Can you use two jumper cables at the same time for more power to jump a car?

Answer 1

Short Answer:

Using two batteries connected in parallel will allow you to charge a dead battery faster and start a car easier. Two cables from one good battery to one dead one is almost useless.

You can absolutely not connect two charging batteries in series (i.e. two 12 volt batteries to get 24 volts) to a dead battery. You can certainly connect two charging batteries in parallel (i.e. two batteries at 12 volts to give more current at 12 volts).

Explanation:

If you look at some of the professional jumper set ups, such as used by highway rescue services, you will notice that some have two or three batteries connected together. There is a reason for that beyond simply the convenience of being able to wheel the batteries up to the disabled vehicle. A single batter is capable of supplying one amount of power but two batteries can supply twice as much.

It takes power, not just voltage, to start a car. (Otherwise, you could start your car with a bunch of flashlight batteries strung together.) Anyone who has done this knows that when jumping a dead battery (really dead, not just anemic) you need to connect the good battery and dead battery for several minutes before there is a chance of starting. The reason is, in large part, that the connected good battery is charging the dead battery and that takes a while. If you try to start the car with the charging battery attached, then the charging battery is doing two jobs, starting and charging the dead battery. If you wait for a while, the dead battery gets some charge and stops being a large drain and if you wait long enough, the dead battery will have enough energy to help start the car along with the help of the good battery.

There is more to the story though. With jumper cables you are often not making a really good connection and the very large power drain necessary to start a reluctant engine has to also overcome the poor electrical connection made by both ends of the jumper cable. The relative importance of the poor connection varies greatly because the quality of the connection depends greatly on the person doing it and the corrosion present and the tightness and area of the clamping contact. If there was enough room to connect two cables between the one good battery and the one bad battery, then that connection area would be larger and offer some potential improvement, though just waiting for five minutes is a lot easier.

There are other considerations. If you are connecting two good batteries to one dead battery, then it would be very desirable that both good batteries are truly fully charged, otherwise the imbalance will tend to have power taken from the battery of greatest voltage to both batteries of lower voltage, though small voltage differences of a few tenths of a volt wont matter since the dead batter is presumably a drain to both good batteries.

To connect two batteries to a dead battery, all the positive terminals need to be connected together and all the negative terminals together. This constitutes batteries in parallel. A few decades ago, when auto electronics were essentially nonexistent, the practice of putting to batteries in series to make double the voltage was called a "hot shot." Such a practice now puts the cars electronic system in danger of being fried, so should never be done.

Finally, it should be noted that when jumping a dead battery, it is almost always done with the good battery in a car with the engine running. This causes the charging system to be active for the good battery. Charging systems typically provide 14 volts when charging and thus when connected to the dead battery, an greater source of power is being provided by the higher voltage at the good battery connection. Indeed, probably more energy is put into the dead batter by the charging system over a few minutes than is put by the actual good battery.

For simple problems corrected by a small addition of energy, most of what has been said is irrelevant and just connecting the jumper cables gets the car started. It is in the case of cold and dead batteries where more power is needed and more of the energy has been depleted from the dead battery that these additional considerations are the key to success. Ultimately, starting a car with a dead battery needs to get enough energy into the dead battery that both it and the good battery working together can accomplish the task. Time is an asset. Give the dead battery enough time, a few to twenty minutes with the good battery in a running car and the charging system doing its job is typical.

Alternate Answer:

Using two jumper cables between two vehicles at the same time would have minimal effect at increasing the voltage transfer between the two vehicles. As long as the one set of jumper cables is in good working order there would be no need to use two sets. However I have seen instances where using two sets of battery cables between three vehicles (two vehicles being used to jump start one 'dead' vehicle) was necessary. These were big trucks with multiple batteries in extreme cold conditions with professionals doing the jump start. Always use caution when jump starting any vehicle.