battery bank question from newbie

[Ed Bickford]

I are currently educating myself (starting anyhow) prior to installing my first PV solar electric system for a small off-grid cabin.

I know there are advantages to having the PV array run at higher voltages in terms of sizing of conductors running from the PV array to the charge controller but what are the trade-offs for higher or lower battery bank voltages (I plan to run a MPPT charge controller so could have the PV array at a higher voltage than the battery bank.) I am trying to decide if I should have a 24V or 12V battery bank but hope to maybe have the solar array at 48 V.

Hope my question makes sense.

Thanks in advance!
Ed

[Rob Beckers]

Hi Ed,

MPPT charge controllers, and running the solar panels at a higher voltage than the batteries, has a number of advantages besides being able to use thinner wires: The panels are run more efficiently (since they are not tied to the battery voltage any more), and you get more energy out of the same panels. You can now also use regular grid-tie panels, you don't have to use 12V or 24V modules, which these days are much more expensive per Watt vs. grid-tie modules that (usually) run at 30 or 35V. So, despite the more expensive controller, you may actually save money overall by using cheaper modules.

For all but the very smallest of off-grid systems I would suggest using 24V or more. You run into really heavy conductors very quickly with 12V, and even with MPPT controllers they are not too efficient if they have to convert 75 or 100V in module voltage down to 12V. For a small/mid-size system 24V is a good compromise (with a choice of things that can run directly on that voltage). For larger systems 48V is the voltage used. I would stay away from 12V though unless you really don't want to do anything more than put two panels on a battery and run a light bulb.

-RoB-

[Ed Bickford]

Thanks Rob!

Re system size and battery voltage: we are planning to live full-time off-grid, first in a small cabin on the property but eventually build a house so I think it will make most sense to go with a system that will be expandable as our demands grow. It sounds like higher voltage battery systems makes more sense, likely 48 volts. We do want to run a fridge that can consume 12 or 24 V and some 12/24V lighting but I guess we will just need some sort of voltage converter (term likely wrong but something to drop the DC voltage as required.)

I hadn't heard about the grid-tie modules you mention which could be cheaper. I found some Kyocera KD 215GX-LPU panels for sale privately and looking around a bit it seems 24V is somewhat common for solar panels. (of course, as I said I am just getting started in my research so might be missing some major info, but I do hope to get things sorted out fairly soon.)

Thanks again for your great input!
Ed

PS - the PV system will be installed on Saltspring Island near Victoria BC where cloudy weather will be an issue

[Chris Olson]

I hadn't heard about the grid-tie modules you mention which could be cheaper.

I got four 240 watt Schott grid tie modules earlier this year. This is a photo of two of them on my shop roof after I got them installed (attached) and the other two were still on the "picnic table mount" but I got those installed now too.

Those panels are pretty big and heavy and I need to tilt them to 55 degrees for winter time here. So I built two racks and put them in pairs on the racks so they're easier to tilt up and down for winter and summer. All four panels on one rack would be over 220 lbs, which is a bit much to handle on that steel roof.

I'm using a Morningstar MPPT-45 controller on them, with all the panels wired in series. They normally run in the range of 115-120 volts on my 24 volt system. The open voltage in really cold weather might exceed the controller's maximum limit of 150 volts, but it hasn't blown up yet.
--
Chris

[Chris Olson]

Hi Ed,
For larger systems 48V is the voltage used.

Rob,

Larger systems do not have to be 48 volt. We lived with a 12 volt system for 7 years. When we upgraded our system this past summer we went to 24 volt. Our battery bank is 2,400 amp-hours, Roll T12's, which is 28 kWh @ 24 volt nominal.

We have dual Xantrex inverters with 120/240 split phase power and our house is totally electric with 240 volt range, 240 volt electric clothes dryer, 240 volt well pump, and dual 55 gallon 240 volt water heaters. Total amp draw by the inverters is 360 amps at full load. But we never let it get that high (except during surge loads) because I got the master inverter set to start the generator at 20 amp load on both legs of the split phase service to help meet the load, then shut it off when the load goes away.

