Home Power Systems: Batteries, by L.K.O.

Monday, Jul 1, 2013

[Editor's Note: This article is part of a series of feature articles by our Central Rockies Regional Editor about alternative / sustainable / renewable energy (RE) solutions for self-sufficiency. Previous related articles in SurvivalBlog that complement this one are "Home Inverter Comparison: Off Grid and Grid Tied," Home Power Systems: Micro Hydro and Energy Efficiency and Conservation. Upcoming article topics in this Home Power Systems series will include: Photovoltaics, Wind generators, Solar Water Distillers, Solar Ovens, and Solar Water Heating.]

First Things, First: Home Power Prerequisites
One important foundation of a cost-effective, sustainable home energy system is energy efficiency and conservation; if you haven't already done appropriate 'due diligence' in this area, you might want to read or review the most recent article in this series, Energy Efficiency and Conservation. The savings one can realize by applying the economies of ideas like these can make the difference between a viable system for your budget and/or location ... or a perceived 'no go' conclusion that might be assumed given current rates of energy consumption. It does require a bit of discipline, research and care in analyzing current and proposed future energy usage, but it can pay off handsomely if, for example, you can end up with a system with half the cost (or even more savings) by judicious selection of appliances, other electrical loads ... and lifestyle choices. It's always a good idea to add in some 'wiggle room' to account for extended periods without sun (solar), wind and/or stream flow (micro hydro), but over-designing a system beyond this can be waste of your financial resources.

This is an ideal time for a reminder that a total 'do-it-yourself' approach is not advised. While renewable energy systems have evolved tremendously over the past few decades, and integrated mostly 'turnkey' systems are much farther away from rocket science and closer to 'appliance status', there's still way too many variables and considerations to advise doing it all by yourself. Experienced renewable energy system designers and installers serve a critically important role to help home energy system owners avoid many other common design mistakes, safety pitfalls, and needless expenditures. It would be an expensive, time-consuming and even potentially dangerous choice to not work closely with a local renewable energy professional. While it might seem more costly than a stubbornly pure "DIY" approach, buying a battery bank twice – before its typical life span is up – because of poor design or incorrect installation is far more expensive. Rely on the experience of a local professional; you'll learn more, save more and benefit from their experience while helping to strengthen the renewable energy industry. If you haven't already done an energy conservation audit, find a local RE professional and ask what they recommend as early in the design phase as possible. Sizing your battery system based on a realistic assessment of both sources and loads will make your project viable and economical. Ask around locally and online among neighbors, nearby friends, etc., to find a renewal energy installer with the right expertise, credentials and references to serve your needs.

Home Power Systems Without Batteries
Before launching into systems that require batteries, it might be helpful to examine home power systems that don't require them, even if briefly. If the prospect of having a remote, self-sufficient homestead with fully off-grid energy isn't on your near-term or medium-term radar, a grid-tied system might offer advantages for city-folk and/or country-folk wannabes. A prior article in this series, Home Inverter Comparison: Off Grid and Grid Tied, goes into greater detail about why one might consider being electrically connected to the local utility grid. A temporary, transitional approach can offer both educational and financial benefits now – and for the duration of however long you might be planning a remote home – as well as decreasing the wait time by saving money in the interim for a future home ... and adding to the resale value of your current home when you are ready to build or buy a fully off-grid home.

Battery Basics
Common to all off-grid systems that use batteries to store energy – from sun, wind, water or whatever source (stationary bicycle generators, anyone?) – are a few key ideas and definitions. Here is a handy glossary of fundamental battery terms. Batteries consist of one or more internal compartments called cells in a given enclosure, and each cell – depending on the chemistry and technology has a typical nominal voltage. For example, a lead-acid cell produces about 2 Volts, so a 12 Volt (12 V) battery internally has six cells wired in series, so that their voltages add up to 12 V.

Batteries are typically wired in series, as is done in a typical flashlight, with the positive terminal of one battery connected to the negative terminal of the next in 'daisy chain' fashion. This provides higher voltages to match inverters and loads, since most inverters designed for home usage have inputs of 12 or 24 Volts. A typical system with 6V batteries will require 4 (or multiples of 4) batteries wired together to provide the nominal 24 Volt DC input that an inverter might require.

