Emergency Power: Some Considerations

From: larryn@ozemail. com. au (Wombat)Date: Sun, Mar 7, 1999 5:04 AM

Perhaps the best way to start this post is some questions and answers to stimulate discussion:

Q: “I have an all-electric home. Can I maintain my present level electricity usage if I decide to become independent of the mains power grid?”

A. Certainly! . . . All you need to do is to buy a large enough generating plant and run it 24 hours a day, as the power companies do.

Q: “Can I maintain a high level of electricity usage using batteries and inverter to change the 12 volts DCto 117 AC, then run a generator a few hours a day to keep the batteries charged? ”

A: Possibly, if you want to buy lots of deep discharge batteries and inverters, plus a very large generator. If talking about lots of electric heating, cooking, and hot water, water pumping, etc. then probably not.

Q: “Can I get by without a generator by just buying a large deep cycle battery and a big inverter?”

A. If talking a short period without mains power, possibly. If want to be independent of mains power for an extended period, or indefinitely, the practical answer for most of us is “no”. To keep the battery charged, have to put more back into it than take out, and this usually involves an alternator and an engine to drive it. If live in the right place, water power may be a feasible alternative to the engine. Solar power and wind power are far more expensive alternatives. However, they may be feasible if can reduce electricity usage to a minimum.

Q: “I want to be independent of mains power. How do I plan for it?”

A: Look at your electricity bills for the past several months to determine your current usage for various times of the year. Electricity is charged by the kilowatt hour. A kilowatt is 1000 watts. If use a 1000 watts for an hour, that’s a kilowatt hour. To get an idea of how much a kilowatt is, look at the wattage rating on a simple electric radiant heater. These are often 1000 watt, or 1 kw, units. Or consider as 10 x 100 watt incandescent bulbs. When you have read the rest of this post you may probably want to start thinking how you can cut down on usage of electricity, which means knowing just how much various appliances use. This can usually be found on a nameplate on the appliance or in its instruction book. The information is usually given in volts and amps, but may be given in watts, or VA, meaning “volt-amperes”. The US mains electricity supply is a nominal 120 volt, 60 Hertz [or Hz. or "cycle"]. In Australia, it is 240 volt, 50 Hz.

Q: “What is the standard for mains power? I’ve noticed that some nameplates say 120 volts, some 117 volts, some 110 volts. Why the difference. And what does “Hz” mean?”

A: There is no universal standard. The US uses a nominal 120 volt system, 60 Hz. Australia uses 240 volt, 50 Hz. The voltage figures are nominal, and vary depending how far you are from the nearest transformer, how many other people are on the line, how much power is being used, etc. For instance, if an American was to check the voltage on a power outlet he might find that the actual AC voltage was anywhere from 112 to 135 volts. “Hz” stands for Hertz, and is the line frequency, the number of times the AC line switches polarity between active and neutral each second. The old term for it was “cycles per second”. [Note to engineers, electricians, etc: Yes, I *know* I'm simplifying the situation by not specifying that the voltage is RMS, not peak. Have also not brought up the question of 3 phase supplies and the splitting of the load between phases. For the moment, am also ignoring power factor, reactance in AC circuits, etc. Am trying to keep this post reasonably easy to understand. ]

Q: “Can you explain electrical terms such as volts, amps, watts, ohms, in simple terms?”

A: Possibly the easiest way to explain is to use a hydraulic analogy. Suppose we have two garden hoses, a 1/2 inch hose and a 3/4 inch hose, and are trying to fill an tank or pool with water. The pressure of water can be compared to voltage, the amount of water to amperage, or current. It will take less time to fill the tank using the 3/4 hose than using the 1/2 inch as the 3/4 inch has less resistance to the flow of the water. The total amount of water delivered in an hour will be a product of pressure x flow, and will correspond to electrical watts, which is the product of volts * amperes. As to the ohm, it is a measure of resistance. Big garden hoses have less resistance than smaller hoses, and bigger copper wires have less resistance than smaller wires. For anyone doing even the most basic work with DC circuits, there is a very simple electrical formula known as Ohms law which everyone should know. E = IR, where “E” is the voltage in volts, “I” is the current in amperes ["amps"], and R is the resistance in Ohms. The other two forms of it are R = E / I and I = E / R. With a given resistance, R, the amount of current I increases in direct proportion to applied voltage, E. So if you apply more voltage to a light bulb than it is designed for, you will burn it out. Apply an overvoltage to a solid-state electronic device such as a transistor or integrated circuit, even briefly, and you will quite probably destroy it. With a constant voltage, E, the current, I, decreases as R increases. This is of importance in determining what size wire you need for connections, making sure connections are low resistance, etc.

Q: “What does the term ‘Ampere hour’ mean?”