All of our RE power sources (two solar arrays and four wind turbines) are separate and the turbines each put out about 60 amps on a good day, and the solar about 40 amps for each array. All the power goes into a big DC bus, and the inverters are fed by the bus.

The reason we went with 24 volt is because of the cost of equipment. All our stuff is SquareD and for 48 volt you have go to 250 VDC rated equipment, which is over the double the cost of the 24 volt DC stuff.

For off-grid I like to use many smaller sources of power instead of, for example, having one big turbine and one big solar array. You got system redundancy when you use multiple smaller power sources, the amp load on wiring is more manageable, and the only real disadvantage is the size of the wire required from the bus to the inverters. But on our system, that's very short - only about 4 feet.

One reason I could think of, to go 48 volt, would be if you want to run a big off-grid wind turbine like a Bergey Excel-R. You can get the Excel-R in 24 volt, but it's derated, while the 48 volt version is rated at 7.5 kW. But even so, at 60 volts 7.5 kW is still 125 amps which takes really big wire on what is usually a pretty substantial wire run. However, with today's MPPT controllers for off-grid wind power, the line loss issues on long wire runs are less significant than they used to be for low voltage systems.

One other consideration is safety with an off-grid system. 12 volt is pretty "safe". 24 volt will make you talk in tongues if you accidentally come in contact with conductors when servicing a battery bank, etc.. 48 volt is outright dangerous and will lay you out on the floor deader than a rock. Being that battery bank servicing is a very real chore for off-grid folks, that has to be taken into consideration.

So there's many things to think about. But a large system on 24 volt is definitely doable with proper planning and design, and my wife and I live pretty comfortable here on it.
--
Chris

[Ralph Day]

Hi Ed
Before you spring for a dc fridge have a look at the merits of a good energystar rated ac fridge. You're going with an inverter and batteries anyway, why not let your renewables power it instead of the constant propane bill (I know my supplier's price varies from fill to fill).

I have a Woods 18cuft all fridge and it's daily usage is rated at .8kwhr, my measured useage is just over .5kwhr per day. They're much cheaper than propane units, and easier to replace/repair if need be. My Woods was about $800 while propane units are in the thousands of dollars. Just be sure you have a large enough pv system (and backup genset) to keep your batteries charged properly.

I know people, myself included, recommend this forum while on other forums...so I must link a "fridge" thread here:

http://www.wind-sun.com/ForumVB/showthread.php?t=5939&highlight=refrigerator+choices

With a larger base of members the naws site has a lot of information on system setups and choices that we just don't have here. But remember to frequent the Greenpowertalk forum too!

Ralph

[Ed Bickford]

Chris and Ralph - thanks for the additional info. It seems there are many things to think about and hearing a range of perspectives is very helpful. The solar power aspect of this major project we are embarking on (leaving the city to build an off grid, passive solar home out in the country) is the part that is most exciting to me and I am enjoying the learning phase and really looking forward to getting a system designed and built.

Thanks again!
Ed

[Chris Olson]

Ed, keep us posted on your progress. To my way of thinking, building an off-grid system is much more rewarding than building a grid-tie system. Grid-tie is all about money. Off-grid is a lifestyle.

Like Ralph mentioned, a good electric Energy Star 'fridge is more efficient than a propane absorption unit. But no matter what, if you live off-grid you need a good standby generator too. And you'll need wind power for off-grid too. If you try to do it just on solar you'll put a lot of hours on your gen unless you live in an usual part of the world where the sun shines 24 hours a day. Off-grid design really requires an integrated system if you want to live comfortably.

As an example, several years ago my wife went to a garage sale and bought a 54" Toshiba big screen TV that draws about 350-400 watts. It was one of the very first big screen sets ever built. She came hauling it home in her truck, pretty proud of the "good deal" she got on it and the fact that we had a big screen TV. Trouble is, we couldn't watch it because we never had the power to run it. That TV weighs about 400 lbs, and fortunately it had wheels on it that made it possible to move it to get it out of our house when we finally bought a new 42" LCD HDTV set that only draws 98 watts, that I carried in the house under one arm.