To provide more power for longer periods of time, strings of series-wired batteries can be wired in parallel, thus extending the time the system can keep loads powered during times when no new source power (sun, wind, or water-generated electricity) is present. The practical limit is three parallel strings in a system, due to slight voltage imbalances between series strings that cause uneven performance and can cause premature system failure. The size of the battery bank, inverter and energy sources (PV panels, wind generators and/or water turbines) are all carefully matched and configured to provide optimum performance, with particular emphasis on optimizing the useful life of the battery system. To get the most life out of a battery – and they all eventually do need replacement just like any other battery – it is best to use identical new batteries made by the same manufacturer, using the same process, materials, chemistry, etc. You might even ask if it's possible to get all identical batteries with the same date code, if it's available. This tends to minimize uneven voltages, and countercurrents (due to those uneven voltages) internal to the batteries that work against optimum efficiency and battery life. Using similar logic to replacing all tires on a car at the same time, replacing all the batteries in a system at once is the best way to get more 'mileage' out of a home power battery bank.

Battery capacity generally depends on the volume of the battery, so larger batteries of the same technology tend to have greater capacity in Amp-hours (Ah), which is a typical term for specifying for how long a battery can supply that amount of current for an hour at the rated voltage. As an example, a 200 Ah battery delivers 10 Amperes (a.k.a. amps) for 20 hours. The generally accepted time rating for most manufacturers is 20 hours. A more important specification is the kiloWatt-Hour (kWh) capacity of a bank of batteries; multiply the nominal voltage of the bank by the Ah rating and then divide by 1000, to get the kWh figure; e.g., 24V x 200Ah = 4800 Wh, or 4800 Wh / 1000 = 4.8 kWh.

Two other terms that describe a battery's state are State of Charge (SOC) and Depth of Discharge (DOD). These provide numbers, usually as a percentage, of how charged or discharged a battery bank or cell is at any given time. The sum of SOC and DOD is always 100%. For example a system that is almost fully charged might have a 98% SOC and a 2% DOD.

There is no direct way to measure a battery’s SOC, but there are several ways to indirectly get a useful approximation, each with it's own level of accuracy. One way is measuring the voltage and comparing it to a standardized chart. This is the least accurate method, but also one of the least expensive, since it only involves an inexpensive digital meter, and often is part of an integrated inverter-battery system. One other method of estimating battery SOC is measuring the density or specific gravity of the electrolyte. (This involves accessing the sulfuric acid in the battery which is quite caustic, so safety precautions are essential.) This is the most accurate test, yet it is only applicable to the flooded types. It involves measuring the cell’s electrolyte density with a battery hydrometer. Electrolyte density is lower when batteries are discharged and higher as the cells are charged. Chemical reactions in the battery affect the electrolyte's density at a predictable constant rate which affords a good indication of the SOC. An amp-hour meter can also accurately gauge an accurate the SOC. Amp-hour meters track all power going in and out of the battery over time; thus comparing flow rates determines SOC.

Battery Technology
Some of the newest battery technology tends to find practical application first in lightweight portable devices of all sorts and transportation (e.g. lithium iron phosphate batteries for electric vehicles). In even smaller devices, using exotic materials and processes in tiny amounts isn't a major concern, while extending useful charge, reducing weight and optimizing portability are huge issues. However, on the other end of the size spectrum, battery technology for home power applications tends to evolve more slowly over time, due to the very different concerns. Unlike cell phones, tablets, etc., a home battery system doesn't need portability, small size or weight. It does, on the other hand, require massive energy reserves. For that reason, the de facto battery standard for cost-effective domestic alternative/remote energy systems is still the tried-and-true lead-acid technology. Lead-acid batteries are heavy, but relatively inexpensive both to manufacture and maintain. Timeframes between installations and replacements of well-maintained lead-acid systems are typically measured in years, so transportation and installation costs, while high compared to their small counterparts in modern electronic gizmos, are infrequent, and small compared to the system life.

Within the lead-acid battery category, there are a few variations. The most important one, as early adopters two or three decades ago discovered, is the difference between deep-cycle (e.g. those historically used on forklifts or other applications where it was expected that the batteries would use up a significant portion of their capacity before being recharged), and shallow-cycle batteries such as a conventional car battery, that is designed to be charged almost continuously by an alternator. The thin plates within a car battery allow for a quick high-current surge during ignition, but, as many of us have learned, leaving headlights on without the alternator 'topping off' the charge can result in a dead battery overnight. It doesn't take too many full discharges of a shallow-cycle system like that to require a new car battery. Even deep-cycle batteries should not be overly discharged. Too many discharges beyond 50% will decrease battery life.

