12V DC System

I am planning a 12V DC system at my house. Can you please help regarding connectors, switches and fuses: specifically, brand names, specification and suppliers?

Thank you,

Warren M.

Warren:

I will provide the answer to your question but first wanted to ask why you want to do this.

The only time I would recommend wiring an entire house for DC power would be a small remote off-grid cabin. In this case you would only have a few ceiling lights, maybe a DC radio or TV, and a DC refrigerator or freezer. If this is all the electrical system you need, and there will be no electric kitchen appliances like a micro-wave oven or AC refrigerator, then you would not need an AC inverter and could get along with just a 12 volt wiring system.

For many years we designed off-grid homes for only DC because inverters were not reliable and had very poor efficiency. However, today most quality inverters like Outback and Xantrex are very reliable and have a very high efficiency, so there is very little energy loss through the inverter and little fear that it will fail and leave you in the dark. This means there is no need to wire a home any differently then a grid connected 120 volt AC wiring system.

This means you will not need the very high-cost DC switchgear. For example, a 15 amp DC circuit breaker is larger than the size of two packs of cigarettes and costs $25 each, and are not stocked by any local electric supplier. A 15 amp 120 VAC circuit breaker is one-third the size, costs about $4.00, and are sold at Lowes, Home Depot, and every electrical supplier.

As an example, four 100 watt 120 volt AC room light bulbs will draw under 4 amps of current and a #14 wire size can easily handle this. If these were 12 volt DC bulbs, four 100 watt DC bulbs would draw 34 amps and would require a very heavy #8 wire. Now you may install smaller light bulbs, but no matter what the wall load, the current will be 10 times greater at 12 volts DC than at 120 volts AC ( 120 / 12 = 10 times ).

Several months ago I was called in to rework a large off grid home that was built 15 years ago when high-efficiency inverters were not yet made, which had all DC wiring, DC wall switches, DC circuit breaker panels, and DC light fixtures. This was a large 3 story home with 4 bedrooms, and all this wiring had to be ripped out from behind the drywall because the owner wanted to convert to using inverters and install all 120 VAC lights and appliances. The existing wiring and DC fixtures were not to code and could not be re-used. This was a real nightmare and very costly.

Unless you are building a remote off-grid cabin, stay with standard 120/240 volt wiring and high efficiency fixtures and appliances.

Good luck,

Jeff Yago

Battery or Inverter Problem?

Dear Jeff,

In May last year, I purchased a 3KVA 24 volts inverter (Victron model) from a local distributor along with 4 units of 12 volts 200AH wet batteries connected 2 in parallel to meet the 24 volts required by the inverter. However less than 2 months into the use of the system 2 batteries failed and the supplier replced then with 2 new batteries of the same make. A short while later (about 2 months again) 2 batteries failed again. The supplier adviced that I should replace the batteries with dry cell deep cycle batteries which cost about twice the former units which I did. Now just about 8 months into the use of these deep cycle batteries, the runtime provided by these batteries have dropped from about 24 hours to less than 4 hours.

I have contacted the supplier and at first he said the inverter needed servicing which he then carried out but the runtime has remained the same. He now says the batteries may have failed since according to him the unit is operating efficiently. My problem now is that I am weary about investing in new batteries which are expensive since they might fail again.

What do you think might be the problem and is there a way I can tell what happened to the batteries.

Regards

Eme Iheanacho

Eme:

You have not provided enough information for me to help. For example, is this inverter being used as a backup power system and only draws down the batteries during a power outage. Is this an off-grid system and the inverter is re-charging the batteries using a generator?

Here are a few points without knowing how this is being used. It sounds like the inverter is over-charging or under-charging the batteries. Have the charger set-points been adjusted for the batteries you have? If I had this many failures in less than 2 months as you describe, I would be packing this equipment up and sending it back for a refund. I would NOT switch to gel or AGM sealed batteries. If you are destroying wet cells this fast, you will really destroy a sealed battery as they are even more sensitive to improper charging.

Good luck,

Jeff Yago

Dear Jeff,

Thank you for your response. The inverter is being used solely as a back up power system when there is a power outage, I also have a generator, I had it before the inverter and now use it to run heavier loads or when the outage has lasted for very long.

And I wish I had packed the inverter up and returned it then but I felt that maybe the batteries were faulty.

Regards

Eme

Eme:

Even though I did not have all the information about your system when I made my first comment, I feel like my suggestions are still true after reading your followup information. In standby mode where a grid-connected inverter is only being used as backup several times a year during a brief power outage, there is very little battery charging going on and the inverter should only be providing a small "maintain" charge to offset any standby losses in the batteries. Wet cell batteries may not be the best choice for this type of application as you will need to simulate a power outage every few months to force the batteries to go through a discharge and re-charge cycle to keep the electrolyte mixed. Wet cell batteries are very forgiving of poor charging performance and when over-charged they usually just need more watering. However, if you switch to sealed gel or AGM batteries which are better for stand-by applications before you solve this charging problem, you will just destroy more expensive batteries because they cannot be re-filled once they are dried out by over-charging.

