Off grid in a travel trailer

BackToBasics

First off, my proposed system is going to be:
400-500 watts of panel
4 Trojan T-105 batteries - 450ah @ 12v

If I find a good deal on higher voltage panels, I'll spring for an MPPT charge controller, otherwise probably just a Xantrex c-35.

I live in Eastern WA so this will be a spring 2012 project. I'm currently just researching and trying to get all my numbers and figures dialed down.

This system will be for a permanent / stationary 12v travel trailer out in the woods. Very basic and minimal. This is much more about nature and freedom than bringing modern life to the woods.

These are my daily power requirements. I would put these numbers somewhere between average and worst case scenario.

Edgestar fp861 80 quart / 2.6 cu-ft 12v freezer - 48 ah/ day
LED lighting

---

10 ah/ day
12v on demand RV water pump

---

6 ah/ day
Small laptop

---

20 ah/ day
Cell phone

---

1 ah/day

Total is 85 ah / day.

Fedex just delivered the freezer today so I'll begin testing it @68 degrees indoors and also @ 15-40 degrees outdoors over the next few days. 48 ah / day is a fairly educated guess, but not exact.

Does anybody see anything wrong with my numbers or the system in general?

Where I'm getting hung up now is portable generator sizing. I'll be picking up an Iota 55 amp charger and that will be the about the only thing I need a generator for once I am completely set up. Iota says 950 watts @ 120v. My math says thats 792 watts to charging and 158 to inefficiencies.

For this alone I would choose a 1200w generator which would put me at a very efficient 80% load. However, I would like to have the option to run some saws and an air compressor. For this I would step up to a 3000-3500 watt generator. I don't like the idea of only loading the generator to 27%-31% for battery charging though.

Those are my first two options.

What does everyone think about sticking with the cheaper, more efficient 1200 watt generator (still with the 55 amp Iota charger) and running power tools off a 2000 watt inverter and my battery bank with the generator running and the Iota charger feeding it at the same time?

Thanks everybody!

PS- This forum has been an invaluable source of information for me over the last few months! I'm glad to finally be active in it.

mike95490

Re: Off grid in a travel trailer

BackToBasics wrote: »

....What does everyone think about sticking with the cheaper, more efficient 1200 watt generator (still with the 55 amp Iota charger) and running power tools off a 2000 watt inverter and my battery bank with the generator running and the Iota charger feeding it at the same time?....

I like it, and that's what I'd do. But I'd do it at 24v at least. There, you can be more efficient when running an inverter with heavy load. Not so much loss in the DC power lines.

Look into one of the Auto Throttle Inverter generators, they can really sip the gas when you have less then full load, and you hear them slow down, which is your clue to switch off, and let solar top off the batteries. If you ever think you are going to go to larger loads, jump now, to 48V.

niel

Re: Off grid in a travel trailer

that may be too much of a battery bank to make the pvs work out at those power levels. with 85ah needed per day by the loads a battery bank of 170ah is necessary to insure not going below 50% soc. now 2 of those will be 12v at about 225ah so there is an acceptable cushion there of 55ah. trojans like about a 10% charge rate and that would be 45a with 4 batteries and only 22.5a for 2 225ah batteries in series. at about 18v for the pv vmp and 22.5a total pv current = 405w. considering the 77% derating, we generally employ to stc ratings on pvs, this gets it to about 526w. it doesn't have to be exactly 10% so +/- a few percent either way is fine imo making the 500w stc area good to shoot for.

if you elect to get the larger battery bank then charging from the generator would periodically be necessary to keep that large bank properly charged. i won't go into the generators much except to say the inverter chargers do very well even if at 27%-31%. this type is more expensive than many of the other standard generators and i think that would necessitate 2 separate generators due to cost constraints. i'll let the guys delve into the generator aspects with you more deeply as i don't have an inv genny to make much comments on it other than what i have.

arghhh

Re: Off grid in a travel trailer

BackToBasics wrote: »

Where I'm getting hung up now is portable generator sizing. I'll be picking up an Iota 55 amp charger and that will be the about the only thing I need a generator for once I am completely set up. Iota says 950 watts @ 120v. My math says thats 792 watts to charging and 158 to inefficiencies.

