That's a mouthful, isn't it? What it means is taking the stored electrical energy out of a 12 volt DC storage battery, and inverting (really CONVERTING, I don't know why this term was used) it to 110 volt AC.
UPDATE: 092315: I noticed that the prices listed in this article were out of date, so I have revised them to their current status.
With the invention of heavy-duty power transistors several decades ago, the development of an efficient system for making AC power out of DC was developed. Prior to that, the motor-alternator was the only way to do it. A 12 volt DC motor was connected directly to an AC generator (alternator) and the output was AC power, but of inferior frequency and voltage control, because as the voltage input to the DC motor varied, so did it's speed, and therefore the frequency (60 cycles per second) of the AC. If AC frequency is not maintained exactly, bad things happen, such as AC motors overheat.
The solid-state inverter eliminates all these problems, and inverters built in the last 10 years even have the correct sine-wave pattern to the AC, along with exact frequency control. This means that you can use them for computers, which have to have a very stable source of power.
There are two means of making your own AC power: you can generate it with a factory-built generator set (genset), or you can make it from battery power via an inverter. Generator sets are very common, but they have restrictions on their use. If you live in an apartment block, you probably can't use one. They require constant supervision, since they consist of an engine running at high speed, and such troublesome things like being maintained in fuel and oil. Oh, did I mention that they are noisy? Yes they are. For most people this is their biggest drawback. They can also kill you, as their engines make carbon monoxide while running.
None of these disadvantages apply to battery-inverter power generation. It is quiet, and in a small room containing your equipment, all you will hear is the cooling fan running on your inverter, if you have a model with a fan. It requires little attention, the only two items of maintenance being to keep flooded-cell (lead-acid) batteries topped up with distilled water, and to occasionally brighten the battery connections to promote perfect contact. Other than that, when you need power, you switch on the inverter and plug your appliances and lights into it.
Here is a generic list of materials which you will need to put a set together: battery (batteries if you want a beefier set), battery boxes for the batteries, an inverter, cabling to connect the batteries to the inverters, a battery charger, and ordinary extension cords.
Like generators (to some degree), battery-inverter set-ups are scalable. That is, you can build them to suit the work. If you wanted to run a few necessities in an average house, you wouldn't need a very big set-up, but if you plan on running refrigerators and freezers, a large TV and most of your lights, you will need more batteries and a bigger inverter.
Let's take this one component at a time.
First the batteries. If you go to your local department or auto-supply store, you will find what are called "RV Deep-Cycle" storage batteries at $65-80. These are usually re-manufactured, some offshore. You don't really know what you are getting. A battery is a simple device, but, as with any device, the quality of construction usually means something. In a battery's case, it definitely does. That's why batteries are sold with guarantees by the month. Personally, I don't buy any sort of battery with less than a 72-month guarantee. Storage batteries are rated as to their total capacity in "amp-hours", but they are also rated in CCA (Cold Cranking Amps) and Reserve Minutes. You may ignore the CCA rating, you won't be starting any engines with these batteries, and if you were, all of them would start a gasoline V-8 with no difficulty. The Reserve Minutes rating means that the battery will be able to put out x-number of minutes of power (at 25 amps draw) until it gets down to 10.5 volts (dead).
Let's remember Ohm's Law: Volts times Amps Equals Watts. Amp-hours are a quantity of amps over time. 120 amp-hours means that your 12-volt battery will deliver 10 amps for 12 hours. Except that it won't really do that. A 120-A/H battery will give you 60 useful A/H. The reason is that you don't drain your batteries below 50% of capacity before recharging if you want them to last, and all inverters will cut out when their input voltage drops below 10.5 anyway. That means that a battery down to 12.06 volts will not operate an inverter, because when you load that partially-discharged battery with the inverter, it's output will drop about 1 1/2 volts. Use this table as your guide. You don't need a fancy battery-condition computer (they sell them for about $250) to tell you how your batteries are doing. Just switch off your inverter for a moment and use a cheapo digital voltmeter on it and compare the voltage reading to the chart above.