Either you have a very good-quality inverter that has the wrong charging setpoints programmed in, or you have a very defective inverter that is over-charging the batteries and I would not keep buying more batteries until you are sure which is true. I am really surprised your dealer is suggesting more batteries when there is clearly a problem with the charging.

Good Luck,

Jeff Yago

Charging batteries

Hello Jeff,

I want to build a small battery backup/inverter system for when the lights go out using 2-4 smaller deep cycle batteries. Is it practical to recharge these batteries from a vehicle alternator with the car running and what hardware would I need to actually do it? Attaching an inverter to the car battery and plugging in an automotive smart charger seems horribly inefficient. Just using jumper cables seems like a good way to burn my batteries up.

Thanks, Jeff

Jeff:

Its possible to do what you want to do, but its not very practical. Charging up a bank of deep cycle batteries that have been depleted can require hours of heavy charge time and running a car engine at medium RPMs for 3 hours each day during a power outage is like taking a 200 mile trip each day and you can imagine what the fuel would cost for that.

You can either use a solar array to keep the batteries charged or buy generator in the 8 kW range and power a quality inverter. Most name-brand inverters designed for off-grid service have very robust battery chargers built in and are designed to maximize the charging to minimize generator run time. You are also correct about jumber cables as most are very light duty and only designed to carry a large current for a few seconds then out away. During battery charging with a good inverter, battery charge currents over 100 amps for several hours is not un-common and that requires #2/0 or #4/0 size copper cables and bolt type connectors.

A small generator will use much less fuel than a car to provide the same charging.

Good Luck,

Jeff Yago

Combining Renewable Energies

Hello Jeff,

First, I wanted to say thank for for sharing your knowledge. This is greatly appreciated!

Right now, we have not yet decided where to settle down as much of this we believe will depend on the answers we find to this:

Is it possible, or even worthwhile, to combine hydro, solar and wind renewables into our house somehow? My thoughts are that we could use solar to recharge our batteries, the wind and water as needed for current power or recharge as needed. I realize the question is very vague, so any guidance as to where to start would be very much appreciated. If needed, we do have an excess of electronics working daily such as 2-3 computers; 2 with the 3rd on in evenings (childs); tv, common appliances etc.

Thank you again!

Bryan

Bryan:

Yes you can combine all of these charging sources if you want, as the battery does not care what is doing the charging and they can all charge at the same time. However, it is very hard to find a good wind power location unless you are right on the ocean coast or on top of a mountain. You may think you have wind where you are, but there are actually very few wind locations that have a strong wind every day of the year and still will require a 30 to 75 foot tower.

On the other hand, it's hard to find a good hydro source as you don't need a raging river, but you do need a fairly large drop from the point of the water entering a supply pipe down to the hydro-turbine which is located hundreds of feet down the river bank to get as much fall as possible. The systems I have completed were in locations like Idaho where there are creeks and streams everywhere that fall hundreds of feet over very short distances. I might add that these locations are usually down in the bottom between high mountains on each side and there is very little wind.

Its easier to find good solar locations between these extremes, but I don't see how you will find a location that is ideal for wind, hydro, and solar all within a few hundred feet of each other. Its not un-common to do solar and wind so perhaps you should limit your search to a higher elevation with a good southern exposure.

Good Luck,

Jeff Yago

Adding wind power

Jeff,

Do you need any special equipment if you want to add a wind turbine to an existing PV system (other than the turbine and support system for it)?

Bill Hiltscher

Bill,

Older model wind turbines required a charge controller to regulate the charging current going to the batteries and to provide a way to load up the turbine to reduce over-speed in high winds once the battery was charged and there was no load on the unit. The solar charge controller and wind charge controller can be used together and even charge at the same time as they do not allow for one charger "back-feeding" into the other one.

Some newer wind turbines include built-in electronics so you do not need a separate charge controller, and even newer models put out 120 VAC which can go straight back into the grid without charging any batteries.

Hope this helps,

Jeff Yago

Gel-Cell Battery Charger

Jeff:

Thank you for all of your help in the past. Is there a particular charger you would recommend for charging a small battery bank of 2 Gel-Cell batteries (183aHr, wired in parallel)? I would really need one that is capable of trickle charging over a couple months unattended, as well as the ability to set a dip switch or other selector to have it charge regular auto/ marine flooded lead acid batteries.

