Devices that use electricity fall into two categories: those that require high-voltage alternating current (AC) and those that require low-voltage direct current (DC). DC devices are often battery powered.

Figure 2-2: Flavors of electricity

LOW-VOLTAGE DC

Low-voltage DC is much safer and easier to generate, use, and store than AC. Low-voltage generally means 12V (volts) or less. I find it helps to think of water flowing through pipes when trying to understand how electricity flows through wires. This image is particularly useful for understanding the difference between voltage and current.

Voltage is like the pressure in a water pipe. A high voltage can supply much more power than a lower voltage can, just as a high-pressure pipe could fill a container with water much faster than a lower-pressure pipe could. But thinking of voltage as just pressure creates an incomplete picture; it’s more accurate to think of voltage as a height difference.

In the schematic (Figure 2-3), the point where the water enters the pipe is above the point where the water leaves the pipe. The higher the entrance is above the exit, the greater the rate of flow. This rate of flow is called the current, and in electronics, the current is the amount of charge passing a point per second. The unit of measurement for current is the ampere, which is abbreviated to just A. It is also common to see current measured in mA (milliamps). One mA is 1/1,000 of an A.

Figure 2-3: Voltage and current

Interestingly, you can work out the amount of power that something uses by multiplying the power in voltage (V) by the current in amperes (A).

When supplying some low-voltage equipment (let’s say an FM radio receiver) with power, it’s important to get the voltage correct. Too much voltage will cause too much current to flow through the radio and may kill it. The last thing you need is a zombie radio! Similarly, if there’s too little voltage, not enough current will flow to make the thing work properly. The range of acceptable voltage can be quite wide, depending on the device. For example, a radio indicated as requiring 6V to operate may work perfectly well at anything between 4V and 8V.

WARNING

When using a low-voltage DC device, make sure you put the batteries in the right way around. Batteries have a positive and a negative connection. If you connect them incorrectly, the current will try to flow the wrong way through the device. If the device does not have internal protection against this (note that most do), the device may be rendered nonfunctional.

HIGH-VOLTAGE AC

High voltage is used to distribute electricity to people’s homes because higher voltage makes power transmission more efficient. High-voltage AC is very different from low-voltage DC. For one thing, the voltage is either 120V (in the United States) or 220V (in most of the rest of the world). Also, AC voltage is alternating: unlike a battery, which has one positive connection and one negative, an alternating current switches the polarity of its two leads between positive and negative at a rate of 60 times a second (in the United States) or 50 times per second (in most of the rest of the world). The unit for frequency, which is the number of times that the electricity switches polarity per second, is hertz (Hz).

How the voltage changes over time with an AC power source can be graphed, as in Figure 2-4. Notice that the voltage doesn’t suddenly switch direction but rather swings gently one way and then the other, gradually increasing to a peak of over 150V and then down to below –150V. Clearly, this is more than 120V on either side of zero. The maximum and minimum are described as 120V because this amount of AC power provides the equivalent amount of power as 120V DC. This way of measuring AC voltage is called root mean square (RMS). For more information on this topic, take a look at http://www.electronics-tutorials.ws/accircuits/rms-voltage.html.

Figure 2-4: Alternating current (AC)

Low-voltage DC devices are often run on AC by using an adapter, like the one your laptop uses or the “wall wart” that you plug your phone into, which converts the AC into DC and drops the voltage at the same time. In our postapocalypse world, unless you have an AC generator, you’re likely to be both making and using low-voltage DC directly. Although you can convert DC to AC with a device called an inverter, converting in either direction is inefficient, wasting some energy, and is best avoided.

If you decide to use an inverter, remember that even though you are powering it from a battery, it is generating high and therefore dangerous voltages. Therefore, exercise the same common sense as you do when plugging devices into an AC wall outlet.

BATTERIES

Batteries, used to store electrical energy, come in lots of different types. Some are small and single-use, like AA cells. Others, such as lithium laptop batteries and lead-acid car batteries, can be recharged. Note that batteries only supply DC.

Both single-use and rechargeable batteries are essential to your survival during a zombie apocalypse, so scavenge as many as you can during your supply runs. As you’ll see in Chapters 9, 10, and 11 of this book, you can use batteries to power zombie-distracting devices and communications devices. Of course, both types of batteries have different merits. Let’s explore those now so you can decide which deserves a spot in your go bag.

SINGLE-USE BATTERIES

AA batteries have a long shelf life, and to operate many small appliances, it makes sense to scavenge a good supply of these. They also run out of power slowly. For example, if your flashlight begins to dim, you’ll still get a few valuable minutes of light before the battery completely dies. Note that rechargeable AA batteries usually give out much more quickly than single-use batteries—and with less warning.

RECHARGEABLE BATTERIES

Lithium polymer (LiPo) batteries have transformed mobile devices because they’re lightweight and can store a lot of energy. Since a cellphone is so easy to carry around, you might think LiPo batteries are a good rechargeable choice for any portable postapocalyptic device. But be warned: they have a few quirks:

• They are prone to catching fire if overcharged, punctured, or cut.

• They require special charging circuits.

• They don’t work well at extremes of temperature.

In short, for storing energy that you generate, it’s better to use the lead-acid batteries that you find in cars. For a start, there should be a plentiful supply of these. They also have the advantage of working at low temperatures, and they are much more forgiving of overcharging or continuing to be discharged after they are empty than other types of rechargeable batteries. The only real downside to lead-acid batteries is that they are really heavy, so when you need to scavenge car batteries, don’t be tempted to load your pack with much else. Otherwise, you’ll quickly find yourself too overburdened to escape a pursuing zombie.

BATTERY CHARGING

Under normal circumstances, the easiest way to charge a battery is to use an AC-powered battery charger. Since you won’t have access to AC (unless you’ve hit the jackpot and found a working generator), you need to consider ways you can generate electricity to charge batteries.

In the project that follows, you’ll learn how to generate electricity and charge batteries using solar power, in many ways the easiest solution to postapocalyptic power problems. You’ll then discover how a stationary bicycle and a car alternator can be adapted to charge batteries. The principles you learn here also govern using water wheels and wind turbines. In fact, anything that can turn the shaft of a car alternator at a reasonable speed and with reasonable force can be used to generate power. A drive belt is a good way to link whatever is turning to the alternator and provide some gearing so that the alternator moves fast enough.