DIY Backup Power System for Home or Off-Grid Cabin
by Papa Bear
The Dallas Observer published a report[i] recently that Texas has the worst electric grid in the nation. This was based on a North American Electric Reliability Corp report[ii]. That’s not very encouraging. They are also reporting that there is the possibility of rolling blackouts in 2013.
For a couple of years, I have had the interest in setting up a backup power system for home use. There is a mobile system that provides power to work a couple of times per month and is a backup system when at home. It provided power for 2 freezers, a refrigerator, and a fan during 3 days of outage one summer several years ago. But we really want a system that has the specific purpose of supporting the house.
When looking for a mobile 120 volt (V) system I did talk to an “expert” who wanted to sell the best system that they had. He wanted to get me excited about selling power back to the electric company. After redirecting the talk with the expert to a mobile plan, and taking some of his advice, it proved to be an exercise in frustration.
Not wanting another talk with another overconfident salesman I decided to plan the home system. One essential requirement is to start with home power needs. There are charts available that will show what most appliances consume.
Another way is to check the sticker on the appliance and see what the manufacturer states is the power usage. Another way is to use a meter that will measure the power usage. The most well known and used is a meter called Kill-A-Watt.
There are other brands that perform the same function. Plug it into a 120V outlet, plug your appliance into it and let it run for a day or two. It does not store the information so you must write it down before unplugging. Check the number of watts (W) that it has used, the total time that it was plugged in and do the appropriate math for 1 day of consumption.
Example: our medium size freezer was tested for 2.76 days giving us an average consumption of .938 KW per day.
Doing this with each 120V appliance can give a measure of our household needs. Remember that the appliances may not be all running at the same time. The total consumption will be one of the concerns in picking batteries and charging them.
How do you pick a battery system?
Let’s look at some battery definitions. Cold Cranking Amps. Cold Cranking Amps is a rating used in the battery industry to define a battery’s ability to start an engine in cold temperatures. The rating is the number of amps a new, fully charged battery can deliver at 0° Fahrenheit for 30 seconds, while maintaining a voltage of at least 7.2 volts, for a 12-volt battery. The higher the CCA rating, the greater the starting power of the battery.
Open Circuit Voltage (O.C.V.) The voltage of a battery when it is not delivering or receiving power. It is 2.11 volts for a fully charged battery cell.
Reserve Capacity (RC) Reserve Capacity, (RC) is a battery industry rating, defining a battery’s ability to power a vehicle with an inoperative alternator or fan belt. The rating is the number of minutes a battery at 80 degrees F can be discharged at 25 amps and maintain a voltage of 10.5 volts for a 12-volt battery.
RC is the number of minutes a new, fully charged battery at 80 degrees F will sustain a discharge load of 25 amps to a cut-off voltage of 1.75 volts per cell (10.5V on 12V battery). This battery rating measures more of a continuous load on the battery and is a much better indicator of how it will operate bilge pumps. An RC number given in the specification indicates that it is more than just a cranking battery and probably a hybrid starting battery. This is a very useful rating for a boater.
Reserve capacity is directly, though not completely, related to battery plate size and quality. As a general rule, cranking batteries have little reserve capacity after cranking operation unless they have thicker plates. If they have thicker plates, it will have a lower CCA rating.
Amp-Hour – battery rating: AH is a common battery rating for batteries. Amp-hour rating of battery capacity is calculated by multiplying the current (in amperes) by time (in hours) that the current is drawn. Variations of the amp-hour battery rating is the most used rating. It most commonly signifies a deep cycle, marine or industrial battery.
Example: A battery which delivers 2 amperes for 20 hours would have a 40 amp-hour battery rating (2 x 20= 40). This is known as the 20-hour rating versus other ratings based on times such as 5, 8 and 100 hours, but also at different amperage rates. Such ratings are given based on what is considered most useful for the intended application.
A battery intended to supply low amperage for long periods, for example, would use the 100-hour method, whereas a 5-hour rating would likely be for a high amperage rate. The 20-hour method is most common.
Marine – It seems as if every battery manufacturer today sells “marine” batteries but, as mentioned earlier, many such take considerable liberty with the term. Some marine batteries are deep cycle, others are hybrids, while others are pure hokum. True marine batteries are designed for dual use of engine starting and house service and are therefore hybrids (not true deep cycle). These will have spongy, porous plates that are significantly thicker than automotive batteries.
They will be larger and heavier than auto batteries. A true marine battery will tolerate up to 50% discharge, whereas a deep-cycle and industrials tolerates up to 80%, whereas an auto battery will quickly die at such discharge rates.
