28.10. Basic electrical theory

To understand electrity, it is easiest to use a water current as an analogy for an electrical current, since most people are familiar with the characteristics of water.

The analogies and definitions used in this section are simplified for the sake of explanation and are not 100% accurate, but they are accurate enough for building a pragmatically useful understanding of electricity.

28.10.1. Electrical current

An electrical current is the flow of electrons through a wire. Much like water flow, an electrical current has similar measureable characteristics, such as pressure, flow rate, and power. It can also perform useful work like a water current.

28.10.2. Electrical pressure (voltage/volts)

The electrical pressure differential between the positive and negative terminals of a battery or other electrical device is the voltage. This is similar to water pressure in a tank or pipe. Water pressure is measured in pounds per square inch (PSI), and electrical pressure (voltage) is measured in volts.

28.10.3. Electrical flow (amperes/amps/milliamps/ma)

When electrons flow through a wire, the rate at which the electrons flow through can be measured, and this rate is measured in amperes. A milliampere (abbreviated milliamp or ma) is 1/1000th of an ampere, so 1000 ma = 1 amp. The equivalent term for water would be gallons per minute of water flow.

28.10.4. Electrical resistance (ohms)

When an electrical current flows through a wire, there is some friction on the current which reduces the amount of electricity flowing through the wire. This friction is called resistance, and is measured in ohms. In hydrodynamic terms, this measurement is similar to the diameter of a pipe. A small straw has a narrow diameter, and requires a lot of suction to pull a certain amount of liquid flow through it. A larger straw has a larger diameter, and requires less suction to pull the same amount of liquid flow through it.

28.10.5. Relationship between volts, amps, and resistance (Ohm's law)

Ohm's law specifies the relationship between volts, amps and resistance. Ohm's law is:

E = I * R

where:

E = voltage (volts) I = current (amps) R = resistance (ohms)

What this means is actually fairly simple.

If you have an electrical current flowing through a wire, and you double the electrical pressure, then you will double the current flow through the wire, assuming you keep the same wire.

The equivalent hydrodynamic analogy is: If you double the water pressure at one end of the pipe, it will double the water flow through the pipe, if you keep the same pipe.

The equation can be rearranged to derive other interesting relationships such as:

E / R = I

So, if you double the resistance of a wire, and you keep the electrical pressure the same, then only half the current will flow through the wire.

The equivalent hdyrodynamic analogy is: if you use a smaller pipe, then less water flows through the pipe, if you keep the same water pressure.

28.10.6. Electrical power (watts)

Electrical power is the amount of useful work which can be done by an electrical flow. This is measured in watts, which is the volts multipled by the amps. Volts and amps independently by themselves do not measure power.

For example, consider a water current of 1 gallon per minute. This water current can either be dribbling out of a six foot drainage pipe at very low pressure, or it can be squirting out of a very small hole at high pressure In the first case, it doesn't have much power, and in the second case, it has a lot of power.

Similarly, consider water pressure of 1 pound per square inch. In a small garden hose, this can water your yard and perform some useful work. However, consider a large river like the Amazon where you may have about 1 pound per square inch of water pressure, but the amount of water flow is enormous - this can perform far more power than 1 pound per square inch in a garden hose.

28.10.7. Battery current capacity (ampere-hours/aH/maH)

The current capacity of a battery is measured an ampere-hours, often abbreviated aH. 1/1000th of an ampere-hour is a milliampere-hour, or maH.

One ampere-hour is the ability to supply one ampere for one hour. Two ampere-hours is the ability to supply one ampere for two hours, or two amperes for one hour.

The efficiency of a battery usually decreases at higher current draws. For example, a 4000 maH battery may be able to supply 400 ma for ten hours, but may only supply 4000 ma for half an hour. Therefore, the capacity of a battery is usually specified at a specific current dependent on the manufacturer.

28.10.8. Battery total power capacity (watt-hours/wH)

The total power capacity of a battery is the voltage multiplied by the ampere-hour capacity of the battery.

Since voltage times amperes equals watts, therefore voltage times ampere-hours equals watt-hours.

28.10.9. Battery maximum discharge rate (C rate/10C/15C/20C)

Manufacturers of lithium-ion batteries usually specify the maximum discharge rate for a particular cell. This maximum discharge rate is specified as a value which is a multiple of the battery capacity.

For example, a 20C battery is rated for a maximum discharge rate which is equivalent to twenty times the value of its total current capacity. If the battery is rated for 1300 maH, then the 20C discharge rate would be 26000 ma, or 26 amps.

Note that this C rate value may be the maximum continuous or burst (usually 10 second) discharge rate depending on the battery manufacturer, therefore it is wise to carefully read the battery specifications.