off grid cabin

Wellbuilt,

I have moved Jason's So. Cal Homestead posts and replies to his own thread. Should be a bit less confusing for everyone.

wellbuilt wrote: »

Hi bill ,
I was just thinking that I would be a little close to my max power needs with no head room and no place to go .
I could run the generator if I need more power but it would be good to have some extra capacity .

A lots of these disisions really are based on your personal choices and lifestyle you want to live.

For example, choosing the size of battery bank (AH*Voltage = Watt*Hour) capacity.

The rule of thumb we typically use for standard lead acid flooded cell batteries is around 1-3 days of storage and 50% maximum discharge (in normal usage). The optimum seems to be about 2 days of "no sun" and 50% maximum discharge.

The reason this works out so well is that Lead Acid have a limited amount of current they can output (and accept during charging). 2 days and 50% (4x daily load) seems to give a battery bank that can output "enough" power to start a well pump, and run tools/lights/etc. relatively well (i.e., matching AC inverter to battery bank capabilities), plus "storage" for bad weather.

But, you could just as easily choose 1 day of storage and 50% maximum discharge (2x daily load)--It will work fine for smaller load (1/2 the surge current and "heavy draws"). And you would use the genset more during bad weather.

Things that suggest a "smaller" battery bank capacity:

• Weekend/seasonal use (not there during winter/bad weather)
• No well pump/surging loads... Just lights, laptop computer, small water pump
• no room for large battery bank (small cabin, RV with limited weight capacity)
• Keep expenses low and don't leave too much for thieves to take when locked up.

Things that suggest a larger battery bank:

• Full time living (9+ months a year)
• Mix of heavy surge loads (well pump, clothes washer) and 24x7 loads (full size refrigerator)
• Need to run power during bad weather
• Don't want too much generator run time (noise, cost/transportation of fuel)

Things that suggest higher rate of charge (larger solar array, AC backup charger):

• Larger battery bank, more solar panels (5% to 13% rate of charge for solar, ~10-20%+ for genset/utility charging)
• More daytime loads (i.e., running irrigation pump, computer, washer, during day, "steals" from solar charging current)
• Batteries are expensive and limited life. Solar panels are relatively "cheap"
• Bit/tall cased batteries "like" 10%+ rate of charge
• Quicker recharging of battery bank after a few days of poor weather/heavy power consumption
• 5% can be OK for weekend/seasonal charging. 10%+ good for full time cabin use (less load/battery management by owner)

The GC battery are 80 bucks each 1 more panel is $ 100.
If I could get a bit more power with out any problems I would do it .
Even if I need a larger controller or a second kid .

Higher voltage battery bank--More power through Charge Controller (rules below are for MPPT type controller).

• 30 amp charge controller * 14.5 volts charging * 1/0.77 panel+controller deratings = 565 Watt array "cost effective maximum"
• 30 amp charge controller * 29.0 volts charging * 1/0.77 panel+controller deratings = 1,130 Watt array cost effective maximum"

So, if you choose 10%+ rate of charge for a 220 AH battery @ 6 volts (golf cart)... You could charge 2 in series with a 565 Watt array @ 12 volt bank or 4x in series @ 24 volt with an 1,130 Watt array.

Saves you a "larger" or several smaller charge controllers in parallel by going with a higher voltage battery bank.

I guess you are saying that 4 6v gc230 ah 460 @ 12volt battery will need to charge @ 46 amps 12 volt or a bit more .
so the 2 230 watt panels on the 30 amp CC will not do the job .

It all goes back to designing a "balanced" system (loads drive battery bank, battery bank drives solar array, loads also drive solar array+hours of sun per day).

if you decide that you want a 4x golf cart battery bank, you have the choice of 2x2 for 440 AH @ 12 volts or 4x1 for 220 AH @ 24 volts. Will save you money on charge controller(s), but you will have to look closely at what 12 and 24 volt AC inverters are out there that will meet your needs. Typically higher voltage inverters ( 12/24/48 ) have a larger "minimum size" (i.e., it is hard to find a 300 Watt 24 or 48 VDC input inverter). But the larger inverters do have more features and options (and need a larger battery bank + solar array to run them).