I have the IOTA 55 amp 12volt charger. It is not very good in the power factor department, pulling almost 1200VA at full load. This will not play nice with a 1200 watt generator. You may need to upsize the generator or downsize the charger. If you upsize the generator, getting a inverter type you can run it at lower loads without problems and have it available for your larger power tool needs as well.

mattbatson

Re: Off grid in a travel trailer

how are you planning on mounting the panels?
on the roof?
Is your travel trailer roof capable of supporting the weight? I know some TT roof's are reinforced for foot traffic, and some are not....

also, how are you heating the TT in that 10 degree weather ?

mikeo

Re: Off grid in a travel trailer

Where I'm getting hung up now is portable generator sizing

Consider a EU series Honda 3000 inverter generator. This gives you the best of both worlds, economy when using at lighter loads and enough surge to run your power tools. They are a little pricy though, but reliability seems to be near the top. They are also convertible to run on propane if that would be more convenient for your use.

westbranch

Re: Off grid in a travel trailer

the primary issue with matching chargers and generators is the inflow draw from the charger, it may only be a blip BUT it can cause the generator to either blow a circuit breaker or stop running.

You need to test before you buy or as MikeO says go for a 3000w inverter type with lots of headroom.

The 2000w might work but it would be borderline if you have drawn the batteries down and the charger is putting out a full 50 A

HTH

BB.

Re: Off grid in a travel trailer

In case you missed it, a good technical thread on picking the optimum charge controller to match a small genset:

Question about battery charger selection with EU2000 generator

-Bill

BackToBasics

Re: Off grid in a travel trailer

mike90045 wrote: »

I like it, and that's what I'd do. But I'd do it at 24v at least. There, you can be more efficient when running an inverter with heavy load. Not so much loss in the DC power lines.

Since this is only a travel trailer with minimal needs, I'm going to stick with 12v.

niel wrote: »

that may be too much of a battery bank to make the pvs work out at those power levels. with 85ah needed per day by the loads a battery bank of 170ah is necessary to insure not going below 50% soc. now 2 of those will be 12v at about 225ah so there is an acceptable cushion there of 55ah. trojans like about a 10% charge rate and that would be 45a with 4 batteries and only 22.5a for 2 225ah batteries in series. at about 18v for the pv vmp and 22.5a total pv current = 405w. considering the 77% derating, we generally employ to stc ratings on pvs, this gets it to about 526w. it doesn't have to be exactly 10% so +/- a few percent either way is fine imo making the 500w stc area good to shoot for.

Interesting, I hadn't read that 10% charge rate for trojans before. I had planned on 5% with panels and around 13% occasionally with the charger and generator.

Personally, I dont mind a smaller bank. What would theoretically last the longest though? Two sets of 2 batteries or one set of 4 batteries?
EG:
My first set of 2 t-105s last 3 years and my second set last me 3.5 years for a total of 6.5 years. But maybe starting with 4 total would yield only 6 years or maybe up to 9. Thoughts, ideas?

arghhh wrote: »

I have the IOTA 55 amp 12volt charger. It is not very good in the power factor department, pulling almost 1200VA at full load. This will not play nice with a 1200 watt generator. You may need to upsize the generator or downsize the charger. If you upsize the generator, getting a inverter type you can run it at lower loads without problems and have it available for your larger power tool needs as well.

That's very disappointing to hear. A quick look around some forums and it appears the PowerMax chargers are more efficient. I need to find what kind of max current they are pulling. They're cheaper too.

mattbatson wrote: »

how are you planning on mounting the panels?
on the roof?
Is your travel trailer roof capable of supporting the weight? I know some TT roof's are reinforced for foot traffic, and some are not....

also, how are you heating the TT in that 10 degree weather ?

Nope, this is going to be my little 10 acre homestead so I'll be doing something more permanent, most likely set in concrete.

I'll be heating with wood. I'm after a nice, air tight stove designed for a small canvas wall tent. Something like this Replace a window with two pieces of sheet metal with a 5" hole in the center and run some double wall pipe out through there. Another option is a marine stove like this, but they're pretty spendy and they'll be stone cold by morning.

mike95490

Re: Off grid in a travel trailer

If your trailer will take the weight (about 1,000lb), look into a soapstone stove, like the vermont bun baker
http://www.vermontwoodstove.com/vermontbunbaker.htm
The walls absorb a lot of heat for slow release for hours.

BackToBasics

Re: Off grid in a travel trailer

I'm not really sold on the inverter generators. I much prefer the simplicity and serviceability of regular generators. The cost of a 3kw inverter generator is not worth it for the couple times a year I'll need to run power equipment. For small projects, I will just use hand tools. Plus, a regular generator loaded to 60%+ (for battery charging) will yield more kWhr / gallon than a 3kw inverter generator loaded to 30%

I would only be into a 1200-1500 watt generator and a 2000 watt inverter to run high wattage tools about $500. That would be all new equipment under warranty. Whereas a used inverter generator of suitable size would be double that.

niel

Re: Off grid in a travel trailer

"Interesting, I hadn't read that 10% charge rate for trojans before. I had planned on 5% with panels and around 13% occasionally with the charger and generator.

