Relativity is usually something said to affect only things moving at speeds close to that of light. But now calculations show that the common car battery works because of relativistic effects.

The work was done by a team from Sweden's Uppsala University and the University of Helsinki. They looked at how much energy a lead-acid battery produces if one doesn't take into account relativistic effects. Then they did the same set of calculations including them.

What they found was that a large portion - some 80 percent - of the 2.1 volts a battery produces was from relativistic effects.

What happens in the battery is that the electrons around the metallic lead sometimes get very close to the nucleus of the atoms. That makes them move much faster. Relativity says that the faster an object moves, the heavier it is.

When the electrons get more massive, the shell that they move around in gets smaller. That increases the amount of energy in the shell, and thus the amount of energy in the electron. Those electrons in the lead get released when the lead reacts with the acid in the battery. The electrons travel to the other battery terminal, which is made of lead oxide.

Without the extra energy from the relativistic effects, the battery current would be much weaker, and cars would not start.

Pekka Pyyko, a professor of Chemistry at the University of Helsinki who led the research team, noted that this explains why there is no such thing as a battery made from tin, germanium, silicon or carbon, all of which have a similar electron configuration to lead. In those cases, the amount of energy the electrons gain by zooming in close to the nucleus is simply too small.

The lead-acid battery isn't the only place that relativity manifests in everyday objects. GPS systems have to take relativity into account to keep accurate positions, and gold is yellow because of relativistic effects when light hits the atoms. But this is the first time such a calculation has been applied to batteries, Pyyko said.