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The RV's Hardware

All this electricity is produced by fifteen ARCO M75 PV modules each rated at 47 Watts. Total peak PV power is about 750 Watts and the array produces about 4,000 Watt-hours daily. This power is stored in six Trojan T-125 lead-acid batteries. Total storage is 705 Ampere-hours at 12 Volts DC. PV power is controlled by a 50 Ampere SCI charge controller. The system also uses a Trace 2012 inverter with battery charger to supply the 120 vac powered appliances.

The installation of all this power equipment on and in an RV could not have been easy. Brint's installation is immaculate. Each panel has its own hand fabricated aluminium mounting brackets. The panels are mounted in the free spaces between the two roof mounted air conditioners and all the other stuff found on RV roofs. Nowhere is there a dangling wire or funky connection. When I walked up to the RV, I couldn't see that the panels were even on the roof!

The batteries are tucked into a slide out compartment accessible from outside, just behind the pilot's seat. Here they are outside of the living compartment, secure and easily maintained. The inverter and controls are tucked under one of the couches in motorhome's forward cabin. Once again the installation is totally ship shape.

A Rolling Solar Power House

No matter where Brint goes in his motorhome he will always have electric power. Brint is often

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Instrumentation for Home Power Systems

Richard Perez

©1991 by Richard Perez

When we make our own electricity we are our own power company. We are our own production crew, our own energy auditor, and our own trouble shooter. Instruments are our eyes into the electric world of our power systems. Without accurate instruments we are flying blind. While instrumentation is not necessary for the system to work, it greatly helps us operate our systems. And when things don't go right, instrumentation is essential for finding out what and where the problem is.

System Measurement

If we don't know how our system is performing, then we cannot effectively use it. We are in the same position as a U Boat captain, we must make operational decisions based on the state of charge of our batteries. If the batteries are full and the power source producing, then we are wasting power in the only way possible in an RE system- by not using it. If the batteries are empty, then we need to ease power consumption.. So battery state of charge is the first and most important bit of info we need.

Monitoring critical system points gives us an at a glance check of major component performance. We don't really need to continually know this data everywhere in our system, just at critical points. The best places to make these mostly voltage and current measurements are on power producers (PV, hydro, wind, or ?), power storage (batteries) and power processing devices (inverters and controls).

Before we measure anything, we need a meter. It could be an analog meter (you know, the older types with a dial and pointer) or a more modern type that displays numbers on a digital display. The instrument may be set up to perform many types of measurements or it may be optimized to perform only one. How accurate does the instrument need to be? As accurate as you can afford. In measurement, accuracy is the name of the game.

A Good DMM vs. Discrete Instruments

A Digital MultiMeter (DMM) can make a variety of measurements. The DMM will measure voltage, current, and resistance. Some DMMs will also measure frequency, duty-cycle, capacitance, test semiconductors, and record data in their memory. There are hundreds of these meters on the market. We use two Flukes at Home Power. The Fluke 77 is rugged, accurate (0.1% on DC), and inexpensive (»$140). The Fluke 87 has all the features mentioned above for about $280. These are highly accurate, capable, and reliable instruments. If you are seriously interested in instrumentation, a DMM of this caliber should be your first purchase. It will be the standard with which you will build other specialized, dedicated instruments. There are less expensive DMMs than the Flukes mentioned here. You will get what you pay for. The higher quality DMMs are more accurate, have many more features, last many times longer, and are very rugged. See HP#15, page 41 for a technical report on the Fluke 87 DMM

Discrete analog meters are inexpensive and usually optimized to perform a single function, like being a battery voltmeter. The accuracy of analog meters can vary from "strictly ballpark" (as bad as ±25%) to very accurate (»1%). Analog meters are inexpensive ($3 to $40) and easily available in the surplus market. They are powered by the circuit under test and generally require no on board batteries. They are extremely easy to tweak into accurate, dedicated meters for virtually any measurement.

So the choice of instruments is up to you. Let your inclination and bank account be your guide.

Battery Measurements

The battery is the heart of the system. The battery is the Numero Uno, first, last, and most essential component subjected to continuous scrutiny. The best single instrument for operating batteries is a dedicated battery Ampere-hour meter. Period. If you don't really care about fully instrumenting your system and want only a single instrument, then get a battery Ampere-hour meter.

Other very useful battery instruments include a dedicated battery voltmeter and a bi-directional ammeter that measures net current into and out of the battery. We use both and find them very informative for at a glance checks on system performance.

Battery Ampere-hour measurement

The constant question in any battery based system is, "How full is the battery?" The easiest to understand and most accurate method uses a digital Ampere-hour meter. It's a "gas gauge" for all types of batteries, both lead-acid and nickel-cadmium. These instruments not only work well, but their information is direct and understandable by even the most nontechnical battery user.