If you get a marine-rated battery (the best ones are all marine-rated), it WILL be a quality battery. The reason is that things move around in the marine environment, and these batteries are built to take some bumping around, so they cost more. They will also weigh more, because they have more lead plates in them which gives them their increased power. A cheapo RV battery may weigh about 60 pounds, but a Trojan of the same size weighs about 72.
As to brands, you can't go wrong with Trojan. They are on the upper end as to price, but they have more plates stuffed into their tough container, so they actually give a little more power (8% more) than a bargain-basement battery. Another good one is Exide Nautilus, specifically the Nautilus Gold. Note that the Trojan is an "Absorbed Gas Mat" type of battery, which is newer (and more expensive) technology than the usual lead-acid. It is not the same as a "gel-cell", which are perhaps the ideal battery, but they have their own issues, the first being that they are triple the cost, and the second being that it takes a special type of battery charger to charge them. They are made in all shapes and sizes, and may be used in any position, not really important to our consideration. They do have one huge advantage, in that they may be drawn down to zero, like a nickle-cadmium or lithium ion battery, without damage, whereas a lead-acid or AGM battery must not be taken below 50% of charge, except in emergency and with the knowledge that you have just seriously shortened it's life.
Battery boxes. The most popular is the Tempo brand, and their ordinary battery box, sized for either the smaller Group 24 or the bigger Group 27 size, is about $12-15 at your local RV or marine supply place. They have an upscale one also, which has a 12VDC outlet wired into the top, as well as a set of indicator lights which give you the state of charge while you are using the battery, or with a press-to-test function when you are not. It is about $50.
Inverters. Inverters are all over the place in size (and price). I have three small ones, a Xantrex 175 Micro, a West Marine 150 and a West Marine 250. I have two larger ones, both older superseded models but similar to this Xantrex 800 and 1500, except that the older models ran on 12 volts, whereas with these new ones you have to tie two 12 batteries together in series for 24 volts. That's probably a lawyer-driven modification, since there is zero functional difference between the two except that with 24 volts, there is half the amperage, therefore less chance of heat buildup in the cabling. You decide how much inverter to get by adding up the AC wattage of your loads. If you have 150 watts of lights (you don't NEED that much, but you might plan for it), a 400-watt fridge and a 400 watt freezer to run, you DON'T need a 1,000-watt inverter, because you aren't going to be running the reefer and freezer at the same time. There is an idle current with these inverters, and it gets bigger as the size of the inverter goes up. For the average user, I would get two inverters, one small one for low power applications and one 800-watt one to run the reefer or freezer. That makes your batteries last longer. If money is no object, and you are going to put together a large battery-inverter set, get one of these. It is a combination inverter and charger, and it has automatic changeover, so you could connect it directly to house power to make an Un-Interruptible Power Supply (UPS). You will need at least 4 batteries, and you'd be best to go to Golf Cart batteries. They are more expensive, but since they are meant to push a 400# golf cart with two 250# blutos around a hilly golf course, they have the ability to discharge more current safely than other types. They can be bought at Costco for $90 each, and since they are six-volt batteries, you need at least two, and preferably four.
Battery chargers. You need to be able to recharge the batteries. You do that under emergency conditions with either a generator set powering a battery charger, or with your automobile alternator. A couple of caveats here: I can hear your first squawk already. You are saying "I thought that the whole idea of this batter-inverter idea was to avoid generators". You're right, the concept avoids generators for the continuous-draw loads, but you have to recharge those batteries somehow. If you have a protracted outage like they have had in recent years in St. Louis, MO and Seattle, WA, you will have to recharge those batteries several times. If the city can't supply that power, you have to. The simplest solution is to have your vehicle outfitted with a 12-volt heavy-duty pigtail lead that you can plug the batteries into and let the car alternator charge them up, which it will do quite rapidly. Most cars have a 50-70 amp alternator, which will recharge your battery in about an hour, two batteries in about 2 1/2 hours (Group 27 storage batteries). You need some way to keep your car's engine revved up to about 2,000 rpm, where the alternator makes the best charge rate. A brick will do nicely, but you could have your vehicle fitted with a hand throttle as well. Any fab shop which services farm equipment can do this modification for you.