I calculated a suggested "maximum array" size for a 30 Amp MPPT charge controller... The suggested array size based on 5-13% rate of charge for 4x golf cart batteries would look like:

• 4x 220 AH * 7.25 volts charging * 1/0.77 rate of charge * 0.05 rate of charge = 414 Watt array minimum
• 4x 220 AH * 7.25 volts charging * 1/0.77 rate of charge * 0.10 rate of charge = 829 Watt array nominal
• 4x 220 AH * 7.25 volts charging * 1/0.77 rate of charge * 0.13 rate of charge = 1,077 Watt array "maximum cost effective"

And if you use a 30 Amp charge controller, you cannot get to a 10% rate of charge. You would need a minimum MPPT charge controller rating of (for a 12 volt battery bank):

• 829 Watt array * 0.77 panel+controller derating * 1/14.5 volts charging = 44 Amps minimum rated MPPT charge controller for a 10% rate of charge on a 12 volt bank

So--Here we are bouncing lots of numbers around... You sort of need to decide if you are somebody that likes to monitor your system daily--And a 5% rate of charge with smaller battery bank is "worth the extra labor" to you.

Or, you would like more of flip a switch and don't stress out type of system... A larger battery bank and higher rate of charge. If you have others that will be living there (spouse, kids, guests, etc.)--Usually the "lower stress" system is appreciated (as well as the ability to run the occasional hair drier, coffee maker, etc.).

Using a nominal set of design rules (2 days storage, 50% max discharge, 10% rate of charge, etc.) and then checking the cost of such a system (and its capability) will save some pencil work for you. Either the system is too expensive or does not meet all of your needs, then you can up size/down size as needed.

I forgot where your system will be installed... Hours of sun per day (by season) will be important too.

-Bill

If you go with a MPPT type charge controller, the minimum size would be:

4x 230 Watt * 1/14.5 volt charging * 0.77 panel+controller derating = 49 amps minimum

You could go as low as 45 amps (over paneling will not hurt a "good" MPPT charge controller--as long as you respect the maximum Vpanel operating voltage).

But, in reality, you are looking at 60 amp controllers. A 45 amp controller would work, but you lose ~4 amps or ~60 watts from your array in the middle of a cool/clear day.

Midnite, Outback, MorningStar, and Schneider/Xantrex all offer MPPT controllers that have a lot of users.

Personally, I like controllers without fans (bigger heat sinks). In an RV, the fan noise may be an issue (plus moving dust/etc. through the controller). MorningStar and Schneider have fanless units.

I cannot really recommend any controller--I do not use off grid power and I am not in the solar business. I would suggest looking at the pros and cons of each unit and call NAWS (and hopefully others here) can make their recommendations.

Note that for some vendors, they make a lot of money on "extras and options". Remote Battery Temperature sensor (I do recommend those), LCD displays, computer/network interfaces, etc. all add up quickly.

http://www.solar-electric.com/inverters-controllers-accessories/chco.html

If your panels are Vmp~30 volts, you have to go with MPPT type charge controllers. If your panels are Vmp~18 volts, a PWM controller can work OK for you too (short cable run from array to charge controller, PWM controllers have less "heat issues" and tend to be smaller, and cheaper).

But there are few (if any) Vmp~18 volt 230 Watt solar panels, so that option is sort of out.

You can also install a pair of charge controllers (like a pair of Midnite KIds:

MidNite Solar The Kid Charge Controllers and Accessories

Won't save you any money, but they do not use fans (as far as I know)--A big plus in my book. You can also hook them up with a Whiz Bang Jr and a 50mv/500A shunt to monitor your battery state of charge (I assume that two kids + 1 shunt would work together correctly--But please confirm--I do not know for sure).