Numerous batteries found in small boats will be labeled “auto/marine” and the only way to tell the type is by cutting it open and examining the plates unless you are buying a reputable brand, but it’s still a pretty good bet that any battery so labeled isn’t going to be very good. There are also very many brand names of this type, and also many of low quality.
Deep-Cycle – These batteries are distinguished by having much thicker plates (1/4″ or 0.270″ for Surette), nearly seven times thicker than an automotive battery, but high-quality batteries will have solid lead plates versus others made of a lead powder composite. Lead powder plates allow for much more rapid charging but also deteriorate much faster, whereas solid or more dense and thicker plates are slower charging but have a much longer service life.
Deep cycle batteries withstand greater abuse and thousands of charging cycles and have much greater service life than the other two types. They do not, however, have as great cranking or burst power, being designed to provide power over longer periods of time. These are best for use with inverter systems. They are identifiable by their cost of 2-3 times that of other types and 20 hour AH ratings. The number of brand names of this type is relatively small since the cost is higher. Good quality ones are usually not found in discount stores or mass retail outlets.
Golf Cart – batteries are generally a quasi-deep cycle similar to marine, and though not as good as batteries with solid plates, they are better than the auto/marine types. Usually set up in banks of six-volt batteries, these have a greater number of plates to provide longer periods of use under a constant power demand and deep discharging. T-105, US2200, and GC-4 are common identifiers. These batteries can discharge up to 80% without being damaged. They are not better for use with inverters than true deep cycle batteries.
Industrial Batteries – “Industrial” or “commercial” has long been used as a designation for deep cycle batteries used in forklifts, sweepers, floor cleaners and similar battery-powered machinery. Similar to golf cart but usually true deep cycle types with much heavier and pure lead plates up to around 0.270″ thick. These batteries can discharge up to 80% without being damaged.
Yet another type name has crept into the lexicon recently, is the RV type. Most RV types sold are cranking batteries or hybrids as indicated by their higher cranking power but lower reserve power.
Obviously, the deep-cycle is the preferred battery type for marine use but for its one drawback of being less able to provide high cranking power. This is overcome simply by increasing battery size.
AGM Batteries – AGM stands for Absorbed Glass Mat which contains the electrolyte absorbed in a mesh of Boron-Silicate glass fibers. Thus there is no fluid electrolyte to leak or spill nor will they suffer from freeze damage. There are two big advantages of this type. First, it can be charged with conventional chargers without fear of damage from modest overcharging.
Second, water loss is reportedly reduced by 99% because hydrogen and oxygen are recombined within the battery. Further, this type has a modestly lower self-discharge rate of 1-3% versus up to 15% with standard lead-acid batteries. The AGM is a true no maintenance battery.
It otherwise has similar characteristics as the standard lead-acid battery. They have yet to see much use in boats, probably due to the higher cost. These are widely used in battery back up power systems and solar systems.
The downside is the cost of around 2-3 times comparable standard batteries. Thus their greatest benefit is for installations where it is hard or impossible to ventilate charging fumes such as the interiors of sailboats.
So which one?
The battery type that you pick will usually be determined by the available budget. Getting the biggest and best there is may be nice but won’t help a bit if it means going into debt to do it. Also, with multiple batteries in a battery pack, they should all be purchased at about the same time and be the same type/kind. Never mix older batteries with newer ones. The older ones will degrade the new ones. It should be apparent, but let’s say it anyway: never depend on a single battery of any kind.
Searching online shows the full range of battery types. The true deep cycle batteries have AH ratings that are high but so is the price tag. Many of the lead-acid and AGM batteries are 6V which means you would need to buy them in pairs. One pair would be hooked up in series to make 12V. That means positive post of one 6V battery to the negative post of the other, and negative post of the first battery to the positive post of the other.
Normally individual 12V batteries will be cabled together in parallel. What this means is that the positive connections will all be linked and all the negative connections will be linked as well. In this way, your 12V battery pack will give you a combined reserve power. If you are working with 6V batteries you would connect the positive connection of one battery to the negative connection of the other, then the negative of the first to the positive of the second. That would produce 12V.
Think of it this way: parallel connection increases amperage, series increases voltage. Standard battery cables are ok to use. Try to keep them all the same length. If your batteries have screw terminals connect to those instead of the battery post.
How many is enough?
There is no easy answer to this question. If you listen to the solar experts they will tell you to buy 2 times the number of batteries that you think you will need. Their justification is that you should not use more than 50% of the battery’s reserve so that you do not shorten its life.
In the beginning, I used a single battery for mobile power. Yeah, that was dumb. It was adequate for the task but that was all. For extended use, I replaced the single battery with 3 marine/deep cycle batteries of the same size. That number was based on available space and budget. Using that trio in parallel connection has shown itself to be a good choice for the task. Based on personal experience, decide how much battery power is needed to meet your expected daily power consumption then add at least 20%. The extra should be enough cover what you did not plan for.