The thick plates of a deep-cycle battery, on the other hand, are designed to deplete a significant portion (but not all) of the available current with hundreds of deep discharges, but not too many significant high-current surges. The heavy plates and bulky design of deep-cycle batteries allows for these deep discharges over extended periods, thus affording an effective home energy battery solution during multiple consecutive cloudy or windless days ... assuming judicious conservation is in effect during these times, which is another example of where conservation measures resulting in an energy-mindful lifestyle and component choices pay off handsomely. Most deep cycle batteries use a 'flooded' or 'wet' lead-acid design, meaning that their internal structure has sulfuric acid (liquid) and a water electrolyte into which submerged lead plates are suspended. Thick plates also maximize the lifespan of a battery which can be decreased by 'positive grid corrosion,' where the positive lead plate slowly wears away. Although plate thickness isn't the sole determining factor resulting in longer lifespan, it is perhaps the most critical variable, assuming batteries are properly maintained and used.

Another battery technology option to consider is sealed (developed in the 1970s) versus unsealed (relatively unchanged since their invention 1859 by French physicist Gaston Planté). Here's more on the history of batteries for those interested. Sealed batteries, also known as valve-regulated lead-acid (VRLA) cells, typically vent less gas and don't require regular addition of water which can be a decided advantage for cabins and locations where less-than-full-time occupancy and/or extended vacations are a concern. Sealed batteries use either a gel electrolyte to surround the lead plates or a fiberglass mat a.k.a. absorbed glass mat (AGM) to contain the electrolyte. When a gel electrolyte replaces a liquid, in some situations it can allow the battery to be used in different positions without leakage, although that's often not an issue, since once batteries are installed in home power systems, they usually stay put for many years. There are varying opinions whether the pros outweigh the cons in these two types (gel or mat) of sealed batteries. Either will typically decrease the routine battery maintenance needed, since unsealed lead-acid cells require periodic addition of distilled water and other maintenance as slow out-gassing depletes the electrolyte, and exposing the lead plates above liquid levels – an unacceptable battery maintenance practice  – decreases battery life far more quickly than a carefully maintained battery.

When comparing sealed versus unsealed lead-acid batteries, there are considerations favoring both that generally hold true. Unsealed batteries usually have the longest life and the lowest cost per amp-hour of any of the other choices (including the newer technologies, often for reasons of economies of scale and market saturation). The downside of unsealed battery systems is that they do require regular maintenance: watering, equalizing charges and keeping their terminals clean.

Other Battery Terms and Specifications
One of the most important battery specifications is the C/Rate, which quantifies discharge rates and charge rates. To calculate the C/Rate value, divide the cell's capacity by the number of hours it takes to either fully charge or discharge it. As an example, a 220 Ah capacity battery, discharged at 22 amps, is being discharged at a C/10 rate (220 / 22 = 10). If the same battery is charged back up by an 11 amp PV system, the charge rate is C/20 = (220 / 11 = 20).

The number of Days of Autonomy is another important specification for a renewable energy system. An effective design ensures that the typical year-round daily charging from all available sources (wind, PV, hydro, generator, etc.) exceeds the typical daily discharging from all loads. A primary requirement for any viable system design is adequate storage between charging periods, including extended days without significant wind, sun or hydro power, as well as night time lulls in input power. The Days of Autonomy figure is a rating that gives the theoretical number of days the system will provide power for the average daily load without any new power input; which also is a useful number when estimating time for replacing major source components if there is a power input subsystem failure.
Of course, each system is different and the selection and number of batteries, just as the sizing and number of PV panels, wind turbines, etc., will vary with each situation and location. Again, consult a local professional before making any cash outlays.

Installing Batteries: Housing and Safety
Locate your batteries in a safe, easy-to-access spot. Most batteries require enclosures that are lockable, sealed, insulated, and vented to the outdoors. Sloped box covers keep things from being piled on top, making the battery system safer and more accessible. Clear viewing windows allow for easy inspection. A removable side of the battery box eases replacement.

Since batteries store considerable zapping power and contain acid and other toxic elements like lead, they are dangerous. They should be accessible only to mature family members trained in proper safety protocols. Keep battery boxes locked but accessible when they need maintenance. Make sure caps and terminals on flooded battery cells are easily reachable. Well thought-out enclosure layout—ideally making all batteries easy to reach without having to lean over one battery to reach another—reduces the chance of accidental shorting, which could reduce the lifespan of both batteries and their owners.

Besides overall environmental factors such as protection from the elements, always consider the average and extreme temperatures (both daily and annual) of the battery housing which should usually be a very short distance from the inverter to minimize power losses and inefficiency. Temperature extremes and averages affect battery capacity and are a crucial part of the design process. Keep batteries out of direct sunlight to avoid uneven heating of individual batteries which could shorten life spans due to uneven currents. Battery manufacturers typically rate capacities at 77°F; name plate rated capacity decreases at lower temperatures and increases at higher temperatures.