The other solution is a small generator powering a battery charger. Forget about the "battery charging" loop on the genset, none of them provide more than a 15-amp charge, and that's too slow (3-4 hours for ONE group-27 battery). The best battery chargers are made by Xantrex, the same people who make the inverters. I have a Tru-Charge 40. It's an expensive charger, but it will do all the charging functions. To charge a battery properly, first you need to stuff in some fast amperes. This is called the Bulk charge, and it recharges the battery up to about 85 or 90%. then you get to the Float charge, which tapers off and does the rest (but takes a lot of time, all day). The very best chargers also have a maintenance cycle, in which they regularily discharge the battery and recharge it though the three stages. This is called "de-sulphating", and it make a battery last years longer. A good battery charger does just that - it makes the batteries last much longer and provide better output when they are called on to do their job. You can get a useful charger for about a third of the Xantrex, though. This one will fill the bill nicely.
Cabling. You need cabling to ties the main components together. All connections should have lug-terminals on them, the kind that go on a bolt-type battery terminal. DO NOT use anything with spring clamps. The actual connection area of a spring clamp is so small that heat build-up is almost guaranteed, with it's attendant fire hazard and loss of efficiency. You could only be drawing 10 amps from the battery (giving you 100 watts of AC), but the clamp might be drawing another 3 amps in heating itself up. That 3-amp heat build-up WILL burn something up eventually. Your cables should be at least #4 size. You can go to a marine supply place or an automotive supply place and they will have a good selection. Use the correct color coding, red for positive and black for negative, and measure before you shop because the longer a cable you have, the more power you waste. You want the shortest cables that will tie your system together.
An alternative. There are ready-made systems available. The best one I have seen is this Xantrex, but they come smaller also, and are available everywhere in big-box stores like Home Depot or Lowe's. The Xantrex will power good-sized appliances with it's 1,500-watt inverter, but it only has a 60 A/H battery in it, so it won't run for long at high loads. It does have inputs, and might be capable of having additional storage batteries put in parallel with it, giving it the legs one needs on output, but I'll bet it's charger won't back-charge those other batteries, so you are back to dealing with battery charging anyway.
Let's put a set together. Let's build a basic battery-inverter set. We will need:
- One battery. Let's get a mid-range Exide Nautilus Gold 105 A/H. We'll pay $160 for it locally.
- One battery box, the Tempo Power Center box. That's about $62 with shipping.
- One inverter, an 800-watt Xantrex Micro, about $110 shipped.
- Cabling, two 14" battery cables, two 24" at $24, buy locally.
- A cheapo DIGITAL multimeter, about $8 locally.
- 2 size 14 extension cords. Price depends on length, about $25 locally.
- Battery charger, about $190 shipped
You are going to fabricate a hold-fast to fasten the inverter to the top of the battery box. Time, with pop-rivets and fender-washers, about 10 minutes with whatever strapping you have handy. While you don't have to fasten the battery charger to the battery box, I'm thinking that this whole lashup will fit nicely in a RubberMaid footlocker, about $40 at Ace Hardware. Just to be sure we can stretch out our recharge capability, we are going to add the pigtail-lead set in, another $90 or so shipped, and if you pay to have it hooked up, whatever those labor charges are.
Gensets? I would only recommend the Honda line, and they are not that cheap. An EU1000 with about the same power capacity as the battery-inverter set we just built will set you back about $650, while a beefier EU2000 will cost you $950. I have the EU2000, and it did just fine when we lost our power for a night during the recent Northwest storms, but I also used a small battery-inverter setup for my computer power and local lights in my computer room. Because I ran the genset continuously (save for refueling it), I didn't get a lot of sleep. With just 3 more batteries, I can lash together a 1500-watt set with my large inverter, so I'm going to do that soon.