Deep cycle batteries will typically last 4-5 years. That is the lifespan not the warranty. Expect to replace them. Knowing that their end of life is coming you could plan your next purchase 1-2 years in advance, using that as the time to make an expansion in capacity if need be.
Is there any maintenance?
With a lead-acid battery, which are sometimes called a flooded lead battery, you must periodically check the fluid level. This means opening a plug and viewing the level. If the fluid level is below a marker then you must fill it until it reaches the marker. In most cases, there is a ring at the end of a short column.
The fluid must touch the ring. Fill the battery with distilled water. Do not use tap water, filtered water or purified water. There will be minerals in it that will shorten the life of the battery. AGM batteries do not have a level to be checked.
Over time all batteries will collect dust on the top. Wipe the dust off as there is a very small possibility that it will conduct power between posts. Also, check the connections to make sure they are still tight. Check the terminals and cables for any sign of corrosion.
How to use the batteries
There are 12V appliances available. If you want to get ideas to go to a large truck stop and look at the 12V accessories that are offered for the truckers. There are fans, slow cookers, toaster ovens, and coffee makers just to name a few. At RV stores you can also see small refrigerators, lights, and other accessories. Online searches can turn up major items such as deep freeze and larger refrigerators[iii].
Feeding 12V power into your home is not difficult. There are 2 basic plans: feed power from the battery pack directly to the location of the appliance, or feed power to a point where it is distributed as needed. The 2nd method is similar to the load center for your household current. We are planning on having some 12V products so a load center will be part of this plan. Electronics supply stores often sell security camera systems.
One accessory for the security cameras is a 12V power load center that is fed from a built-in transformer. This should be easily adaptable as a load center for a 12V system. The 2 terminal blocks at the top of the box are for connecting the devices needing power. You can make one yourself or buy one.
Most of the time we will need 120V power for the existing appliances. There will be a need for an inverter. It will take the 12V power and produce the 120V that need. Since we have estimated or measured the amount of the appliances will use we can use this to decide how large of an inverter is needed.
The 120V power that comes into our homes is alternating current (AC). It is graphically illustrated as a sine wave because the power flows forward then backward by the same amount. In the USA this flow, or cycle, occur 60 times per second. In many other countries, it cycles 50 times per second.
Inverters are made to produce either step wave or sine wave. A step wave imitates the 60 cycle sine wave but in incremental steps. The sine wave inverter makes a smooth flow of power. If your power needs include medical devices such as a CPAP machine then you will want a sine wave inverter for the device power supply. Note that the sine wave inverters are more expensive than the step wave inverters. You can run 2 inverters, step wave, and sine wave, from the same battery pack if need be.
The first mobile inverter I had was a 1000W major brand that came from the solar expert’s store. It proved to be completely inadequate. What I later learned was that the total wattage is split among the outlets. So if the inverter produces a total of 1000 watts and has 2 outlets, each outlet can support a load of 500W.
Some inverters have 2 values in their specifications: continuous load and surge. In that case, the surge amount is divided by the number of outlets. So if the inverter has a 1250W capacity with 2000W surge each outlet will have a capacity of 1000W. Unless the inverter specs show a surge, there is none. The total value is all you have to work with.
Many inverters have LED indicators that show the amount of power that is being taken from the batteries and voltage level. Most will also have cooling fans that are thermally controlled. Those that do not specifically say they have thermally controlled fans will keep the fans running all of the time. This means that the inverter is taking power from the batteries when it may not be needed. Most inverters will also have a low power shutoff. If the voltage drops below 10.5V the inverter will not run.
There are inverters that go to 240V of you have a need. These can be used on a well pump for example. These require 24V supplied by the batteries.
Inverters can power devices that are plugged directly into them. A good extension cord running between the inverter and the appliance is one method of doing this. Another is to connect to the household wiring via a transfer switch. The transfer switch is usually an external switch that cuts out grid power and supplies power from the inverter.
Ways to recharge them.
Charging batteries is not done at 12V. Most charging methods will be about 15.5V. Most charging systems talk about charging in amps not watts. There is an easy way to convert:
Amps x Volts = Watts
This first one may sound silly but the basic battery charger found in most automotive or farm stores will work. If your backup power system is waiting for the next grid outage this will keep the batteries peaked.
Make sure that it is an automatic charger as they will start at their maximum value and make the charging current taper to a minimum until the battery system is fully charged. It will then act like a “maintainer” to keep the batteries from self-discharging.
