The "self-recharging" features of batteries is most noticeable in a car battery. In some cases you can crank the engine until the battery seems totally dead, then come back an hour later and crank it again. The higher the drain on the battery (a car's starter motor is an incredibly high-drain device!), the greater the effect.

To understand why this happens, it is helpful to understand what's going on inside the battery. Let's take the simplest zinc/carbon battery as an example. If you take a zinc rod and a carbon rod, connect them together with a wire, and then immerse the two rods in liquid sulfuric acid, you create a battery. Electrons will flow through the wire from the zinc rod to the carbon rod. Hydrogen gas builds up on the carbon rod, and over a fairly short period of time coats the majority of the carbon rod's surface. The layer of hydrogen gas coating the rod blocks the reaction occurring in the cell and the battery begins to look "dead". If you let the battery rest for awhile, the hydrogen gas dissipates and the battery "comes back to life".

In any battery, be it an alkaline battery found in a flashlight or a lead acid battery in a car, the same sort of thing can happen. Reaction products build up around the two poles of the battery and slow down the reaction. By letting the battery rest, you give the reaction products a chance to dissipate. The higher the drain on the battery, the faster the products build up, so batteries under high drain appear to recover more.

Many battery-operated appliances use two or four cells in series to create higher voltages. If one of the cells has a problem (for example, it does not dissipate reaction products as well as the other batteries), it can make all of the batteries appear to go dead. If you test the batteries individually, however, three of the four may be fine. If the batteries seem to go dead too quickly, testing all four batteries is a good idea. Throw out the bad one and re-use the other three.

Precession

If you have ever played with toy gyroscopes, you know that they can perform all sorts of interesting tricks. They can balance on string or a finger; they can resist motion about the spin axis in very odd ways; but the most interesting effect is called precession. This is the gravity-defying part of a gyroscope. The following video shows you the effects of precession using a bicycle wheel as a gyro:

The most amazing section of the video, and also the thing that is unbelievable about gyroscopes, is the part where the gyroscopic bicycle wheel is able to hang in the air like this:

The ability of a gyroscope to "defy gravity" is baffling!

How can it do that?

This mysterious effect is precession. In the general case, precession works like this: If you have a spinning gyroscope and you try to rotate its spin axis, the gyroscope will instead try to rotate about an axis at right angles to your force axis, like this:

In figure 1, the gyroscope is spinning on its axis.
In figure 2, a force is applied to try to rotate the spin axis.
In figure 3, the gyroscope is reacting to the input force along an axis perpendicular to the input force.

The Cause of Precession

Why should a gyroscope display this behavior? It seems totally nonsensical that the bicycle wheel's axle can hang in the air like that. If you think about what is actually happening to the different sections of the gyroscope as it rotates, however, you can see that this behavior is completely normal!

Let's look at two small sections of the gyroscope as it is rotating -- the top and the bottom, like this:

As forces are applied to the axle, the two points identified will attempt to move in the indicated directions.

When the force is applied to the axle, the section at the top of the gyroscope will try to move to the left, and the section at the bottom of the gyroscope will try to move to the right, as shown. If the gyroscope is not spinning, then the wheel flops over, as shown in the video on the previous page. If the gyroscope is spinning, think about what happens to these two sections of the gyroscope: Newton's first law of motion states that a body in motion continues to move at a constant speed along a straight line unless acted upon by an unbalanced force. So the top point on the gyroscope is acted on by the force applied to the axle and begins to move toward the left. It continues trying to move leftward because of Newton's first law of motion, but the gyro's spinning rotates it, like this:

As the two points rotate, they continue their motion.

This effect is the cause of precession. The different sections of the gyroscope receive forces at one point but then rotate to new positions! When the section at the top of the gyro rotates 90 degrees to the side, it continues in its desire to move to the left. The same holds true for the section at the bottom -- it rotates 90 degrees to the side and it continues in its desire to move to the right. These forces rotate the wheel in the precession direction. As the identified points continue to rotate 90 more degrees, their original motions are cancelled. So the gyroscope's axle hangs in the air and precesses. When you look at it this way you can see that precession isn't mysterious at all -- it is totally in keeping with the laws of physics!

Uses of Gyroscopes

The effect of all this is that, once you spin a gyroscope, its axle wants to keep pointing in the same direction. If you mount the gyroscope in a set of gimbals so that it can continue pointing in the same direction, it will. This is the basis of the gyro-compass.

If you mount two gyroscopes with their axles at right angles to one another on a platform, and place the platform inside a set of gimbals, the platform will remain completely rigid as the gimbals rotate in any way they please. This is this basis of inertial navigation systems (INS).

In an INS, sensors on the gimbals' axles detect when the platform rotates. The INS uses those signals to understand the vehicle's rotations relative to the platform. If you add to the platform a set of three sensitive accelerometers, you can tell exactly where the vehicle is heading and how its motion is changing in all three directions. With this information, an airplane's autopilot can keep the plane on course, and a rocket's guidance system can insert the rocket into a desired orbit!