I almost purchased a Samlex SEC1215 but figured I should check w/ the expert first.

Respectfully,

Matt Houghton

Matt:

There may be a charger that does both, but it does not seem very energy efficient. Most battery-maintainer type "trickle chargers" are very small and use very little electricity. They are designed to just off-set the standby losses for a battery bank not being used, and they are designed to use only a tiny amount of purchased electricity.

There are larger battery chargers that have switches to select from heavy charging to lower charge rates, but most do not go as low as you are needing and even if they did, they most likely would require much power to run in this low charge mode since you are dealing with larger capacity electrical components not being used at their rated output. There is nothing wrong with having two chargers on the same battery bank. Keep the trickle charger on full time, and use a larger charger when you need a fast re-charge after a heavy deep discharge.

The following link is for the unit I have used on several projects with good results. Its not a high dollar model, but seems to do a good job at a fair price. http://www.batterytender.com/default.php?cPath=11

Good Luck,

Jeff Yago

Biodiesel for generator

Hi Jeff,

My weekend cottage is powerless at moment, but running the lines to attach it to 'the grid' is estimated to cost $16,000 ! I am approx. 300 yards away from nearest power box. So instead of doing grid tie solar, I am now contemplating off grid solar and looking at generators for backup.

Sounds like diesel is the way to go for reliability and low maintainence, which is my biggest goal. I am willing to pay a little more for this.

My question is, what do you think of running BIOdiesel in most modern diesel generators? Is this feasible? Is this for summer time only? (I live in northern Wisconsin, and wonder if Biodiesel mixes would freeze!).

I'm trying to be as 'earth friendly' as possible.

Thanks!

Mark

Mark:

Being totally off-grid is not as easy as you think unless you have lots of solar modules and batteries, and really limit the use or quantity of larger appliances. Having said that, depending on your solar exposure and location, the more solar you have the less generator run time so fuel type may not be a big issue.

Some diesel engine manufacturers will void their warranty if you use bio-diesel, although this is changing as manufacturers get more experience with bio and the quality of the fuel processing improves. However, not only bio-diesel, but regular diesel fuels can be a real problem during the winter unless you use crank case heaters, fuel pre-heaters, and other heating devices to prevent the fuel from turning into gel. If you think solar is expensive just to run a few appliances and lights in an off-grid home, wait and see what it will cost you in batteries and solar modules just to power a crank case heater on a diesel generator all winter !

This is why many backup generators are propane. The fuel does not go stale, it does not freeze, and you don't have to worry about it turning into a semi-solid in the fuel lines. A fill up once a year to a 500 gallon underground tank should not only give you many months of backup power, but can also be used in a gas stove since you will not be able to have an electric stove in an off-grid home.

Hope this helps,

Jeff Yago

Batteries

I have a solar system (4-100 watt panels, voltage regulator, 6-6volt deep cycle batteries wired in series and parallel ( a battery "pack"), a low power and high power (3000/6000 watt Vector Inverter with soft start technology)).

This system is at my cabin which I use only 10-15 times/year for 4 days at the most per visit.

I use the low power (400 watt inverter) for lights, radio, a small TV and other miscellaneous needs and don't use much power from the low power inverter. In fact the blower internal to this inverter rarely comes on and I have never had a problem getting enough power out of the battery "pack" to supply my low power needs.

I use the high power inverter for my well pump only which draws high current ( i.e. 60 amps a.c for about 1 second) at start-up and then for 2-3 minutes of a 12 amp a.c. current draw, till the pressure tank is full. I estimate that I start the well pump anywhere from 3-30 times per visit ( over a period that might range from 2-4 days and thus often get a solar charge during this time).

Lately the battery "pack" will support the high current draw at 12 volts (measures above 13 volts at these attempts) without dropping below the 10.5 volt minimum required by the inverter for only 1-2 times to start the well pump. After 1-2 times of starting the well pump the battery pack voltage drops to about 12.4 volts and when I attempt to start the well pump, the voltage drops below the 10.5 volts required by the inverter and the inverter goes into a fault mode. The batteries seem to have enough capacity right after they have experienced a charge cycle from the solar panels and the voltage from the combined 6-6volt battery "pack" above 13 volts.

I suspect the batteries need to be replaced even though the specific gravity of all cells in all of the batteries measures above 1.270. The batteries are about 5.5 years old.

Considering my application, where I don't use the batteries frequently and probably don't run them down to less than 60-70 % of full charge, what type of batteries would you recommend? Deep cycle, marine, automobile? 6 or 12 volt? Any other recommendations?

Thanks for any advice you can give me.