Running all this? Easy as pulling the extension cords out, hooking up the things you are going to power, and throwing the switch on the inverter. Just a quiet hum tells you that it's running, plus the lights on the battery box come on. You can do the math if you have a steady draw, and calculate when you will be down to 50% on the battery, or you can hook up your multimeter and just eyeball the battery voltage occasionally. When it's time to recharge, you run out those 30 feet of jumper cables, plug into the pigtail lead, start your car and you're packing amps! Run the car for an hour, then shut it down and repeat as necessary. If the mains power comes back on, plug in the battery charger to get the battery back up, but don't leave it plugged in. For long-range maintenance, have a bottle of distilled water and a battery-bulb handy and refill the cells to the ring as necessary. Once a year or so, take apart all the connections, and using a small welder's steel-bristle brush, polish them bright and re-assemble.
You're prepared for a power outage now, my friends, but if you want to get educated in more depth, read this excellent 12VDC primer.
In "RV-speak", where motorhomes and many travel-trailers have AC/DC systems, the "inverter" refers to a system or device for producing AC from DC; a "converter" - or "converter-charger" as it is sometimes called - produces DC from AC "shore-power" for operating the RV's low-voltage systems (lights, water pump, ignition voltages for refrigerator, furnace, and water-heater propane burners, blower voltage for furnace, sensor voltage for system-health management monitors, and ventalator fans) and the required charging voltage for the "house" batteries.
The most common converter-chargers are rated between 35 and 65 amps and can comfortably handle all the DC load of the RV for which they were spec'ed - while providing a modest output to charge the deep-cycle marine batteries used to power the RV while not "hooked up."
Due to the voltage/current relationship and DC-to-AC ineffeciencies inherit in voltage inversion, operating high-current/power consumption appliances still remains problematic. Most RV's have addressed this by equipping the RV with a generator rated for the high-consumption appliances.
In terms of power budget, RV's combine the best of both worlds: systems or devices with large current/power requirements are typically non-essentials and are powered by shore-power or generator, the two best sources for high-voltage/high current power. Under most conditions, people can live without TVs, VCR's, microwave ovens, and air conditioners; things like RV refrigerators, furnaces and water heaters by necessity are designed to be very effecient and to operate from low-voltage sources (house batteries) and gas when AC is not available.
Posted by: JT | January 04, 2007 at 07:46
I wrote some about this a few months back in relation so solar power, and a couple times a few more months back; once for camping power, and once jsut on generators.
I'm actualy saving in the fund for the eu2000i; both for emergency home power, and bugout power (only 43lbs); as well as general ouitdoor power needs. Quiet (relatively), clean, and efficient, but yeah not cheap.
Anyway, in the plan for us, is a large battery setup (I'm thinking a bank of large marine storage batteries), wired in to a solar system (living in Arizona it'd be silly not to), and with leads for the generator setup to act as backup and surge power. I figure 6-8 or so of the long reserve yacht batteries, or solar accumulator batteries will do us pretty good; serving our emergency household loads for several days at least, for about $2000-2500, plus another $1000-$1500 or so for the charger, inverter, wiring, and cutover switch (Depending on how the new solar technologies pan out, we may wire in the system as part of our normal household electric as well).
If the budget allows, I'm going to pick up both an eu2000i, and a larger generator (probably not the 3000. Though it is excellent, it's also a lot more expensive than comparable power rated gensets from other major manufacturers) for fixed site power; dedicate the 2000 just to mobile power.
Actually first step here though, is to pick up a large deep freezer; somethign I've been meaning to do for a year now. With a deep freezer and a goodly supply of ice, that's a fair bit of your power load reduce already.
Posted by: Chris Byrne | January 04, 2007 at 08:55
In dealing with power emergencies, one has to remember that appliances like air conditioners, freezers, and refrigerators have one common characteristic: all have high-current-starting motors in the refrigeration compressors. Although you may have an inverter rated for the current draw listed on a freezer or fridge's electrical rating plate, the >surge< current of the motor starting up must also be taken into account.
Posted by: JT | January 05, 2007 at 06:29
Great comments, people.
JT brings up something that I haven't thought about before: the possibility of buying an AC/DC/LPG refrigerator/freezer for permanent home use. It might be considerably more expensive to buy, and might not be as efficient when run as an AC-only model, but wouldn't it be great to just have a bottle of propane and a small 12VDC battery to ignite the flame and not even have to worry about that load or it's surge-start problem?
