Take a meter, please Therese Peffer Finally, the last piece fell into place. I'd grown used to my pretty blue photovoltaic module out on the grass soaking up the sun's energy and filling up a car battery. I'd installed a charge controller, fuses, and a 12 Volt circuit breaker, but one thing was missing. I didn't feel comfortable about unplugging from the Home Power Office & Power "grid" until I had my meters Ä a voltmeter and ammeter Ä in place. I knew the panel was charging the battery Ä the regulator has LED lights, so I know when I'm "regulating" (shunting power from the battery when it is full). But just how much of those flowing electrons are going in and out of the battery? What is the battery voltage? Why Me(ter)? So why have a round-the-clock, 24-hours-a-day voltmeter on a battery? I suppose I could continue to use a multimeter to measure my battery's voltage every so often. After all, it's 12 Volts, not going to change much, right? Boy have I learned a lot about batteries! I don't have the space or expertise to discuss battery state of charge here (Richard promises to write an article for the next issue). But I learned that the voltage of a battery varies depending on whether it is being charged or discharged. The voltage also says what the state of charge is, that is, how full the battery is or how close it is to its capacity (measured in Amp-hours). And the voltage will vary at different temperatures. If your battery's voltage differs from the voltage of a "healthy" battery under these conditions, you may have a "sick" battery. So constantly monitoring battery voltage is vital to a renewable energy system. The ammeter I installed for the heck of it. I was curious how many amps of current my panel produces when it's cold or very hot, during a bright shiny day or a day that's overcast. I also wanted to see how much current some of my loads draw (a few lights and a small stereo). Meter, meter on the wall... I decided to buy analog meters rather than digital. Digital meters are more accurate, but, well, I don't even like digital watches! I find analog meters easier to read at a glance, and my analog voltmeter will give accuracy to a tenth of a volt, which is good enough for me. Another decision was to buy surplus meters that I had to alter rather than ready-made meters I could just plug in and use. Richard assured me this would be a cheap and easy solution and it was! I bought two meters from C&H Sales (see access). One was a 0-1 milliamp DC meter, with a scale from 0-10, and cost $2.95. The other was a 50-0-50 microamp DC meter (Nullmeter) that cost $4.95. Both meters are ruggedized and can stand to get moved around and jostled Ä great for a portable system. I used the milliamp meter as a voltmeter. The meter is mounted in my small plywood power box. Small (#22) wire travels from the positive and negative posts to the positive and negative posts of the battery (see diagram). The scale (0-10) is not useful to read a 12 Volt battery's voltage, which normally ranges from 11 to 16 Volts. But having ten divisions was perfect for modifying the meter to an expanded scale voltmeter that reads 11-16 Volts (see sidebar). The 50 microamp meter I used as an ammeter. It is also mounted in my power box. The positive and negative leads attach across a shunt connected to the negative post of the battery. This way I can monitor current coming into and out of the battery. The meter didn't have any numbers on it, so I decided what the scale would be. The amount of current through the meter depends on the resistance of the meter and the shunt. A shunt is basically a piece of metal with a known resistance. The ones you buy are rated in terms of Amps and Volts, and have bolts for easy attachment. Use Ohm's Law (Voltage = Current x Resistance) to figure the resistance of a shunt. For example, a 500 Amp-50 milliVolt shunt has a resistance of 0.050 Volts divided by 500 Amps equals 0.0001 Ohms (ê). The current rating tells you the ampacity of the shunt, or how much current it can take on a continuous basis. The Test When you buy surplus anything, you never know what you're going to get, so the first step was to test the meters. Chris and Richard helped me set up the test procedure (see below). We used a AA nicad battery in a holder, a 50 kê ten-turn potentiometer (a resistor that you can vary from 0 to 50 kê), two Fluke 87 Digital MultiMeters (DMM), and various wires with alligator clips. One DMM measured voltage, the other measured current. We were very careful with the DMM that measured current Ä if you accidentally leave the multimeter in current reading mode while you measure voltage, you will blow the internal fuse of the meter!! We checked the 1 mA meter (soon to be voltmeter) for a smooth linear progression from 0 to 1 mA. For example, the meter should read zero when no current was flowing, half scale at 0.5 mA current, and full scale (10) with 1 mA of current. We did the same with the 50 æA future ammeter for both positive and negative scale, but also needed to measure the resistance of the meter. We started testing with the potentiometer turned