Battery bank for weekend use vs full time

Joe94
Joe94 Solar Expert Posts: 42 ✭
in Solar Beginners Corner #1
I'm finishing up my off grid system. I will not be using this system full time but just for weekend. I have 500 watts of panel through a midnight classic on a 12 volt system. My plan is to build the battery bank with 4 T105s.

450 * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 423 Watt array minimum

I know it would be ideal to 1 or 2 more panels to increse the rate of charge to 7.5 or 10% but do I really need it as the system will have all week to charge with virtually no loads to pull it down?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,642 admin
    #2
    Re: Battery bank for weekend use vs full time

    Do you have times where you will be spending a week or more at a time?

    Seasonal use in "summer"--Or will there be times of use when bad weather/poor sun?

    How do you feel about running a Honda eu2000i (1,600 watt) generator at times (or Yamaha similar inverter/generator)?

    For weekend use, you probably will age out the batteries before they cycle (wear) out--So having more panels may not really "save" you on battery life for a weekend/seasonal cabin.

    Having more solar panels is really good for those times when you need more power (and keeping the battery bank on the "small side" and minimizing generator run-time). A 10% rate of charge is a pretty nice setup for long(er) battery life and more power if/when you need it.

    Panels are "cheap" when compared to 10 years ago... Batteries are getting more expensive.

    The down side is vandalism and theft--I hate to leave too much gear out there in remote locations for people to steal. The small gensets are pretty easy to take with you at the end of the season/at the end of each trip. A few people take down their panels--But it is not easy or fun.

    If you have lots of snow, a couple small panels on the south side of the building (above typical snow levels) will keep the batteries well charged through the winter.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #3
    Re: Battery bank for weekend use vs full time
    Joe94 wrote: »
    I'm finishing up my off grid system. I will not be using this system full time but just for weekend. I have 500 watts of panel through a midnight classic on a 12 volt system. My plan is to build the battery bank with 4 T105s.

    450 * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 423 Watt array minimum

    I know it would be ideal to 1 or 2 more panels to increse the rate of charge to 7.5 or 10% but do I really need it as the system will have all week to charge with virtually no loads to pull it down?

    Well the math is actually: 500 Watts * 0.77 derating / 12 Volts = 32 Amps peak charge rate.
    On two strings of T105's (450 Amp hours) that's 7% charge rate (32 * 100 / 450) so it will just about do it.

    What I'd wonder about is how you've sized the battery bank for usage requirements.
    And if it's not going to be in use for the week set the Float Voltage on the low side.
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  • Joe94
    Joe94 Solar Expert Posts: 42 ✭
    #4
    Re: Battery bank for weekend use vs full time

    Right now most of my usage is lighting and entertainment. All the lights are DC LED's with every light in the whole house on I use about a 100 watts an hour. I don't leave them all on at once but I figured that 3hrs per day of them all being one at once should cover the average day so 300 watts for lights. I have a LED TV that uses about 40 watts an hour and watch it maybe 4 hours a day for 160 watts a day. For stereo I have two Bose Sound Link Bluetooth speakers they are rechargeable and run about 4-5 hrs at normal volume. they recharge in about 2hr at 10 watts per hour. so 20 watts a day there. My wifi modem uses 15 watts an hr. its on a power strip shared with the TV that I turn off at night so I use it about 4hr a day or 60 watts. Cooking will be gas with a Brown range so now power draw for that and the water heater will be some form of on demand gas. I haven't set up the water heater or pump yet so I can only estimate those loads. I'm not planning to use a refrigerator just yet, but will probably install a Sundanzer when I do.

    So right now I'm at
    300 lights
    160 for TV
    20 Stereo
    60 Internet
    40 recharging phone and Ipad

    So I'm only at 540 watts a day not factoring the pump and water heater. So with 450 * 12= 5400 watts * .3 = 1620 minus whatever derating facture for inverter loss and such. I think I should have enough power for my planned use. I also have a 2000 watt Champion inverter generator as a back up to run additional loads as needed. I don't think I'm forgetting anything major. I'm planning to add another panel or two at some point but right now that money is better spent on other aspects of the construction.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,642 admin
    #5
    Re: Battery bank for weekend use vs full time

    Just to clarify... Watts is a "Rate" (like miles per hour) and Watt*Hours is an amount (like miles driven)
    Joe94 wrote: »
    Right now most of my usage is lighting and entertainment. All the lights are DC LED's with every light in the whole house on I use about a 100 watts (not watts per hour).

