Things that Work! Home Power Tests Cruising Equipment's Digital Ampere-hour Meter Richard Perez The constant question in any battery based system is, "How full is the battery?" There are many ways to determine a battery's state of charge (SOC). The easiest to understand and most accurate method we've ever seen uses Cruising Equipment's Digital Ampere-hour meter. It's a "gas gauge" for all types of batteries, both lead-acid and nickel-cadmium. It not only works, but its information is direct and understandable by even the most nontechnical battery user. The Cruising Equipment Ampere-hour meter has become our prime instrument for determining our battery's state of charge. If you are going to use only one instrument to fly your system, then this is the one. Ampere-hour Measurement There are many ways to measure a battery's SOC. In lead-acid cells, you can measure the specific gravity of the electrolyte with a hydrometer. But this is inaccurate as it depends on temperature, and risks contamination of the cell. In nicads, specific gravity of the electrolyte is meaningless for determination of SOC as it doesn't change with the cells' state of charge. We've always used a specially made, expanded scale 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 batteries State of Charge. Sound confusing? Well, it is. And all this confusion is cleared up by Cruising Equipment's 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, and doesn't care whether the battery is being charged or discharged. Consider this example of a 100 Ampere-hour battery. If the 100 Ampere-hour capacity battery has 10 Ampere-hours withdrawn, then its state of charge is 90% ([100 A-h- 10 A-h]/100 A-h, or [100-10]/100, or 90/100=90%). If we can measure the actual number of Ampere-hours of electricity withdrawn from the battery, then we will know exactly how much power remains (its state of charge). Ampere-hour meters come in many types. Most are totalizing types that add up the Ampere- hours flowing in a single direction. The Cruising Equipment Ampere-hour meter is optimized as a Battery SOC meter. The meter is bi-directional it measures current flow when the battery is being charged or discharged. The Cruising Equipment Ampere-hour Meter Concept The meter should be 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 Cruising Equipment 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 since it is already full. What makes the Cruising Equipment Ampere-hour meter so slick is that it is designed for only one job-- battery state of charge. This meter works so well because it does this one job better than any instrument. It isn't designed to measure the long term power production of PV/Wind/Hydro sources or to measure the long term consumption of appliances. It will work for short term Ampere-hour measurement, for example, we've used it measure the actual consumption of our microwave powered by the batteries. Shipping, Packing, and Installation/Operation Instructions The unit arrived via our local UPS service in fine shape. It was packaged very well and should survive shipment just about anywhere. The instructions are detailed and very complete. They include well written, step by step installation instructions for the novice electrician and an installation schematic for impatient techies. Operation instructions included are complete with a "what does it all mean?" section with electrical basics such as Ampere-hour definition, battery capacity explanation, and use in multiple battery systems. Test System We installed the meter on our main system at Agate Flat. This system is described in detail on page 7 of this issue. This system uses a 384 peak Watt PV array (Å2kWh daily) feeding a battery of four Trojan L-16W lead-acid batteries (700 Ampere-hours at 12 VDC). The major DC consumer in this system is a 2.3kW Heliotrope inverter. Installation The Cruising Equipment meter is easy to install. There are five wires involved. Two of them go to the battery PLUS and MINUS (either directly or to the battery's buss). Two go to the shunt in series with the battery's (or buss's) negative terminal. And the last wire can be connected to battery PLUS via a switch to turn the meter's backlighting on or off. A schematic of the wiring is shown below. INSERT INSTALLATION SCHEMATIC The Cruising Equipment Ampere-hour meter uses a shunt to detect current. A shunt is a very small amount of resistance inserted in a current path. This small resistance has a voltage loss that is directly proportional to the amount of current flowing through it. See HP#6, page 35, for a technical discussion of shunts. The shunt used here is a precision shunt (1%) with a resistance of 0.001½. The shunt is available in 100 Ampere maximum current for small systems, and 300 Ampere maximum for larger systems. We chose the larger 300 Ampere shunt because our inverter often consumes well over 100 Amperes on surges (like starting the microwave or bench grinder). Since the shunt is installed so that all the electrons moving either in or out of the battery MUST flow through it, the shunt must be capable of handling the largest sustained current flow that the system will ever undergo. Performance We installed the meter on our fully charged battery pack on 15 January 1990. We have been watching it like hawks ever since. We measured its accuracy against a Fluke 87 (reading true average current simultaneously through the same shunt at the same time). The Cruising Equipment Ampere-hour meter agrees with the Fluke 87 test setup to within 2%. And that's close enough to be within the limits of error in our testing procedure. The Cruising Equipment meter has replaced our homebrew expanded scale battery voltmeter (see HP#2, page 31) as our main instrument for flying our system. Every morning the first thing we check is this Ampere-hour meter and it tells us at a glance how much we've used from the battery. During the day, we watch meter measure the gradual refilling of our battery by the PV array. If the Sun doesn't shine, then the meter tells us when the battery is getting low and thereby, when its time to fire up the generator. We use the overcharge measurement feature to determine when equalization charges are completed on our lead-acid pack. If I could only have one instrument for operational information in a battery based system, it would be the Cruising Equipment digital Ampere-hour meter. As a techie, I enjoy and use the information provided by the some 14 meters (not including DMMs and portables) staring me in the face. But this Ampere-hour meter now has the final say and the rest of the instrumentation is strictly for fun and/or special jobs. On a nontechnical note, Karen has always had trouble determining SOC with our dedicated battery voltmeter. She easily grasped the concept behind the Cruising Equipment digital Ampere-hour meter and now has no trouble flying our system on her own. While all the techie info here is well and good, you don't have to understand any of this stuff to effectively use this meter. When it comes to "how full is the battery?", the Cruising Equipment's digital Ampere- hour meter has definitely got the answer. Cost The digital Ampere-hour meter costs $199.00, the 300 Ampere shunt costs $39.95 and the 100 Ampere shunt costs $19.95. I recommend the 300 Ampere shunt for any system using a 1,000 Watt or greater inverter. Conclusion The Cruising Equipment digital Ampere-hour meter is definitely a "Thing that Works!". It is the best instrument for measuring a battery's state of charge that we ever seen. It works equally well on both our lead-acid and nicad batteries. And best of all, you don't have to be electrical engineer to understand its information. Access Contact: Cruising Equipment, Inc., 6315 Seaview Ave. N.W., Seattle, WA 98107 or call 206- 782-8100.