again. There were a few really bright customers who when they saw the radio was blowing the 7 amp fuse they would replace it with a 10 amp fuse and just keep going to higher and higher amperage fuses. It was usually when they got to a 30 or 40 amp fuse that the fuse would not blow anymore but the radio would not power up because the trace between the power input and the diode was burned out.
The safest way to jump start a vehicle is first start the engine on the vehicle that is going to be used to power up the inoperative vehicle. Connect the cable to the dead battery first and then, observing polarity, carefully connect the cables to the battery on the running vehicle. Give the dead battery time to charge. Shut the engine off and with the cables still attached try to start the down vehicle. As soon as the vehicle starts remove the cables being very careful not to short them. If you have a ham radio, stereo, or some other electronic add on remove the fuse to them before starting the process of jumping the batteries.