Mobile Solar

Re: Mobile Solar

You are probably between a rock and a hard place here... The risk from theft vs damage of packing and unpacking a trailer weekly, and the chance of a wind storm blowing the panels way if you can't get them properly tied down (or put away) in time is probably roughly equal.

Motion capture video, IR, and a cell phone based alarm may not be worth the costs or get somebody there in time to stop the thieves. Putting panels on a roof (or 2nd story roof, or high structure) may help.

But any determine thieves with enough privacy and time will not stop until you are stripped clean to the foundation (or just party out and destroy anything of value):

Who stole my house? - Thieves pillage family's remote vacation spot

It was never recovered:

EL DORADO COUNTY - Family's happy reunion at site of stolen cottage - Donor erects new one -- theft still unsolved

But he has installed a steel shipping container to put his valuable tools in it (and keep the temptation down).

In the end, the above event is very rare (one time event, 2003, in the US?).

Keeping the the visible items of value minimized (not too large array, installed on roof facing away from the road, or on the ground behind the cabin), don't keep irreplaceable objects at the cabin, a watchful neighbor, and having good insurance is about the best you can hope for.

-Bill

Re: Mobile Solar

Very simply... Figure out how many panels you will need, and then design something to hold 1/8" 66"x33" 34 lb each tempered glass sheets with a 2" aluminum rail around the edge (like aluminum frame windows) that can open, close, survive a trailer and high winds. BP (205 watt panel example) uses 50 pounds per square foot for wind and 113 PSF for snow loading.

Just wind loading would give you 756 lbs per panel that the structure would have to withstand. If you are doing 2-4 panels--may be OK to do... Larger folding/portable system would be very difficult.

For batteries, just figure out how many amp*hours you want, and go to the Wind-Sun store and see how big and how much they would weigh.

A small system (which will not supply much standby power), will not be too heavy... A large one--will be difficult and expensive to two. If the trailer has some sort of suspension (required for the glass panels), the batteries, if well mounted so the wires don't pull, will probably be just fine (like car batteries take the vibration OK too).

Honestly, if you are going to just need a few hours of power per day, for weekends, a small $1,000 Honda eu2000i generator (1,600 Watt, 4 hours on 1.1 gallons of gas or 400 watts for 15 hours on 1.1 gallons of gas) and a few gallons of gasoline will be much easier to use and probably cost you less to run than just the fuel for towing the trailer.

-Bill

You could also build your own hybrid system... Put the batteries in a locked box/cellar and use the generator to charge them while you are there (plus a small solar panel for trickle charging), then add solar panels later when you are there much more and the price/security justify the added costs.

Re: Mobile Solar

You should take a look at the Wind-Sun's battery FAQ to understand how best to charge your battery bank...

Battery charging takes place in 3 basic stages: Bulk, Absorption, and Float.

Bulk Charge - The first stage of 3-stage battery charging. Current is sent to batteries at the maximum safe rate they will accept until voltage rises to near (80-90%) full charge level. Voltages at this stage typically range from 10.5 volts to 15 volts. There is no "correct" voltage for bulk charging, but there may be limits on the maximum current that the battery and/or wiring can take.

Absorption Charge: The 2nd stage of 3-stage battery charging. Voltage remains constant and current gradually tapers off as internal resistance increases during charging. It is during this stage that the charger puts out maximum voltage. Voltages at this stage are typically around 14.2 to 15.5 volts.

Float Charge: The 3rd stage of 3-stage battery charging. After batteries reach full charge, charging voltage is reduced to a lower level (typically 12.8 to 13.2) to reduce gassing and prolong battery life. This is often referred to as a maintenance or trickle charge, since it's main purpose is to keep an already charged battery from discharging. PWM, or "pulse width modulation" accomplishes the same thing. In PWM, the controller or charger senses tiny voltage drops in the battery and sends very short charging cycles (pulses) to the battery. This may occur several hundred times per minute. It is called "pulse width" because the width of the pulses may vary from a few microseconds to several seconds. Note that for long term float service, such as backup power systems that are seldom discharged, the float voltage should be around 13.02 to 13.20 volts.

Assuming you have flooded cell batteries, you can adjust your generator to output at least 14.2 volts (remember temperature correction, colder than ~77F, the voltage will need to go up) and charge until see or hear the batteries begin to bubble (or use a battery monitor) for most weekends. Then once in a while (once or twice a month), crank the voltage up to 15.5 volts or so and equalize charge them.