A trickle charger is the same thing as a maintainer. It puts a very small amount of amps, usually 1A, when the battery needs it. This also works if your power system is on light duty until the next outage. There is a manual charger but you must monitor the charger. When the charger shows that the batteries are fully charged you must disconnect the charger.
The next type of the charging method would be 12V automotive alternator that is mounted on an exercise bicycle or some similar device. There are a couple of alternators to choose from but the easiest to work with is a model that has an internal regulator. Another way is to use a small gas engine to run the 12V alternator. There is a web site[iv] that shows how to make your own small engine charger and sells parts to do this with.
Many of the AC producing gas generators, usually those below 4000W, have a connection for 12V. The documentation for these generators say 2 things that are of interest: the 12V connections are only for charging batteries and they are not regulated.
Using the 12V output from one of these generators to the batteries would easily lead to an overcharge situation which would be very bad for the batteries. To prevent this means connecting an automotive regulator to the 12V output then connecting the regulator to the 12V battery pack.
So at this point, you might wonder, if there is an AC generator why use the inverter? If your batteries are very low connect the AC appliances to the generator while the batteries are charging. Unless there is an unlimited supply of fuel for the generator, and it is completely silent, you will not want to have it running all the time.
Next up in the power chain would be a windmill. They are great for producing power, can be configured for 12V or 24V and have very little upkeep. One little problem: they require a minimum of 10 mph of constant wind speed in order to produce electricity. Some of us cannot get that much wind speed. A secondary problem is visibility. They are up in the air, rotating blades, acting like a beacon to anyone who is without power.
Finally, there are photovoltaic or solar panels. In most towns and cities they can be seen attached to the flashing stop signs or school zone signs. It is the same concept as home use.
Solar panels can be purchased in single panels or in sets. They are as small as 5W or currently as large as 290W. The choice is not easy and neither is the price. When selecting panels remember that the higher values may mean a faster charge but the batteries usually respond better to a slow steady charge.
The maximum power of any panel will be achieved when the panel is in “ideal” position. Ok, what does that mean? Simply put, the panel must be directly facing the sun. In other words, if the panel is perpendicular with the sun then it is directly facing the sun. When the sunlight strikes the panel at an angle the power output drops. Open circuit voltage for solar panels can be anywhere from 17.2V up.
In the 1980s there were experiments done with solar panels. One of these tests was to devise tracking systems so that the panels were kept facing the sun. Most of the early methods proved to be difficult to work with.
Another thing that was done was to place the panels at an angle that more closely matched that of the sun. For example: if the location was Denver, CO the latitude[v] is 39 degrees meaning that the panels should be mounted at the same angle from horizontal.
Today people generally point them in a south facing compass direction and put them at a 45-degree angle. Not ideal for maximum utilization but it is easy to set up. Modern tracking systems are available.
Another possibility is to have multiple panels but not all pointed at the same compass direction. Place them at several compass directions so that a lower peak current will occur but be spread out over a longer time. For example, if there are 5 panels in the solar array place each of them at 5 compass angles along the path of the sun. With solar panels, it is possible to mix panels with different output.
The solar panel approach will require a charge controller. This is a device that takes the input from the panels and feeds it to the battery system until the batteries are charged. The smallest one will handle a total of 105W or 7A. Better ones can support 30A or more and will show the state of charge of the batteries. All of the charge controllers prevent the batteries from discharging through the solar panels when the sun is down.
There is a small amount of upkeep with solar panels. You should keep the surface clean. Dust and dirt that accumulates will cut down on the amount of sunlight that the panels get.
Let’s look at all this as 2 possible choices – one where finances are unlimited, and the other where there is a budget.
Description Type Quantity
AGM batteries 210AH 8
Inverter 7000W 24V 1
Transfer switch 1
Solar Panels 95W 12
Charge controller 24V 1
Tracking system 1
The batteries would be wired into a 24V system in order to feed the inverter. The transfer switch would perform the cutover during a grid failure. The tracking system would make sure that maximum power is delivered to the batteries.
While convenient it becomes too easy to live in the house with a full power system. It does not give the incentive to conserve during difficult times. The movement of the tracking system may attract attention.
Description Type Quantity
Deep cycle/marine batteries #29 5
Inverter 3000W 12V 1
Solar Panels 40W 5
Charge controller 1
The budget system would be manually connected to appliances. It is possible to run some things, such as freezer and/or refrigerator from the inverter all the time. That way there is no loss of food in the event of an outage while away from home. The panels do not all have to be purchased at the same time, plus they could be mixed with smaller ones that turn up on sale.
I do not have all the answers. This discussion does not include all the possibilities or all the details. Any product that may appear in a web link is not an endorsement.