Ampere-hour Measurement

There are many ways to measure a battery's State of Charge (SOC). In lead-acid cells, you can measure the specific gravity of the electrolyte with a hydrometer. But this is temperature dependent and risks contamination. In nicads, specific gravity of the electrolyte is meaningless as it doesn't change with the cell's state of charge. We can use a voltmeter to determine SOC. But this is not very accurate, and varies with the battery's temperature. Measuring SOC by voltage is also dependent on the current flow through the battery. If the battery is under charge, then the voltage is higher. If the battery is under discharge, then the voltage is lower. And after you have compensated the voltage measurement for current and temperature, then you must still consult a SOC versus Voltage chart to accurately determine the battery's State of Charge. Sound confusing? Well, it is. And all this confusion is cleared up by an Ampere-hour meter.

Ampere-hour measurement is the best way to determine battery state of charge. The measurement doesn't depend on temperature, cell type, or whether the battery is being charged or discharged. The Ampere-hour meter provides a digital readout of exactly how many Ampere-hours have been withdrawn from the batteries.

Ampere-hour meters come in two types. Some are totalizing types that continually add up the Ampere-hours flowing in a single direction, say the yearly current production of a PV array. Other Ampere-hour meters are optimized as Battery SOC meters, and are bi-directional, net reading meters. They measure and count current flow to and from the battery.

The Ampere-hour Meter- a "Gas Gauge" for Batteries

The battery Ampere-hour meter is installed on a fully charged battery. At this point the digital display will read zero (0). This makes sense since the battery is full and we haven't yet withdrawn any power from it. As the battery is discharged, the digital display counts the Ampere-hours withdrawn from the battery. For example, say our battery is full in the afternoon and during the night we withdraw 40 Ampere-hours. In the morning, the Ampere-hour meter will read -40 (that's MINUS 40) to indicate that we've withdrawn 40 Ampere-hours from the full battery. As the Sun comes up and the PV array (or any other power source) starts recharging the battery, the Ampere-hour meter begins counting up (from -40, it counts to -39, -38, -37, etc.) to zero as the battery refills. When the battery is full, the meter again reads zero. At that point any additional recharging of the battery is read as positive numbers on the display. For example, after the battery is full, if we put 20 Ampere-hours more through it, then the display will read 20 as "overcharge Ampere-hours". After charging stops, the meter resets itself to zero regardless of the number of overcharge Ampere-hours. This makes sense since overcharge Ampere-hours can not stored by the battery because it is already full.

For "Things that Work!" tests of two battery Ampere-hour meters see HP#16, page 40 for a review of Cruising Equipment's meter. See HP#20, page 40 for a "Things that Work!" review of the Ample Power's Ampere-hour meter. The Cruising Equipment model is a straight Amp-hour meter for about $200, while the Ample Power version is also a battery voltmeter and battery ammeter for about $300.

Battery Voltage

Let's face it, although battery Ampere-hour meters are the best tool for the job, they are also expensive. Many of us still use battery voltmeters to aid us in determining battery state of charge. If you own a DMM, then you can use it to measure battery voltage. Most of us, even those with several DMMs, still like to have a dedicated battery voltmeter on line. You can buy these as digital meters for around $50, as analog meters for about $20, or you can make a very accurate analog model as follows.

The Expanded Scale Analog Battery Voltmeter

The idea here is to use an analog dc milliammeter to accurately measure battery voltage. This circuit produces an expanded scale voltmeter. Most analog voltmeters start reading a 0 volts. This is really a waste for battery systems as a lead acid battery will have about 10 to 11 volts (20 to 22 VDC in a 24 VDC system) even when just about empty. So the portion of the meter's scale between 0 and 10 volts is never used. Wasting this portion of the meter's scale decreases its resolution and thereby the accuracy of the meter. This circuit allows the meter to start reading at 11 volts and to display full scale at 16 volts (a very fully charged 12 Volt battery while still under charge). The 24 VDC version starts reading at 22 VDC and displays full scale at 32 VDC. This is called an expanded scale, and makes the meter much more accurate to use.

Expanded Scale Analog Battery Voltmeter 11 to 16 VDC (or 22 to 32 VDC)

Battery Input 11 to 16 (22 to 32) Volts DC

Expanded Scale Analog Battery Voltmeter 11 to 16 VDC (or 22 to 32 VDC)

Battery Input 11 to 16 (22 to 32) Volts DC

Resistor values in paranthesis () are for 24 VDC model.

All resistors 1/4 W. unless otherwise noted All capacitors 50 Volt rated

Designed by Richard Perez

Resistor values in paranthesis () are for 24 VDC model.

All resistors 1/4 W. unless otherwise noted All capacitors 50 Volt rated

Designed by Richard Perez consumption. We've had one on line since 1976.

