Solar PV Backup - Backup

BBDawg
BBDawg Registered Users Posts: 6
Can't seem to ask the right question to get the answer i am looking for so I will try to describe what I am asking.
I am looking for a backup battery charger for my 600 watt PV system. I have 3ea 200 watt panels at 24 volts with a Xantrex 1,000 watt 24 volt inverter connected to a 30 amp (24volt) charge controller with 4ea 100 amp AGM batteries. (I am running a large side by side refrigerator/freezer)

My load allows the batteries to charge fine on clear days but on cloudy days the load draws more power than the panels can replace and after 4 days, the inverter shuts down do to low voltage. (< 22 volts)

I would like to add an A/C battery charger to keep the batteries at max when the sun is blocked.

With a 24 volt system, how can I do this?
Can it be set to cut on/off automatically?

Comments

  • Urbandialect
    Urbandialect Solar Expert Posts: 107 ✭✭✭✭
    Re: Solar PV Backup - Backup

    So you need a 24 volt charger you can plug into the grid or generator to charge your battery on cloudy days, right?

    I'm sure if you just google 24 volt battery charger you'll find what your looking for...

    also, i'd would put your freezer and refrigerator on a timer.. I'd set it for 1 hours on 2 hours off.. that way you can extent the amount of time you can go over cast on your batteries. (so basically the timer will run your freezer /refrigerator for 1 hours then cut if off for 2, then turn it back on for another hour.. you might have to play with setting, maybe on for 2 hours off for 1 to get it were you want)
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Solar PV Backup - Backup

    24 Volt battery chargers are available: http://www.solar-electric.com/bach1.html
    The size would depend on the capacity of your battery bank. It sounds like you have 200 Amp hours at 24 Volts. That would "like" at least 20 Amp charger. You should factor the continuous loads in to that as well.

    As for making it automatic, it can be done. But you may find it requires a rather substantial investment in new equipment - such as an inverter/charger unit with auto gen start and a generator to match.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Solar PV Backup - Backup

    In general, you can connect any good quality 24 volt AC battery charger to your bank in parallel with other chargers.

    The problem is that measuring battery state of charge by measuring voltage is not very accurate... You could put in a voltage controlled switch set for ~23 volts and turn on the AC battery charger and it would help...

    Or, you could install a Battery Monitor (the Xantrex and Victron Monitors have an external output) with a programmable switch (turn on at 75% state of charge and turn off at 90% state of charge) and use it to control an AC battery charger.

    It is practical--probably not.

    The problem is that when you draw the battery bank down towards 22 volts--you are near/at 0% state of charge and can damage the batteries.

    Have you thrown a Kill-a-Watt meter on your refrigerator to check the kWH per day usage?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BBDawg
    BBDawg Registered Users Posts: 6
    Re: Solar PV Backup - Backup
    24 Volt battery chargers are available: http://www.solar-electric.com/bach1.html
    The size would depend on the capacity of your battery bank. It sounds like you have 200 Amp hours at 24 Volts. That would "like" at least 20 Amp charger. You should factor the continuous loads in to that as well.

    .

    I had no idea the answer was so simple! That's why I couldn't find the answer..
    Thanks much for the reply and the link..
    I have a couple of other questions and will post them if I can't find the answer first..

    Thanks Randy!:D
  • BBDawg
    BBDawg Registered Users Posts: 6
    Re: Solar PV Backup - Backup
    also, i'd would put your freezer and refrigerator on a timer.. I'd set it for 1 hours on 2 hours off.. that way you can extent the amount of time you can go over cast on your batteries. (so basically the timer will run your freezer /refrigerator for 1 hours then cut if off for 2, then turn it back on for another hour.. you might have to play with setting, maybe on for 2 hours off for 1 to get it were you want)
    This is GREAT and might actually solve my issues by itself! I am getting a reading of 4.85 KWH over 72.27 hours for the fridge. (67.2 watts per hour ??) (from my Kill-a-Watt hour meter)

    Using this 2 hr on/1 off setup, will there be surges in power when the fridge restarts each cycle? (what effect might this have?)

    Thanks MUCH
    Randy
    :D
  • BBDawg
    BBDawg Registered Users Posts: 6
    Re: Solar PV Backup - Backup
    BB. wrote: »

    It is practical--probably not.

    The problem is that when you draw the battery bank down towards 22 volts--you are near/at 0% state of charge and can damage the batteries.

    Have you thrown a Kill-a-Watt meter on your refrigerator to check the kWH per day usage?