Personally, I dont mind a smaller bank. What would theoretically last the longest though? Two sets of 2 batteries or one set of 4 batteries?
EG:
My first set of 2 t-105s last 3 years and my second set last me 3.5 years for a total of 6.5 years. But maybe starting with 4 total would yield only 6 years or maybe up to 9. Thoughts, ideas?"

if you intend to charge say once a week or so with a genny and give it a full charge with around a 10% rate then you could get away with about a 5% rate from pv in the meantime. don't forget that we derate the stc values as those conditions are rarely met so if you specify a 5% rate of current that to get this from a pv array you should take that current and divide by .77 to see the current required from an stc array.

now it is the matching of charge sources and load sources to a battery bank that allow for better life. just because you add 2 more batteries does not necessarily make the bank last longer even though it won't cycle as deeply into it, which is another lifespan factor. i know this is confusing and i'm probably not helping in that area much.:-) basically if you opt for a larger battery capacity and the loads don't draw as deeply into it it will last longer, but won't necessarily go 2 to 1 like you indicated. the problem isn't the lighter loads taken off of the larger bank as it is the problem of getting enough charge back into it as it will still need to be large at least at times.

hmmm. maybe i should let somebody else take a stab at what i'm trying to say here as i'm definitely not doing well at it today i'm afraid. normally, i would not post something like this, but i didn't want it to seem i'm ignoring you either.

BackToBasics

Re: Off grid in a travel trailer

mike90045 wrote: »

If your trailer will take the weight (about 1,000lb), look into a soapstone stove, like the vermont bun baker
http://www.vermontwoodstove.com/vermontbunbaker.htm
The walls absorb a lot of heat for slow release for hours.

Oh wow, those are cool. Not at all in the budget though. Plus too big for trailer. I did plan on reinforcing the floor to add up to about 600 pounds of brick and block for thermal mass and to keep the heat off the wall.

Cariboocoot

Re: Off grid in a travel trailer

BackToBasics wrote: »

My first set of 2 t-105s last 3 years and my second set last me 3.5 years for a total of 6.5 years. But maybe starting with 4 total would yield only 6 years or maybe up to 9. Thoughts, ideas?

Okay, right there something is wrong with your battery utilization. T105's should last 6 years with normal deep cycle use. Either they've been habitually drawn down below 50% SOC or they've been left below full charge too often or they haven't been charged up quickly enough.

And yes Trojan recommends a 10% (of the total Amp hours @ the '20 hour rate') peak charge current. That is net - i.e. not including any current going to run loads at the same time the battery is charging.

An example of what you'd be looking at for solar to achieve this with one set of T105's (225 Amp hours @ 12 Volts):

22.5 Amps * 12 Volts = 270 Watts less the 77% typical efficiency of panels + charge controller (as Niel mentioned) = 350 Watt array.

If you're going to go for 5% "maintenance" charge rate and give it a good charge with the generator once a week you'd need half that, 175 Watts, providing there are no loads used during charging.


These numbers also assume you are able to "put back the used Amp hours" daily. Usually with the 10% peak charge rate this balances out to about 50% DOD if there's the minimum 4 hours of equivalent good sun:

350 Watt array derated = 270 Watts * 4 hours = 1080 Watt hours / 12 VDC = 90 Amp hours which is (90 * 100 / 225) 40% of capacity. (Note that if this were to be converted to AC there would be additional power loss in the conversion and additional consumption by the inverter.)

It's the old off-grid saw again: know your loads. As you can see, one set of T105's and 350 Watts of panel will make a happy 1kW hour DC only system.

As far as the generator is concerned, a 1200 Watt generator would have no trouble recharging up to 60 Amps (allowing for a pf of 0.60 on the charger, which is worse than they are). Remember that the batteries are not going to take full current all the time. With the panels in place chances are very good you will never see a need for maximum current from the generator & charger.

BackToBasics

Re: Off grid in a travel trailer

Cariboocoot wrote: »

Okay, right there something is wrong with your battery utilization. T105's should last 6 years with normal deep cycle use. Either they've been habitually drawn down below 50% SOC or they've been left below full charge too often or they haven't been charged up quickly enough.

And yes Trojan recommends a 10% (of the total Amp hours @ the '20 hour rate') peak charge current. That is net - i.e. not including any current going to run loads at the same time the battery is charging.

An example of what you'd be looking at for solar to achieve this with one set of T105's (225 Amp hours @ 12 Volts):

22.5 Amps * 12 Volts = 270 Watts less the 77% typical efficiency of panels + charge controller (as Niel mentioned) = 350 Watt array.

If you're going to go for 5% "maintenance" charge rate and give it a good charge with the generator once a week you'd need half that, 175 Watts, providing there are no loads used during charging.


