Since both batteries should generate the same voltage, the voltage between the two positive terminals or between the two negative terminals should be 0, meaning no electricity will flow. So how does this result in the dead battery being charged? The answer lies in the side effects of draining a battery. Batteries die not because they lose voltage or the ability to store charge, but because resistance between the terminals increases as the internal electrodes corrode. However, most battery types — especially the lead-acid batteries used in cars — also lose a bit of voltage as they lose charge this is how battery indicators know when a battery is dying.
In the case of car batteries, a fully charged battery will measure about Thus, there actually is a voltage between the two batteries which will conveniently cause charge to flow from the full battery to the dead one.
Though you can connect the terminals in any order, you never should. Here is the usual order for connecting battery terminals:. There are two reasons for this ordering: first of all, creating a short-circuit could cause the battery to overheat and explode, spewing dangerous acid all over the place — this ordering minimizes the chances of a dangling cord accidentally causing a short-circuit.
Overcharging a car battery produces hydrogen gas, the explosive gas responsible for the Hindenburg Disaster. This not only explains the ordering, but why you connect to the chassis instead of the negative terminal — the circuit stays complete either way because the negative terminal and the chassis are electrically-connected in all cars unless a wire broke! Two minor points: The minor voltage difference is between a full and an empty car battery is not enough to charge the empty one in reasonable amount of time.
Also, if you waited long enough then you would end up with two batteries that are half full since the net capacity has to stay the same. Tip 6: Timely Up-Keep Always have a qualified professional maintain your electrical system and make sure to have quality replacement parts installed when existing electrical system parts fail.
Quick Tips: Jumping a Battery Tip 1: Never connect the black cable to the negative — terminal on your dead battery. Top Questions and Answers. How do I jumpstart a battery?
Quick Tip: Never connect the black cable to the negative — terminal on your dead battery. This is very dangerous, could result in a possible explosion. How do I reconnect loose battery wires? Knowledge is power. To keep batteries powered-up. Stop in for a complete electrical check. Read up on our current battery deals and specials.
Test your car battery life with our Virtual Battery Tester. Find the right battery for your exact vehicle — at the right price. Enter your zip code to find a store near you. Up Next. Get to know your car's battery and how it powers your journeys.
Published January 29, Updated December 28, Best Car Batteries of More From Consumer Reports. Common Steps to Jump-Start a Car.
The vehicles shouldn't touch one another. Turn off the ignition on both cars. Next, connect the negative cable to the negative terminal on the good battery.
Finally, connect the other end of the negative cable to a ground on the vehicle with the dead battery. This can be the engine block or another metal surface away from the battery. Be careful not to touch the two ends of the cable together while doing this. Start the rescue car that is providing the electricity. Start the car with the weak battery. If it does start, let the problem car run for at least 20 minutes to allow the battery to recharge before shutting it off. Call a local service station for help.
Disconnect the cables in the reverse order.
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