It'll be an adventure for you, for sure :)
--
Chris

[Rob Beckers]

Ed, to echo Ralph's comments: There are a number of efficient regular (grid type) fridges out there. To get at actual usage numbers take a look at Consumer Reports (libraries have them for free). They test under real-world conditions, rather than going by the more optimistic manufacturer's numbers.

There are also special 'off-grid' electrical fridges that run directly from 12/24V batteries. They are very, very efficient (we sell Sundanzer fridges and freezers, they are amazingly efficient). These are expensive though, for their size, but work well (in the 6 years we've sold them I've never heard of one breaking).

If you're looking at inverters to run the house: The two big brands are Outback and Xantrex/Schneider (there are others, cheaper too, these are the most heavy-duty ones). Outback just came out with a new line. No experience with them (yet) though the specs look very interesting. The Xantrex XW series works great, we've sold a number of those.

Let us know how it goes!

-RoB-

[Joe Blake]

I'd agree with 12/24 volts being doable. I've been running a 12 volt stand-alone system in parallel with grid-tied 1600 watts system. As mentioned, one of the plus sides of using 12 volts is the availability of 12v gadgets, including relatively inexpensive battery chargers so I can tap into my on-grid system on a good day. And the safety factor cannot be overestimated. Very important.

If you are considering a low voltage cooling system, might I suggest getting a freezer and just keeping it filled with bottles of ice, which I then swap daily into my 240v refrigerator. The fridge then acts more like an old-fashioned "ice-box" for minimal expenditure. (Using an in-line meter I determined that this 10 year old fridge, over a period of 9 days, consumed 7.5 Amp hours of grid power. (I can't say how much it consumes without the daily addition of the ice.)

I've got two 12/24/240 volt Engel camping freezers, which when running full blast (ie at 240 volts via an inverter) they draw about 75 watts combined. They'll cool down to about -18 degrees (C) in this fashion. When running off the 12 volt battery bank (150 A/h) they'll cool to about -8 degrees, unless the batteries are fully charged then the freezer will go down as far as -12 degrees while the sun is shining. (They'll seldom, if ever, drop below -6 degrees.) All year round, at sunset, I'll disconnect the majority of my appliances from the grid and run off the batteries (unless I use my BIG computer, then I leave the charger running, but I'll shut that down and the freezers run off the batteries overnight at 12 volts).

In a way the ice bottles act like a "power storage" battery/ capacitor, when there's plenty of sunlight the ice gets very cold and keeps the fridge cooler longer.

The freezers ARE expensive, but for me they're worth every cent. They use a "swing motor" which apparently has only one moving part (and that is self lubricating) so virtually nothing can break.

http://www.sawafuji.co.jp/english/tech/shindou.html

I've had mine for several years now, running 24 hrs a day, and apart from having to defrost manually every couple of weeks they're great.

Several years ago I even managed to track down a Pentium IV full featured computer which was designed to run directly off a car battery, but these days I think most laptops draw fairly small amounts of power (and include a screen) and would probably go quite well, given reasonable usage.

For my (stand alone) inverter(s) I have three of them; 150 watts, 300 watts and 600 watts. The smaller ones I keep mainly for emergencies. They're SEA brand (Solar Energy Australia) and have been very reliable. My only incident was when a rather large ant got inside the case and anchored onto a "live" contact and earthed itself, resulting in a rather catastrophic internal power surge. Even though the repair knew what the problem was, the damage was replaced under warranty. :D

There's plenty of scope for innovation in this area. Have fun.

Joe

[Chris Olson]

Several years ago I even managed to track down a Pentium IV full featured computer which was designed to run directly off a car battery, but these days I think most laptops draw fairly small amounts of power (and include a screen) and would probably go quite well, given reasonable usage.

When we had our 12 volt system we had 12 volt power jacks in the house and used them to run our laptop computers using those 12V->19V DC converters that you can buy to plug into your car's cigarette lighter socket.