-Bill

I am sorry to hear that your place has been used for target practice (too many stupid people out there these days).

Yes, you can put your two solar panels in parallel--They should have Vmp~17.5 volts or so... If they have way different voltages, that could be an issue (less power from your MPPT controller).

13.6 volts for float is fine (if you have find a manual/settings for your battery, you can confirm or adjust as needed).

For fulling recharging your battery bank, if flooded cell you want around 14.5 to 14.8 volts held for ~2 hours (if batteries are not discharged too much) or 4-6 hours if batteries are well discharged.

So, take a good DMM and see what your battery charger is doing for "bulk" charge voltage (i.e., 14.5 volts or so) and how long is holds that voltage.

A hydrometer is a great tool for understanding how well your batteries are charging. A good glass hydrometer is nice... Or you can get one of these newfangled devices (very nice). Make sure you have distilled water so you can rinse out the hydrometer after you are done (they will get sticky over time if you don't clean them... And glass hydrometers are very easy to break).

[h=2]MidNite Solar Hydrovolt Battery Hydrometer[/h] -Bill

wellbuilt wrote: »

Ive only had the solar panels up and running for 2 weeks but the battery was a 12.5 volt when I bought them and I charged the full when I brought them home, 12.7 sitting for a few days .
The acid looked full and ive been wanting to get a hydrometer .
The cc floats at 13.6 and charges at 14.6 most of the time in full sun .
If I start using power when in float the 13.6 stays the same and the amps will run up to 9.5+-
Im almost always in float 13.6 1.5 to 4 amps I think the battery is full or above 90%.

Sounds good. Besides logging specific gravity once in a while--Watch your distilled water usage... If you have to refill the cells every 1-2 months or so (for standard flooded cell batteries)--Charging sounds good.

If you have to refill every month or less, dial back the charging voltage (and absorb timer, if it has one) a bit.

If you don't have to refill after 6+ months, crank up the charging voltage a bit.

I will run them down this weekend ( going fishing )
The only way to change the setting is to fool with the temp sensor on the unit it self .
The battery , cc inverter are in a locker in the bathroom the battery are in a box with a 3" inlet on the bottom and a 3" out let on top with 1"+ air space all around ( no fan )
The door has some vents in it . nothing gets even warm .
The battery being vented to the outside and the CC being in side temp the controller could be off .
I can adjust the temp setting to trick the controller and move the charging voltage up and down .
should I try this ? The CC is on default setting now .

If you don't have one yet, get a remote battery temperature sensor and put it on one of the batteries (under a hollowed out Styrofoam block helps with accurate measurements). Mucking with manual offsets, I believe, detracts from the the whole automation/precision measurement tools that are designed into the controllers.

As above, monitor the battery bank and see if you need to "nudge" the absorb voltage one way or another.

I am in a 26' air stream trailer for now what should I do for grounding
The solar panels are on a deck roof in front of the trailer
I have green 10 gage wire run to the frame of the trailer
I grounded the CC case , the morning star inverter from the neg lead as in the manual , solar panels from the frame to a bus bar to a fat 12vold ground and a solid 8 gage ac ground .
is this ok ? Thank bill

John

Sounds good John... The grounding wire(s) are primarily there to provide a path back to the power source (DC Battery bank, AC inverter, etc.) so that if there is a worst case short circuit (AC or DC hot to metal sink/RV frame/etc.)--That the wire is heavy enough to pop the upstream breaker/fuse.

Another reason for the grounds are to keep lightning "outside" of the living area. So, I would recommend running the outside ground wire down the outside of the RV to the frame ground underneath.

If the RV is parked for long periods of time/in a lightning prone area, you should think about running a 6 AWG or heavier ground wire from the frame bond to a rod/copper plate buried in the earth. Just in case...

-Bill