     I don't leave them all on at once but I figured that 3hrs per day of them all being one at once should cover the average day so 300 watts*hours for lights. I have a LED TV that uses about 40 watts (not watts per hour) and watch it maybe 4 hours a day for 120 watts*hours.

    For stereo I have two Bose Sound Link Bluetooth speakers they are rechargeable and run about 4-5 hrs at normal volume. they recharge in about 2hr at 10 watts (not hours). so 20 watts*hours a day there.

    My wifi modem uses 15 watts (not hours). its on a power strip shared with the TV that I turn off at night so I use it about 4hr a day or 60 watt*hours per day.

    Cooking will be gas with a Brown range so now power draw for that and the water heater will be some form of on demand gas. I haven't set up the water heater or pump yet so I can only estimate those loads. I'm not planning to use a refrigerator just yet, but will probably install a Sundanzer when I do.

    So right now I'm at (Watt*Hours per day)
    300 lights
    120 for TV
    20 Stereo
    60 Internet
    40 recharging phone and Ipad

    So I'm only at 540 Watt*Hours (or WH) a day not factoring the pump and water heater.

    So with 450 * 12= 5400 watts * .3 = 1620 minus whatever derating facture for inverter loss and such.

    Not sure--Is 450 a typo for 540? And 540 is already in Watt*Hours so you would not multply by 12 (volts?)--But divide by 12 volts to find out AH per day of battery use (on a 12 volt battery bank)... So, assuming all your loads are AC, and a standard 2 days of no-sun and 50% maximum discharge on 12 volt battery bank:
    • 540 Watt*Hours per day * 1/0.85 inverter eff * 1/12 volt battery bank * 2 days storage * 1/0.50 max discharge = 212 AH @ 12 volt battery bank recommended
    Next, sizing the array for the 5-13% rate of charge:
    • 212 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 200 Watt array minimum
    • 212 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 399 Watt array nominal
    • 212 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.20 rate of charge = 519 Watt array "cost effective maximum"
    And depending on season/location, 4 hours a day minimum sun for ~9 months a year works for many sunny US locations:
    • 540 WH per day * 1/0.52 system efficiency * 1/4 hours a day sun minimum = 260 Watt array minimum
    So, the rough array size would be from 260 to 519 Watts with ~399 Watt array being a healthy nominal size.

    Note Golf cart batteries are ~200 to 220 AH @ 6 volt and would be a nice first battery bank for this system.

    And my numbers are not "that accurate"--Just carrying digits so you can follow/check my math. Anything within 10% is pretty much "the same" in solar calculations.
    I think I should have enough power for my planned use. I also have a 2000 watt Champion inverter generator as a back up to run additional loads as needed. I don't think I'm forgetting anything major. I'm planning to add another panel or two at some point but right now that money is better spent on other aspects of the construction.
    A 45 Amp @ 12 volt Iota charger would be a nice match to your 2,000 watt Champion (a separate IQ4 module would be nice if you plan on running the charger from 120 VAC power at times).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
    Re: Battery bank for weekend use vs full time

    Example clarification: 40 Watt LED TV * 4 hours a day = 160 Watt hours.
    That sort of thing.
    Getting the terms and the math mixed up can give you a system that won't work.

    Looking at it from the other direction, 450 Amp hours @ 12 Volts should be able to supply:

    450 / 4 (for 25% DOD) = 112.5 * 12 VDC = 1350 Watt hours DC - inverter consumption & conversion efficiency = approximately 1150 Watt hours AC.

    500 Watts of panel @ 4 hours of good sun * 0.52 over-all efficiency = 1040 Watt hours AC.

    So you should be able to count on 1kW hours per day, no problem. More if you can make use of daylight power (panel capacity not used to charge batteries). And of course you have the generator. Always good to have the generator.
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