battery charging in cold climates

bobber

I'm new to this so bare with me. I have a small cabin off the grid in U.P. of Michigan. I have been using a Honda 2000 generator 2 deep cycle batterys and a 800 watt inverter to run a few lights, T.V. and and a RV water pump system. I am thinking about a solar system to reduce the generator noise and gas usage. The cabin is small so the battery's and inverter are in a shed that is not heated.I have read that using Solar panels and charge controller in cold climates can cause overcharge problems and that the battery's and equipment should be in heated building. Any thoughts on this would be greatly appreciated. Thanks

vtmaps

Welcome to the forum,

You need a charger that has a battery temperature sensor.  It will raise the charging voltage to compensate for low temperatures.  You need an inverter that can handle the higher voltages (you might be up to 16 volts while charging).

The batteries won't freeze as long as they are not discharged too far.  The batteries should be in an insulated box and they will keep themselves 10-20° above ambient. 

The batteries will have lower capacity and greater voltage sag under load when they are cold.  As long as you design your system with that in mind, it's not a problem.

There are some marine rated controllers that might be suitable for an unconditioned environment.  

When designing a system we start with the loads... you have not specified a number of kilowatthours per day.  In some systems we start with the batteries and figure out how much load they can handle and how much solar array and controller they need.

What sort of batteries do you have?  Do you have a clear area near the cabin?

--vtMaps

bobber

bobber said:

Thanks for the quick response. The cabin sets on a hill side with an unobstructed view to the sun. I now have 2/12volt deep cycle  marine battery's that I have just replaced w/2/6volt golf cart battery's (Trojan T-105). I think I need 2 more Trojan battery and will buy soon. The new battery's have not been put in service yet.The camp is a 4 hour drive from home and don't plan on going up until the middle of May so to give you actual usage numbers will take a couple of weeks. I replaced the light bulbs w/60wattLed's. I would estimate 3 light bulbs at 6hrs. per day. The RV 110 volt AC water pump pumps water from a 100 gal.tank for washing dishes and showers. The T.V. is a 32" Samsung that is used 2 or 3 hrs. a day on average. I forgot to mention a sleep app . machine that I use on avg. 5 hrs. A night. I came up with .6 kilowat hrs. per day. This is not a metered number but estimated. Thanks again for any help you can provide. Bobber

K

BB.

Bobber/K,

6 kWH per day (6,000 Watt*Hours) is quite a large amount of power for an off grid summer cabin. When I was trying for fairly extreme conservation in my metropolitan home--I could get down to ~6 kWH per day (fridge, tv, computers, lights, central heat, etc.--using natural gas for cooking/hot water/drier).

More or less, I would suggest ~1,000 WH (1 kWH) per day as a "small" cabin size system (using propane for refrigerator/heating/cooking).

And if you use an energy star rated full size refrigerator, clothes washer, deep well pump, TV, computer (still very conservation minded) closer to ~3.3 kWH per day.

We can suggest a "paper design" for a 6 kWH per day off grid system--But it would probably be too expensive to be "practical" for your needs. Also, if the cabin in a weekend/summer cabin (with some weekends in winter)--A solar power system is pretty expensive unless it is use ~9 months of the year or more.

So, some questions/suggestions:

• When will the cabin be occupied (figure out how much sun you have when you are there).
• Get a Kill-a-Watt type energy meter and measure your 120 VAC loads.
• You can also get DC WH/AH meters too and measure your DC loads.
• A 1,000 WH per day system works nice with a 12 VDC battery bank (smaller/cheaper system/supports "RV" appliances). A 3.3 kWH per day system is probably 24 VDC minimum. And a 6 kWH per day system would be (most likely) a 48 VDC battery system. Do you have a need for 12 VDC loads (you can get some 24 VDC pumps/other stuff for trucks/marine use).
  
• Do you have "good" security at the site? Installing $x0,000 worth of solar equipment at a remote site can be too much for thieves to ignore.
  
• After you measure your loads, I highly suggest looking for the most energy efficient devices you can. It is almost always cheaper to buy/use equipment that is designed to minimize power usage vs trying to build out a larger solar power system.
• Look at alternatives for power types... For example for weekend/few weeks per year usage, look at a propane refrigerator instead (salvaged RV units can be pretty cheap). Or build a solar+battery system to power small loads (laptop, LED TV, DC water pump, lights at night, CPAP machine designed for camping, etc.) and use the genset for larger loads (pumping to cistern, running vacuum and washing machine, etc.).

-Bill

bobber

I meant 6/10 of kWH per day, I thought Sorry

BB.

Sorry I missed the decimal point.

0.6 kwh or 600 wh per day. Very reasonable.

I will type a longer reply when back to a computer.

-Bill

bobber

Sorry about the .6 should have gone w/600 wh per day. Ilke I said I'm new to this. Would appreciate any help on sizing and equipment. The cabin is mainly used from June through Early December. Thanks again for any help you can provide. Bob

BB.

yea Bob--Getting old here. Why I (and engineers) put a zero in front of decimal points as in 0.6 kWH per day... Makes it easier to see (and I need to clean my laptop screen too ).

So, the basic numbers/math rule of thumb design: First the battery bank--2 days of storage (1-3 days typical), 50% maximum discharge (for longer battery life):

• 600 WH per day * 1/0.85 AC inverter eff * 1/12 volt battery bank * 2 days storage * 1/0.50 maximum = 235 @ 12 volt battery bank

2x 6 volt @ 220 AH batteries (golf cart batteries) in series for a 220 AH @ 12 volts bank would be nice.

Then there is charging the battery bank. Two calculations, one based on 5% to 13% rate of charge (5% for weekend/seasonal cabin, 10%+ for full time off grid cabin):

• 14.5 volts charging * 235 AH * 1/0.77 panel+controller deratings * 0.05 rate of charge = 221 Watt array minimum
  
• 14.5 volts charging * 235 AH * 1/0.77 panel+controller deratings * 0.10 rate of charge = 443 Watt array nominal
  
• 14.5 volts charging * 235 AH * 1/0.77 panel+controller deratings * 0.13 rate of charge = 575 Watt array "cost effective" maximum
  

Next, based on hours of sun per day based on location. Guessing near:
http://solarelectricityhandbook.com/solar-irradiance.html

Green Bay
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 46° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
3.03	3.82	4.51	4.86	5.09	5.19
Jul	Aug	Sep	Oct	Nov	Dec
5.32	5.04	4.61	3.81	2.93	2.53

If you would be OK with some generator use during winter/bad weather, pick November at 2.93 hours of sun:

• 600 WH per day * 1/0.52 off grid AC system eff * 1/2.93 hours of average sun = 394 Watt array "break even" for November
• 600 WH per day * 1/0.52 off grid AC system eff * 1/2.53 hours of average sun = 456 Watt array "break even" for December

So, a 443 Watt array to 575 Watt array would be a good fit... If you really want to avoid using a genset--You should probably run 20-30% more solar panels (closer to 575 Watt) (do not plan on using 100% of predicted output every day.

How does this look?

-Bill

westbranch

If you are not using the cabin over winter, make sure before you leave the last time in the year that your batteries are FULLY  charged, before closing up for the winter...

bobber

Bill, Once again thanks for all the info. I don't have  problem running the genset so would go with the lower end 400 to443 Watt. Not knowing alot about the system would I be out of line asking your advice on equipment?  Once again any help would greatly appreciated. Thanks Bob

BB.

Please ask. I can give some starting point suggestions, but others here can give you real life operational answers.

-Bill