Battery Current

A bi-directional Ammeter is a great instrument to have. The instrument measures current into or out of the battery. Since the ammeter is in series with the battery, the ammeter must have low insertion loss. Every amperage measurement scheme has some electrical resistance. At the high currents commonly found in battery systems, the insertion resistance must be low (less than 0.001 W). For example, an inverter starting a big electric motor may require over 800 Amperes of current from the battery. All this current must pass through the ammeter.

All the components for this meter are available at most Radio Shack stores, or from just about any electronics supply house. Cost of the parts should be between $15. and $40., depending on your hardware sources. Construction time is about 1 hour for an experienced assembler. This circuit is powered by the battery under measurement.

We don't have space here to give an electronics primer for those not familiar with electronic construction. What I do offer is the schematic for the circuit. If you can't figure out how to build this meter from the schematic, then please seek out an electronics person who can aid you.

Electronic Nitty-Gritty

This circuit uses a 1 mA. DC Ammeter as an expanded scale voltmeter. The meter has its ground elevated to 11 Volts (22 Volts in a 24 VDC system) by the use of an LM 723 voltage regulator in shunt mode. This makes the meter very accurate as there are no series semiconductors in the measurement circuit. Full scale reading and the 11 Volt (or 22 Volt) ground level are both adjustable by using the potentiometers in the circuit. R1 adjusts the shunt regulator. Adjust R1 until Test Point 1 (TP1) is at 11 Volts (22 VDC in a 24 Volt system). Then adjust R2 until the meter reads the battery's voltage at the time. Use an accurate DMM to calibrate this circuit.

Average power consumption is about 5 milliWatts. When on line 24 hours a day, power consumption is less than 0.1 Watt-hours per day. This meter is super-efficient and can be left on line all the time with a minimum of power

Shunts

In high current situations, use a shunt for measuring battery current. Shunts are very low resistance, precision resistors designed specifically for current measurement. Shunts are relatively inexpensive ($10 to $40), accurate (0.1%), and can handle large currents (10 A. to >1000 A.). Shunts are used for current measurement by every Ampere-hour meter and most ammeters. If you can live with accuracy losses <10%, then you can use the copper wiring in your system as shunts. It all works by the magic of Ohm's Law.

In Theory

Ohm's law tells us that any electrical current flowing through a material (like a piece of wire or a shunt) suffers a loss in voltage. This voltage drop across the material is due to its resistance and the movement of the electrons (current) through that material. The amount of current flowing through the material can be determined if we know two things. One, the voltage loss across the material, and Two, the resistance of the material. Or in algebraic terms using Ohm's Law:

where

I= the amount of current in Amperes

E= the voltage drop in Volts

R= the material's resistance in Ohms

Everything in the system is wired with copper wire. The wiring is necessary to move current from place to place. If we consider these wires as resistors, then we can use the voltage loss across a wire to determine the current flowing through the wire.

How it Works

All we need to perform current measurements is a Digital MultiMeter (DMM) and the already existing wire in our systems. And help from Ohm's Law.

The DMM is used to measure the voltage drop across a piece of wire carrying current. The DMM should be capable of making measurements in the millivolt DC range. Such resolution is necessary as this technique involves using lengths of wire with resistances from 0.01 W to 0.0001 W. The resultant voltage drops across such small resistances will be low, and we'll need a DMM that can make accurate measurements in the milliVolt range.

We also need to know, as accurately as possible, the resistance of the piece of copper wire we are using. To find this resistance first determine the wire's size or gauge. Most wire has its gauge number printed on its insulation. Or the wire's gauge can be determined by using a wire gauge measuring tool. Once the gauge number is known, then measure the length of the wire. Copper wire has its resistance, in Ohms per foot, specified by gauge number. Once we know the gauge, we can look up the resistance (W/ft) on a Copper Wire Table. This value is multiplied by the number of feet of wire we are using to make the measurement. And the result is the resistance of that particular piece of copper wire or shunt.

This technique can be used on wire of any size, and of any length. There are certain resistance values for shunts that have distinct advantages. Consider the following resistances: 0.01 W, 0.001 W, and 0.0001 W. If these values are used for R, then we are performing division by a decimal fraction of 1. This means that the measurement taken by the DMM can be read directly and a calculator is not needed to perform the math. Only the decimal point of the reading of the DMM need be shifted to obtain the amperage measurement.

What follows is a Copper Wire Table that is optimized to display the lengths of various gauges that have resistances from 0.01 W to 0.0001 W. Find the wire gauge size of the wire you are using, and the lengths necessary to produce the shunts are shown across the table. Measure the indicated length along your wire and you have a shunt with a resistance that is a decimal fraction of 1. Attach the leads of the DMM across this length and you're ready to make current measurements.

At the head of each shunt column on the table, there is a reminder to shift the decimal point on the mV. reading taken from the DMM. For example, let's consider a 12

Copper Wire Shunt Table
DIY Battery Repair

DIY Battery Repair

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