Flooded battery life can be extended if an equalizing charge is applied every 10 to 40 days. This is a charge that is about 10% higher than normal full charge voltage, and is applied for about 2 to 16 hours. This makes sure that all the cells are equally charged, and the gas bubbles mix the electrolyte. If the liquid in standard wet cells is not mixed, the electrolyte becomes "stratified". You can have very strong solution at the top, and very weak at the bottom of the cell. With stratification, you can test a battery with a hydrometer and get readings that are quite a ways off. If you cannot equalize for some reason, you should let the battery sit for at least 24 hours and then use the hydrometer. AGM and gelled should be equalized 2-4 times a year at most - check the manufacturers recommendations, especially on gelled.

So, for example, Say you have 3 days of store power--and you discharge down to 60%, for a C/10 (1/10 battery rating as charge current), it would take you 4 hours to recharge with the generator set.

You have a 500watt generator with DC connections--is that a 500 watt DC generator or a 500 watt generator with 10 amp 12 VDC connections (120 watt)?

Ideally, you would like the output of your DC generator (or your AC generator with DC charge controller) to output about C/10 rating of your battery bank (more current than C/10, and you could over heat your batteries, less current that C/33 and you will not be able equalize your batteries--plus it will take you 3+ times longer run time to charge your bank).

Then, for trickle charging your battery bank while you are not there, you could get enough solar panels such that they would be somewhere between C/33 to ~C/80 with charge controller that you can adjust (to float voltage) or a several stage charge controller (to finish up the charging of your battery, then switch to float).

A MPPT (Maximum Power Point Tracking) charge controller is wonderful, but for small arrays (typically less than 400 watts), a full sized MX 60 (60 amp) charge controller ends up being less efficient than the less expensive PWM type controllers (there are some new, smaller MPPT units intended for RV/cabins that might work well on a smaller panel set).

Read through the FAQ and look closely at the battery charger / generator... Not all are equal and for an AC charger on a generator, depending on the charger, you may need to oversize the generator by 2-3 times the charger rating because of the "poor power factor" of many AC chargers.

You will probably want to oversize the generator anyway--use it in the morning to charge your batteries and run the vacuum, do the wash, make coffee, etc. Don't make the generator too large, they tend to waste fuel when running with light loads (some less than 50% load, other brands/models with less than 25% load).

My personal choice for a small, very quiet, fuel efficient generator is the Honda eu2000i. Between 25% and 100% load (400 watt to 1,600 watts) the fuel flow is very linear with load (1,600 watts, 1.1 gallons of gas, 4 hours. 400 watts, 1.1 gallons of fuel, 15 hours). It does this because it has an internal inverter that allows the engine to throttle back (slow down) when the loads are light.

If you have really light loads, you could also look at the eu1000i).

A regular 5kW $400 genset will use many times the amount of fuel when running at light loads). I would stay the heck away from one of these... $500 price difference (vs eu2000i) is about 120 gallons of gasoline--it may take as little as 10-40 days of use to "drink through" your savings (depending on how long you run it at low power). Plus they tend to be obcenly noisy when used in quite locations. The Honda eu family, you can hold a normal conversation right next to it.

If this where a full time home with larger loads, then a real "prime mover" type generator running off of propane or diesel would be a better pick. And looking at RV generators might be an intermediate solution (quite, automatic start, pretty fuel efficient). However, I tend to find that almost all of these other solutions are way to large for my needs... I can live with an eu2000i quite nicely even for my 3 bedroom home (would have to watch my power use or stop using a microwave and toaster oven)--The prime mover types seem to be 10-20 kWatt for a small one. Way too much power (and cost) for me.

Anyway, that is how I would approach the problem. Some others may have other generator suggestion (especially for the mid to large size units).

-Bill

PS: For your 4x 6 volt L16 batteries--assume 12 vdc @ 420 amp*hours...

Battery bank @ 12 volts=840 AH rating
Battery Charger: 84 amps, or 1,302 watts
Solar Panels C/33 to C/80= 25 amps to 10.5 amps (or ~425 to 178.5 watts of panels)...

If you go with a 1,300 watt battery charger, you may need to go with a larger Honda eu3000i generator (with electric start)...

Solar panel sizing is based on 17 volt rating, and a C/33 sized system (~3%) may even let you get away with skipping the generator if your cabin loads are very light and you don't draw down the batteries much (say, no generator when you are there alone and spending most of your time fishing--and fire up the genset when the family is there playing video games, have the fan on, and using a lot of well water for showers during cloudy weather).

Sizings are approximate and you can certainly go up or down on many of the suggestions.

-BB

PPS: Sorry, I missed that was a 5,000 watt genset, not 500... Just substitute your numbers...