The other thing I've been thinking of is to look into having a couple of big bottles of CNG, although it would take a very serious type of SHTF situation to end NG gas supply, the usual severe storm doesn't affect it.
Chris, you're right about solar, and in the PHX area, wind also. My daughter's sailboat which she lived and cruised on for 3 years was equipped with both, a 64 square foot solar array and a wind generator. The solar array could be counted on for almost 10 amps output (and it was 1st generation), and the wind generator would put out 18-20 in a strong breeze, and 5-6 if it was turning at all. Since the boat had only one long-term load, a cold-plate refrigeration system, and one occasional one, the autopilot, the two sets of golf-cart batteries would last for weeks without any sort of A/C input.
There is one caveat to off-grid in the SouthWest, and that is cooling. If living off-grid, you simply have to give up air conditioning and maybe even swamp cooling, you just can't supply that may kilowatts. That calls for personal cooling (wearing cotton clothing and misting water and making breezes on yourself).
Posted by: Rivrdog | January 05, 2007 at 13:03
I seem to remember that one type of battery needs to be stored at 50% charge for long term storage. That's not lead-acid, correct?
Posted by: Anon | January 07, 2007 at 19:55
Anon, I don't know about any storage battery with such a requirement. When some lead-acid models are sold, they have no acid in them, and must be filled at the dealer, then charged. I know of no such requirements for LIon, Ni-Cad, Gel or AGM batteries.
You may be thinking of self-discharge rate. Lithium Ion will discharge at almost 30% per month, but the others self-discharge at no more than 5% per month.
I have a lot of LIon batteries, and I must say it is definitely a pain in the patoot to remember to charge them on a calendar basis, but I put up with it, because where else can you get a AA cell with 2.8 amp-hours capacity? I have a marine walkie-talkie that I replaced it's Ni-Cad battery with LIon, and now the radio will operate three times as long as with a NEW Ni-Cad.
Posted by: Rivrdog | January 08, 2007 at 18:33
I was looking to buy a food slicer, and then I noticed it sayed It had a D.C. motor. Does that mean I have to run it off a battery? Or maybe a converter? The info. on the appliance, witch is on the internet, does not give any info????
Thanks,
Norm
Posted by: Normand a Kerr | February 18, 2007 at 11:08
You may be thinking of self-discharge rate. Lithium Ion will discharge at almost 30% per month, but the others self-discharge at no more than 5% per month.
Posted by: Juno888 | June 25, 2007 at 01:49
i am interested in building a power system that could handle all applainces that ia use in my house.
Posted by: Harold Galley | July 09, 2007 at 05:05
i like to use the inverter in my car, that battery is 12volts, and if i do attach the inverter to my car battery in car then can it be possible that i can play the ac appliances in the car, for example home theater system or any lights,,,,that we use in home.i want to generate the dc power to ac. so i need some assistance to do that,and if u help me i will really be obiliged.please to assist me what r the equipments i need and how to do,,,i heard from some one that if the car will be in running condition then will i be able to use the power of the battery to use for ac current or only when i switch off the engine i will be able to get the power....?
ED NOTE: You can do this, Sanjeeb, but remember the numbers. The average car battery is about 60 ampere-hours or less, and a moderate load will drain that battery in minutes. Also, if you are using the accessory plug in the car to power the inverter with, you cannot use more than a 120-watt load through them, or you blow the fuse. You best way to go is to put in a second battery of the deep-cycle variety, then use a battery isolator to connect it to the primary battery. The load will always be on the secondary battery, and your car will still start and run properly on it's primary battery. If you get a Group 31 deep-cycle battery of 115 amp-hours, you can drain it at 25 amps (300 watts) for almost 3 hours without recharging and without risking damage to the battery (the 50% drainage rule). Don't forget the capacity of your car's charging system. It has to be up to the task of putting all those amp-hours back into the battery, which usually means long engine runs to do it. If you can, use an AC battery charger and plug the car in to it to recharge. Don't forget that you will need at least 4-guage copper cables to hook up a set-up like this.
Posted by: sanjeeb | September 14, 2007 at 09:08