Battery Interconnections
Solid metal bus bars come with some industrial batteries for making the series and parallel interconnections. However, most battery banks need cables for these inter-battery connections, as well as cables to connect to an inverter or DC load center. Size battery cables big enough to handle their maximum rated continuous current. Protect them with fuses or circuit breakers rated for high amp-interrupt current. Determine cable size from inverter specifications and/or any DC loads that the battery bank powers directly. 2/0 or 4/0 cable is common for residential-sized systems.

Don't use welding cable! This used to be a common practice for batteries, since listed cable was not available and welding cable was relatively inexpensive, flexible, and could handle ample current. However, welding cable is not designed for this application and is not listed by the National Electrical Code for battery system use. Do use flexible, UL-listed, NEC-approved battery cable – now readily available – for all battery wiring. If you are hiring a renewable energy system consultant – highly recommended, regardless of your level of proficiency and expertise, even if only to double check your work – you can ensure that your entire system, not just the battery subsystem, is safe, meets all applicable building and regulatory codes, is the most cost-effective, energy-efficient and long-lasting design possible.

Wiring Best Practices
Equal charging and discharging across all cells keeps batteries healthy; resistance differences within a battery bank can lead to premature failure. Poor lug crimps, loose terminal connections, unequal parallel cable lengths, and too small a wire gauge can all affect the equal treatment of cells, thereby shortening battery and system lifespan.
Electrons follow a variety of paths when entering or leaving a battery bank with multiple parallel strings. Therefore, it’s critical to minimize the number of parallel connections and ensure their lengths are equal. When wiring parallel strings, always make series connections first. Then parallel the positive ends of the strings, and finally connect the negative terminals. Connect inverter cables to opposite corners of the battery bank to keep electrical paths between strings as equal in length as possible.

Ongoing Battery Maintenance and Operation
In Home Power Magazine's article "Managing Your Batteries," author Dan Fink says: "Take the initial cost of your battery bank, and divide by the number of years until it needs replacement. That’s your annual 'battery bill.' If you can stretch battery life to eight or ten years, the bill is minimal. If you ruin them in a year, that’s a big bill, and you probably were not paying much attention to them. Overcharging, undercharging, and high and low temperatures can all count as 'abuse.' " As indicated, careful maintenance of batteries can make a huge difference in their lifespan, and thus the effective prorated cost of this highly variable component in the overall renewable energy system.

Before doing battery maintenance, remembering that you're dealing with strong acid, first put on protective eyewear, rubber gloves, old clothes and/or a protective apron. As noted above, monitoring the SOC (State of Charge) is not a simple process, but there are viable approaches, such as using temperature compensated hydrometers (the most accurate but most complicated method). Other methods, best used in combination hydrometer readings, are voltage measurements taken after a rest period (with no incoming or outgoing currents for 2 hours to allow the electrolyte to stabilize), and coulomb counting, which calculates Ah by measuring accumulated charge or discharge amounts, typically by use of highly accurate shunts (very precisely chosen low resistance elements in a circuit whose purpose is to measure current with sensitive volt meters. Meter choices include internal meters (built into modern inverters), system-integrated meters (for networked systems) and stand-alone meters. Many monitoring systems are integrated with computer interfaces for logging and analysis to automate some of the tedious aspects. The Home Power Magazine "Managing Your Batteries" article lists several options for each of the monitor/meter technologies just mentioned.

Batteries and Renewable Energy System Planning: Always Get Expert Help
This article should be considered merely an introduction to the subject. Along with exploring and educating yourself on as many of the appropriate links and references as possible, when you're getting serious about considering a renewable energy system, do consult a local renewable energy professional before spending too much time or money, even before you do a detailed energy conservation assessment. There's no substitute for knowing someone with industry expertise and working with their recommendations for a viable, safe, regulatory-legal and cost-effective system. Even if you plan to do some or much of the work yourself, 'reality checks' from seasoned experts early on and at key points in your planning and implementation phases is a sound investment you won't regret.

Additional References
Batteries for Alternative Energy Systems
Off-Grid Batteries: 30 Years of Lessons Learned
Choosing the Best Batteries
Battery Installation and Maintenance
General Information on Deep Cycle Batteries
Battery University
Lithium-Ion Batteries for Off-Grid Systems: Are They a Good Match?
No Batteries Required: Grid-Direct PV, Wind and Hydro-Electric Systems

Vendor Contact Info
Here are a few manufacturers of batteries and related components for home energy systems; there are many more online:
RE Battery Manufacturers (Home Power Magazine article, Access appendix)
Free Sun Power
The Inverter Store
Trojan Battery Company
Battery Monitors, System-Integrated Amp-Hour Meters, Data Acquisition, Networking & Internet Monitoring


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