We also had 12 volt florescent lighting in the kitchen, hallway and living room. And we had two 12 volt ShurFlo pumps. We ran our 240 volt well pump with the generator and filled a 100 gallon tank in the basement with it. The ShurFlo pumps provided pressurized water for the kitchen sink, bathroom and clothes washing machine.

We had a 12 volt RV furnace blower on our central forced air wood furnace.

We had a Tecumseh 6 hp snowblower engine driving a 12 volt Delco 10SI alternator:

https://lh3.googleusercontent.com/--EuwfLeALYE/TRagEyHV-gI/AAAAAAAAFEY/V9Rn3BSi0Ok/s640/100_0225.JPG

That thing sat outside year 'round with a plastic tub over it. In the evenings in the winter time when the wind wasn't blowing so the wind turbines weren't working, when we wanted to watch TV or use more power, I'd start it up and let it run just above idle charging the battery bank to keep the bank up so it didn't get sacked. That little DC gas charger would run for two hours on three pints of gas. And it would start at 30 below zero F.

We had three 2,000 watt Schumacher inverters for 120 volt power in the house. When my wife washed a load of clothes on a bad power day, I'd start up that gas charger and run it at full throttle to keep the bank up while the washing machine was running. And it took all one inverter could put out to run the washing machine on spin cycle.

When we put in our new power system and battery bank, all that stuff is gone now. But we lived with that stuff for seven years and never had a single problem with it.

Where there's a will, but not much money, there's always a way when you got a 12 volt system. 12V = not much $. 24V = more $. 48V= more $ yet, and then some. And when you get down to where the rubber meets the road, the only real difference is in how many amps the inverters pull on the lower voltage systems. And these days, with MPPT, you can bring wind and solar power in to your bank at 100 volts and step it down to 12 or 24 just as efficient as you can with 48.
--
Chris

[Ed Bickford]

Thanks everyone, much appreciated!

I had originally said I wanted to put together a system that could be used at first when we are in a small cabin on the property and then expanded later when we build a full-size house and shop. I now think I will have 2 separate systems: one short-term small, likely 12V but maybe 24V, system for the cabin when we are there only part time for the first year or so and then a much larger 'whole' house and shop' system to last many years.

Likely not possible to go with the long term solution now due to various reasons. For long term I envision (at this point) a 48V system, lots of PV panels, MPPT charge controller, maybe Xantrex XW inverter, larger Honda inverter-generator.

The short term solution will only need to power a few 12V lights, 2 shur-flow pumps, a modem and lap-top computer, maybe a 12V/24V fridge but not sure about the fridge at this point. We also have a 240V well pump and a storage tank that gravity feeds the cabin so only need the Shur-Flow pumps for pressure for sink and shower. The property came with a loud and obnoxious 10 horse briggs and stratten generator for the well pump and I can stand listening to that while filling the water reservoir but not while showering. (currently need to have B&S running to have a shower or get anything more than a dribble out of the kitchen sink tap - noisy!!!)

A couple of things I still need to look into/consider:
1/ fill out a system load worksheet (will do tonight)
2/ consider the fact one shur-flow pump will be about 120 feet from the battery bank (maybe a reason to go 24 volt and buy 24V versions of the Shur-Flow pumps, or determine what size conductors would be required for that distance with 12V.)
3/ consider that the property also came with a Honda EU2000i which apparently can charge 12 volt batteries, maybe a reason to go 12V? although I suppose if the inverter includes a battery charger then the Honda's 120VAC output could be used with the charger in the inverter to charge 12 or 24V systems.

Anyhow, comment and suggestions welcome. My understanding of the options is slowly improving.

Thanks again all!
Ed

[Chris Olson]

2/ consider the fact one shur-flow pump will be about 120 feet from the battery bank (maybe a reason to go 24 volt and buy 24V versions of the Shur-Flow pumps, or determine what size conductors would be required for that distance with 12V.)

Just put a marine deep cycle battery right by the pump so it's got full amps when running, and run 120 feet of #10 wire to the battery from the main bank to keep it charged up. It'll work fine and voltage drop in the long wire run won't be an issue.
--
Chris