These numbers also assume you are able to "put back the used Amp hours" daily. Usually with the 10% peak charge rate this balances out to about 50% DOD if there's the minimum 4 hours of equivalent good sun:

350 Watt array derated = 270 Watts * 4 hours = 1080 Watt hours / 12 VDC = 90 Amp hours which is (90 * 100 / 225) 40% of capacity. (Note that if this were to be converted to AC there would be additional power loss in the conversion and additional consumption by the inverter.)

It's the old off-grid saw again: know your loads. As you can see, one set of T105's and 350 Watts of panel will make a happy 1kW hour DC only system.

As far as the generator is concerned, a 1200 Watt generator would have no trouble recharging up to 60 Amps (allowing for a pf of 0.60 on the charger, which is worse than they are). Remember that the batteries are not going to take full current all the time. With the panels in place chances are very good you will never see a need for maximum current from the generator & charger.

Thanks for the reply, Cariboocoot.

That mention of battery life was just an attempt to better explain the question I was asking about battery longevity. Whether four batteries would last longer used all at once or with two being used and then two more being purchased when the original two fail. I just made the numbers up off the top of my head.

I will definitely scale back to just two t-105 batteries. 85ah / day is probably never going to happen, I expect an average of 70 and even less in the winter when the freezer is outdoors and cycling on very infrequently.

BB.

Re: Off grid in a travel trailer

More or less, if you look at the cycle life charts for a battery, 2x the battery AH (and 1/2 the cycle depth) equals something like ~2.2x longer life.

Or, you can look at it as 1/2 the number of charging cycles, and therefore ~2x the life.

Some folks have argued that having a bank the lasts 8 years instead of two banks 4 years each--is less work and less time spending nursing a starting to fail 4 year old bank.

Batteries tend to be less efficient at higher charge/discharge rates (Peukert).

With solar panels, we have a bit of an issue in that there are only so many hours of sun in a day to do the recharging--So higher charging currents for shorter periods of time are necessary, especially during winter (unlike batteries that have utility power for recharging).

Trojan recommends a fairly nominal charging voltage of 2.35 volts per cell until the last 95-100% and finish at 2.45 to 2.7 volts per cell (at 10-13% * C rate of charge as the current limit) until the battery hits around 90% state of charge. (page 12 of this PDF for flooded cell profile).

Flooded/wet batteries will gas (bubble) towards the end of charge to ensure the electrolyte is properly mixed

"Dave Sparks" here has recommended tracking arrays because they not only collect more energy, the extend the charging time too is really good for battery life (if I understand correctly).

Also, from Dave's (Dave Sparks, poster here and has lots of off grid customers) experience, he has said several times here that (in his opinion/experience) high charge rates are hard on battery life:

Dave Sparks wrote: »

1200 / 10% / 3.5 = 34.3 amps AC

This is the maximum charge amps that you should set the XW to.
Unless you have very big generator your probably fine. Remember you can and should charge at a lower rate if you want more than 10 years on this battery! Just because Surrette says this is fine remember they are in the business of selling batteries. Batteries like the slow charge of a solar day and their life will be shorten by fast generator charges. Good Luck!

But remember Dave also is an advocate of having the battery in a fairly narrow range of state of charge for long life too:

Dave Sparks wrote: »

I learned this strategy from Dave Surrette (Rolls) in the late 70's. Pretty much the bible on how I design my systems for off-grid.

Assume that the system will never reach more than a 90% state of charge.

Try not to go below 50% SOC, ever! Complete absorption over 90% of the year.

Use the energy stored from 70% to 90% SOC for your daily cycles.
Save the energy from 50% SOC to 70% SOC for aging to get long battery life.

I know Surettes has changed their recommendations over the years but I also know they are in the business of selling batteries! If you do the above you will get 10 to 15 years on their batteries with decent maintenance. ...

-Bill

PS: Should add that Surrette's (Rolls') batteries are high end (and expensive)... So you would expect them to last longer than some other brands/models.

BackToBasics

Re: Off grid in a travel trailer

Hello again everybody.

I just spent two days and a night up at my property. Two very clear, sunny days that were perfect for tracking the sun and choosing possible array locations. I've got two very good sites that will only require removing 2-3 trees for perfect peak sun.

Since I determined these sites near winter solstice (longest shadows), is it safe to assume that the same locations will not be shadowed in the spring, summer, and fall?

mike95490

Re: Off grid in a travel trailer

not likely. Winter, the sun is the lowest, so you should be safe.

Track the course of the moon in winter, and it's the same course the sun takes in the summer ! (and the reverse in summer)

niel

Re: Off grid in a travel trailer

yes, they can get shaded as the winter sun does not travel as far east and west as a summer sun does. it will be higher in the sky at solar noon in the summer, but an adjacent tree to the east or west can impact the insolation of the pvs as the sun rises and falls farther east and west in the summer. ideally, the view should be clear to the horizon from east to west through south. few of us get that ideal.