to the highest resistance. We measured current in milliAmperes and voltage (V) at zero, quarter scale, half scale, three quarter scale and full scale. The ten-turn potentiometer allowed us to position the needle of the meter precisely on the scale. Full scale reading showed 1 mA of current at 0.104 Volts. The progression was smooth. I measured the resistance at each data point (R=E/I) and found a steady 104 ê (the face of the meter read 105 ê). The microammeter was a little tricky since the needle starts in the middle and can go right or left. I measured right scale (positive) at quarter, half, three quarter and full scale. Then I swapped the leads on the positive post of the meter with those on the negative post to measure the negative scale. The microammeter of course was more sensitive Ä at first we had the DMM set to read current in milliAmps, but discovered we needed to read in microAmps. I measured the resistance at each data point and found an average of 1908 ê. I decided that I wanted the ammeter to read about 10 Amps full scale. The next step was finding the total resistance to get this reading. Back to Ohm's Law: I measured the voltage at full scale (0.1 V), and I want the scale to read 10 Amps, so Rt=0.1/10 = 0.01. Now I have the total resistance and the resistance of the meter. I used the following equation to find the resistance of the shunt: 1/Rt = 1/Rm + 1/Rs. Solving for Rs gives RtRm/(Rm - Rt). Actually the resistance of the shunt was approximately the total resistance. I first thought that I would use wire as a shunt. I made a spreadsheet of wire sizes, Ohms per 1000 feet, and the number of feet necessary to produce the desired resistance. I would need 17.7 feet of #10 wire Ä out of the question. I'd only need 2.76 feet of #18 wire, but worried whether #18 could handle the current. Since space in my power box is at a premium, I ended up using a 10 Amp-100 mV shunt. The resistance is R=E/I = 0.1/10 = 0.01. The shunt can handle 10 Amps continuous and 20 Amps surge. I was curious how the ammeter worked and whether it really read 10 Amps full scale. Chris and I took the DMM down to the trailer, set the meter on millivolts and took a reading across the meter. (A 1 mV reading across the shunt corresponds to 0.1 Amps). As the sun kept popping in and out of the clouds, we saw 1 to 4.4 Amps! We discovered that the scale was not linear. At quarter scale the meter read 1 Amp, at half scale it read 2.5 Amps, and the 4.4 Amp mark was less than three quarter scale. Then we unplugged the trusty Solarex MSX-60 PV panel and started plugging in loads. We used my 20 Watt DC halogen light and large resistors to draw up to 10 Amps from the battery. At 10 Amps, the needle was not quite at full scale. I marked the readings. Even though the ammeter does not give a linear reading, with my calibration marks I can tell about what the current is. I don't require the meter to be accurate to a tenth of an ampere. Now starts the fun... I can't help it. Every time I'm in the trailer, I'm constantly looking at the voltmeter and ammeter. It's hard to describe the feeling of producing your own power, watching the power come in and out. I feel, well, empowered! Access Therese Peffer, c/o Home Power, POB 520, Ashland, OR 97520 ù 916-475-3179 Side Bar Need an inexpensive but accurate analog voltmeter that can be left on 24 hours a day? Many analog meters you can buy have a scale from 0-20 Volts, which is not very useful for a 12 Volt battery Ä only a small portion of the scale is used. Even lead acid batteries will read 10 to 11 when empty, so using part of the scale decreases resolution. You can buy meters that have a scale from 10-16 Volts for about $20 to $80 depending on how accurate and rugged a meter you want. Or you can buy a ruggedized surplus 1 milliAmp DC meter and build this circuit for an accurate, cheap, and rugged voltmeter with an expanded scale that reads from 11-16 Volts. This circuit was designed by Richard Perez and first appeared in Home Power #2. I am not an experienced "homebrewer", but once I got the parts together, this circuit took me about 3 hours to put together. I attached the small piece of perforated board the circuit is on to the meter with a bolt. "This circuit uses a 1 mA DC Ammeter as an expanded scale voltmeter. The meter has its ground elevated to 11 Volts 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 ground level are both adjustable by using the potentiometers in the circuit. R1 is the adjustment for the shunt regulator. Adjust R1 until the (voltage)???? is at 11 Volts. Then adjust R2 until the meter reads the battery's voltage at the time. Use an accurate DMM to calibrate this circuit. All the components for this metering project are available at most Radio Shack stores, or from just about any electronics supply house. Cost of the parts should be between $20-$40 depending on your hardware sources. This circuit is powered by the battery under measurement and never requires the use of small batteries. Average power consumption of this meter 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 consumption."