A: Defined this in an earlier post, but will repeat here for those who may have missed it. The amp/hour is fairly rough measure, but basically it was a rating of how much current could be supplied continuously over a specified period of time without the voltage falling below a minimum level. The time was usually taken to be eight hours. So the formula was amp hour rating / 8 = current in amps. As an example, a 120 ah battery is supposed to be able to supply 120 / 8 = 15 amps continuously for 8 hours. It will supply more current for less time or less current for more time. Theoretically, if will supply 15 amps for 8 hours it should supply 30 amps for 4 hours or 7.5 amps for 16 hours. However, the curve isn’t linear, especially at higher discharge rates. At 60 amps, won’t last two hours, and certainly won’t deliver 120 amps for more than a few minutes. On the other hand, if only drawing a couple of amps from it, will last longer than the curve would indicate. Temperature has a pretty marked effect, though. The amp hour rating was usually calculated for 80 deg F. [27 C] Higher temperatures increase the chemical reaction, hence the output, but operation above 110 F [43. 3 C] shortens battery life. The amp hr capacity was said to reduce about 0. 75% for each one degree F [~ 0. 55 C] below the rated temp. At 0 deg F. [minus 17. 77 C] the available output would only be 60 percent of the rated output. The measure isn’t used as often today as used to be, possibly because of its imprecision. As the battery ages, its capacity can decrease for various reasons.

2==> That should be enough questions and answers for the moment. Since looking at DC systems, let’s look a system many readers would be reasonably familiar with, the electrical system of an automobile. [We'll ignore the ignition system, of no importance here.] Basically, the electrical system of a car or truck consists of a battery, a means of charging it such as a generator or alternator, and a regulator to prevent the battery from being overcharged. For practical purposes, can almost ignore the generator, as almost all vehicles use alternators. To most survivalists, its main interest is that it can act either as a DC generator or a motor if feed power to it, which could be useful in some situations.

3==> The alternator isn’t a DC machine. It has 3 coils and acts as a three phase alternator. The output from the three phases feed through rectifying diodes [usually inbuilt] to a common output. For the alternator to produce power, the field coils must be energized, and this is one of the functions of the car battery. The voltage and current output of the alternator are directly proportional to its speed of rotation. To charge the battery, the voltage output must be greater than the battery voltage.

4==> The battery has a couple of other important functions. Most people see it as just a device to store power. However, an equally important function is to regulate the voltage on the system. For this reason, it is definitely NOT a good idea to disconnect the battery while a car engine is running.

5==> The main function of the regulator in the circuit is to protect the battery from overcharging. In its simplest form it is a relay coil with a set of contacts. Those of you who have an ammeter on your vehicle rather than an unreliable “warning light”, can easily see what is happening with the system. When you switch on the ignition, the ammeter shows a slight discharge. When using the starter, it may flicker slightly, but doesn’t show the discharge directly as the starter current doesn’t pass through the ammeter. [For obvious reasons: The ammeter only usually reads to 30 or 40 amps of charge or discharge, the starter current may be several hundred amps.] When the engine starts, the battery will be partly discharged. So system power will energize the field coils of the alternator through the relay, and the alternator will start feeding power back into the battery. It will probably do so at 25 or 30 amps for a minute or so until the battery charges and its voltage rises. When it reaches the “set point” of the regulator, usually about 12. 7 volts, the regulator relay operates, opening the circuit to the field coils, and the alternator, although still turning at high speed, is producing no appreciable power.

6==> Why, incidentally, is an ammeter better than an “idiot light”? Because it gives you more detailed info on what is happening with your system. Why is a light used? Because it is cheaper than an ammeter and saves the manufacturer a few dollars.

7==> If you turn on various loads such as stereo, headlights, etc. when the motor isn’t running the ammeter will give you an indication of the amperes you are drawing from the battery. Headlights, for example, might draw 20 to 30 amps. If you have driving lights, even more. Now if the engine is running when you switch them on the regulator coil will release, closing the contacts to feed current to the field coils and the alternator will take up most or all of the load. Suppose, though, you are driving more slowly than usual due to poor road conditions, and the alternator can only supply 20 ampsof the 35 you may be using. The ammeter will then show a 15 amp discharge, showing that the other 15 amps is coming out of the battery.

8==> A note on the wiring: If you look at your car wiring, you will see that some wires are much thicker than others. The cable running from battery to starter relay is thickest of all, as it may have to handle several hundred amps for a few seconds. The next thickest is the one from the alternator, as it has to handle the amperage that the alternator can supply. If you install a set of high power driving lights, you will note that they have a fairly thick wire in their supply as well. What would happen if you substituted a smaller wire? Its resistance would be higher, less current than needed would get through it, so the lights would be dimmer than they should be. In addition, the smaller wire will heat up, and, given enough time, the insulation may melt shorting it out to the frame of the car, which is the negative side of the circuit. The wire may even catch fire. So correct wire size is important.

9==> Getting back to the vehicle battery, it is designed to be cheap, rugged, and low maintenance. It will tolerate both extreme discharges and high rates of charge. However, it isn’t really designed to supply lower rates of discharge for long periods. For this you need to handle several hundred amps for a few seconds.

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