What happens if you connect negative to negative car battery

“Positive first, then negative. When disconnecting the cables from the old battery, disconnect the negative first, then the positive. Connect the new battery in the reverse order, positive then negative.”

When you are replacing your car battery, It isn’t always easy to remember the order in which to disconnect and reconnect the terminals. Nevertheless, it is important to connect them in the right order.

When removing the old battery, remember to first disconnect the cables from the negative terminal, which is usually black and has a minus (-) sign then disconnect the cables from the positive terminal, which is usually red and has a plus (+) sign. Take caution at all times to never touch a metal object to both terminals of the battery at once. 

After disconnecting the terminals, unhatch the clamps securing the battery in place and slowly lift it out of the battery tray.

Before placing the new battery in position, ensure that both terminals and the cables are rust-free. If they are rusty, you can clean them with water, baking soda, and a wire brush. 

Next, lower the battery onto the battery tray with the terminals in the right position, and clamp it into place. Now connect the new battery in the reverse order, connecting the positive terminal first then the negative. Make sure that the battery is secured in place!

Take a look at any battery, and you'll notice that it has two terminals. One terminal is marked (+), or positive, while the other is marked (-), or negative. In normal flashlight batteries, like AA, C or D cell, the terminals are located on the ends. On a 9-volt or car battery, however, the terminals are situated next to each other on the top of the unit. If you connect a wire between the two terminals, the electrons will flow from the negative end to the positive end as fast as they can. This will quickly wear out the battery and can also be dangerous, particularly on larger batteries. To properly harness the electric charge produced by a battery, you must connect it to a load. The load might be something like a light bulb, a motor or an electronic circuit like a radio.

The internal workings of a battery are typically housed within a metal or plastic case. Inside this case are a cathode, which connects to the positive terminal, and an anode, which connects to the negative terminal. These components, more generally known as electrodes, occupy most of the space in a battery and are the place where the chemical reactions occur. A separator creates a barrier between the cathode and anode, preventing the electrodes from touching while allowing electrical charge to flow freely between them. The medium that allows the electric charge to flow between the cathode and anode is known as the electrolyte. Finally, the collector conducts the charge to the outside of the battery and through the load.

On the next page, we'll explore how the cathode, anode, electrolyte, separator and collector work together to produce an electrical current and keep your portable devices going strong.

The - end of the charged battery is already connected to the chassis, the engine, and, particularly, the starter motor. The whole car is designed to work that way. Everything use earth return. All the lights have one wire, and one connection to the chassis. The spark plugs have one wire, and one connection to the engine block. And so on.

OK, on a modern car you use a second wire for the starter motor, because it draws a lot of current and you would get a significant voltage drop and power dissipation in the earth return path. And diesel engines don't have spark plugs. And high power driving lights are driven through a relay like the starter motor is. And lights mounted in plastic fittings need 2 wires.

Given that the -ve end of the battery is connected to the chassis and the shell and the engine, where is the best place to connect the return wire?

Traditionally, you connected it to the chassis or the shell or the engine, so that you didn't have to lean, with a live wire, into the engine compartment, across the engine (hot moving machinery), on to the battery (hydrogen explosion risk, with sulphuric acid as well). But this assumed that the battery was difficult to reach, and that the engine and chassis had a very good connection to the battery (required for the starter motor earth return).

Nowadays, some people make the return connection direct to the battery, if they can reach it easily.

Even if you connect one end of the return wire to the chassis, you normally connect the first end direct to the first battery. This because until the wire is connected to the first battery, it is not a live wire, will not spark when you touch it to something, is not particularly dangerous. After the return wire is connected to the first battery, it is live, and is dangerous. So you connect it to something safe that is easy to reach.

No, you can not connect the return wire to a third car. Cars sit on rubber tires, and the tires insulate each car from other cars. You need a complete circuit from your battery to the other battery.

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