Dave Keese

Dave:

I suspect what you have is a battery bank that has out-lived its usefulness. When batteries get old or have had too many long periods of low charge, they develop a "false" charge. The old plates become covered with calcium buildup which insulates the lead from the acid and reduces the charge capacity of the battery which is like making it smaller. With no load connected to batteries in this condition, the meter can read a fairly high voltage and make you think the battery is fully charged. This same problem can also make the solar charge controller think the batteries are full and will stop the solar charging. However, as soon as you connect a large load the voltage will drop in only a few minutes to the level of a discharged battery due to the limited capacity of the built-up plates.

Taking a voltage reading of a battery that is not supplying power and one that is under heavy load will both give you a false voltage reading.

The more accurate voltage reading is when the battery is under a small load. Normally a specific gravity reading is more accurate, but you would need to check every cell as one or more could be really low and causing the rest of the cells to never reach full charge.

The old lead-acid deep cycle battery is still the lowest cost battery you can buy (although all battery prices have recently increased about 40% due to world-wide lead shortages) and they are very forgiving of over-charging, which only causes some water loss. Since your cabin is empty for many days each month and the system is not providing any power, you may want to consider a deep-cycle AGM sealed battery. However, remember a sealed battery must be charged with the correct charger settings or you can boil out the moisture and it cannot be replaced.

Time to buy batteries!

Jeff Yago

Dave: I would only use deep cycle batteries and yes, there is a big difference between using 6 volt and 12 volt batteries. Almost all deep cycle 6-volt batteries like golf cart and L-16 sweeper batteries have double the thickness in their plates since the 6 volt battery only has 3 cells yet takes up the same physical base size as a 6 cell 12 volt battery. Most "deep cycle" 12 volt batteries are usually the RV/marine type, which are much heavier than a car battery which is why they are called "deep cycle", but they are about half the capacity and life of a golf cart or L-16 "deep cycle" industrial battery. I would not use the lighter weight 12 volt batteries in this application, and if you need a sealed battery, the 6-volt golf cart battery is also available in a sealed version. Good Luck, Jeff Yago

Fesnel lens

Hi Jeff,

I'm a disabled senior citizen who has discovered solar energy, love it!

I am toying with the idea of using a fresnel lens to heat a large piece of concrete or steel (maybe railroad track) and have the heated object on a movable system so that in the evening I could transport it into my garage and extract the heat to my home. I thought I could use a Heliostat to track the sun for heating purposes. Am I nuts? Suggestions?

Thanks,

Bob

Bob:

Sounds like a lot of work on your part. You can obtain the same energy with a lot less effort. For years people have heated greenhouses by placing large 30 and 55 gallon metal drums of water on the back walls. The drums are painted black and absorb the heat of the sun that has entered the glass and shines on the back wall away from the plants. Then at night, the heated drums give up their heat to offset the heat that is being lost.

Remember that any lens does not "increase" the solar energy available, it just concentrates it on a smaller area. For example, if you take a 1 sq. ft. thick steel plate and place it in direct sunlight, it will absorb the exact same solar energy as taking a 1 sq. ft. concentrating lens and shining the sunlight onto the same metal plate. Yes, the point where the lens has concentrated the sunlight will get hotter, but the rest of the plate that would have received at least some sunlight will not. The lens cannot increase the energy falling on each square foot of area, it only concentrates it onto a small point when increases the temperature. However, the same total energy (BTU) of heat is available.

Hope this helps and good luck dragging that railroad rail in and out of the house,

Jeff Yago

Book on solar power

Hi Jeff,

I'm a long-time fan of BHM and your articles.

Please advise, is there one single book out there that will best explain to a middle-aged single gal how best to go solar powered at home? I don't have the large sums to spend on big systems, but a smaller project, perhaps, or a system in steps?

I know you get tax credits but I don't have the cash flow upfront to spend, let alone to wait for the rebate or credit. What can people with no mechanical skills and modest means do to get the benefits of solar power other than making 'sun tea'?

Regards,

PJ in Hillsboro, OR

PJ:

I am going to help you feel better about yourself and what you are wanting to do. I would bet over 75% of the initial phone calls and email I receive from potential solar clients are women. Yes, most are married, but they seem to be driving this effort in their household and are very aware of what is involved. I would also point-out that our two most recent totally off-grid solar projects were for a single middle-aged women and a 84 year old widow, both living by themself. Both were very capable and very much into solar. You will also find that a good solar dealer can design a system with a reduced solar array to keep down initial costs, but have the wiring there to add more solar modules later in several stages which avoids having to finance a larger up-front system cost.

Please see the Jan/Feb, 2009 issue of Backwoods Home magazine as it will include a book review on a solar text I am recommending, plus an article describing the new solar tax credits includes several financing methods.

Good Luck!

Jeff Yago

Read More Ask Jeff Yago

Read Articles By Jeff Yago

Read Energy Articles