    -Bill

    My Zantrex Prosine 1000 cut off the system when it reached a certain level. I only guessed at the 22 volt. The night before shutdown the voltage was around 23.6 but still running. I don't have a manual for my Zantrex so I am not familiar with the "auto switch function" if it even has it.
    Here is my output from my Meter - 4.85 KWH over 72.27 hours for the fridge. (67.2 watts per hour ??)
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Solar PV Backup - Backup
    BBDawg wrote: »
    My Zantrex Prosine 1000 cut off the system when it reached a certain level. I only guessed at the 22 volt. The night before shutdown the voltage was around 23.6 but still running. I don't have a manual for my Zantrex so I am not familiar with the "auto switch function" if it even has it.
    Here is my output from my Meter - 4.85 KWH over 72.27 hours for the fridge. (67.2 watts per hour ??)
    • 4,850 Watt*Hours / 72.27 Hours = 67 watts (average power)
    • 67 watts * 24 hours per day = 1,610 WH per day = 1.6 kWH per day = 588 kWH per year
    That is not too high of power usage for a good size fridge. But, as you can tell, still not easy for running on an off-grid system.

    Using PV Watts; a 1,000 watts (1kW) of panels for an offgrid system with derating of 0.59 (AGM batteries) for Datona Beach FL:
    "Station Identification"
    "City:","Daytona_Beach"
    "State:","Florida"
    "Lat (deg N):", 29.18
    "Long (deg W):", 81.05
    "Elev (m): ", 12
    "PV System Specifications"
    "DC Rating:"," 1.0 kW"
    "DC to AC Derate Factor:"," 0.590"
    "AC Rating:"," 0.6 kW"
    "Array Type: Fixed Tilt"
    "Array Tilt:"," 29.2"
    "Array Azimuth:","180.0"

    "Energy Specifications"
    "Cost of Electricity:"," 9.0 cents/kWh"

    "Results"
    "Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
    1, 4.35, 74, 6.66
    2, 4.96, 77, 6.93
    3, 5.81, 98, 8.82
    4, 6.14, 98, 8.82
    5, 5.98, 97, 8.73
    6, 5.67, 87, 7.83
    7, 5.74, 92, 8.28
    8, 5.65, 91, 8.19
    9, 5.51, 86, 7.74
    10, 4.84, 79, 7.11
    11, 4.67, 76, 6.84
    12, 4.23, 72, 6.48
    "Year", 5.30, 1027, 92.43
    For July, looking at ~92kWH per month per 1kW of panels:
    • 92kWH per July * 0.6kW/1kW of panels / 30 days = 1.84 kWH per day
    So--your array, if everything is working OK is just large enough, on average.

    Any cloudy weather, or other months with less sun--and you will need AC/Genset backup power....

    The other question is about the inverter. Inverters have "standby" power--and could be 6-20 watts or so--and you have to account running the inverter 24x7 too in your total power consumption.

    To better power your fridge, you should be looking at almost 2x the size of your current array and allow for other seasons and stretches of bad weather.

    Regarding the 1,000 watt inverter--the fact that it is reliably running your fridge is very nice... 1,200 to 1,500 watts is the normal minimum recommendation to run a full sized fridge (motor starting loads + defrost heater draw).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Solar PV Backup - Backup

    For what it's worth, I run a refrigerator off-grid on a 24 Volt system. It has 320 Amp hours of battery and 700 Watts of panel. Were it only the 'frige, that would do. Over-all the system is more marginal than I care for. It is not possible to run it in Winter and along about Fall the generator gets used more and more.

    The 'frige itself is 16 cu. ft. and runs on average about 1/3 of the time (8 hours per day). The start-up surge is significant, but not impossible to handle. The 500 Watt defrost element is more troublesome as you can't control when it comes on and you can't disconnect it or you soon have clogged air vents inside.

    The absolute minimum you can run a 24 Volt system down to is 21 Volts. Not recommended if you don't like replacing batteries often. Mine is set to shut down at 22, and only does so when the kids are there on their own and run the water pump at night. Like last year. Every night for a week. Good thing I was going to buy new batteries anyway. :cry:
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Solar PV Backup - Backup

    i am not sure if the inverter you have is a sine wave inverter or msw type. if it is msw then it is certainly eating up even more power than is necessary to run the frig than compared with a sine wave inverter. the compressor and fan(s) will also have a shortened lifespan on msw. with that said if you have a msw inverter then i'd recommend getting a sine wave inverter. i should point out that some of them have built-in chargers and a transfer relay so things can run off of the utility or battery and can automatically switchover. this is more $ of course and you can just get the inverter without a built-in charger and use an external charger as you already planned to do. the agms will be better at charging at lower levels (as low as 5% recommended) because of their efficiency and should not need to account for the extra load if you operate the loads at that time from utility ac. i am assuming you have utility ac and i guess i should not assume that, but it applies to any ac source to run the loads direct to avoid too many stages that drop the efficiency.

    it is of course a good idea to add more pvs if you can to help offset the lowered charge rate in off seasons or bad weather.

    i might add that you may want to be sure the voltage does not drop below at least about 23v as agm batteries by their nature have a higher standing voltage and a higher 0% soc (dead) voltage too.