how many hours runtime for my Air Conditioner with 200Ah Solar Battery?

hmong2017
hmong2017 Registered Users Posts: 39 ✭✭
edited August 2018 in Solar Beginners Corner #1
I have a small 12V system setup. 

4 of 100Watt 5A Mighty Max brand going into a MPPT Charge Controller 30A, charging my (2) 100Ah Acid Sealed Battery in parallel unbranded. The inverter is 3000Watt 12V output 110V. 

My batteries are 100Ah each. I paralleled them, so I should have 200Ah. The Batteries are 2 years old. 
My Air Conditioner is 8000BTU and use or draw 7A and uses 120V. 
The inverter output is only 110V but it works and powers my AC.

So... I have been using it but I only get an hour MAX of runtime for my air conditioner. Is something wrong or is it really just an hour of runtime for real?  

Can someone give me a math to explain? Thank you
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #2
    I would have said half an hour.   But if you are getting an hour, you are likely flattening your batteries to zero, and will damage them.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #3
    Nominally the calculations would be (using a Kill-a-Watt type meter will help nail the numbers):
    • 7 amps * 120 volts = 840 Watts
    • 840 Watts * 1/0.85 inverter eff * 1/12 volts nominal = 82 Amps @ 12 volts
    • 200 AH battery bank * 0.50 max recommended discharge (can go deeper if recharged right away) * 1/82 amps = 1.35 hours
    Note that the battery bank is rated 200 AH @ 20 hour discharge rate... A ~1-2 hour discharge rate, the apparent capacity of the battery bank will be less...

    For example, a Trojan T-105 FLA battery has a capacity of 225 AH @ 20 hour rate, but only 185 AH @ 5 hour rate:

    https://www.trojanbattery.com/product/t-105/

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Marc Kurth
    Marc Kurth Solar Expert Posts: 1,165 ✭✭✭✭
    #4
    Expanding on Mike's point about shortening the battery life:

    Remember that for most SLA batteries, 10.5v under load is a dead battery, because you have drawn out all of the rated capacity. (11.6v no-load)  Think of it like "red line" on your tach. You don't want to hold it there and you sure don't want to go past it!

    Deep cycle batteries are rated to provide a given number of charge-discharge cycles.
    A few SLA's can go 500 to 600 cycles at 100% depth based on a 5 hour discharge.
    Most will only provide 75 to 150 cycles at the 5 hour rate.
    Discharging the battery capacity in an hour will reduce those numbers

    Adding to Bill's point about capacity reduction on fast discharge:
    A typical lower cost 100 ah SLA battery will be rated 60-65 ah when discharged in 1 hour.
    So on fast discharge, you effectively have a battery bank rated at 120 ah to 130 ah.

    Lastly, many inverters are set up to "drop out" at 10v, which will give you another few moments of running, but cut your rated cycle life in half - or even lower.

    Marc
    I always have more questions than answers. That's the nature of life.
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  • jonr
    jonr Solar Expert Posts: 1,386 ✭✭✭✭
    edited August 2018 #5
    With heating and cooling, consider water storage instead of batteries.  Eg, chill the water while the sun shines.

    Also consider a more efficient air conditioner.

    I am available for custom hardware/firmware development

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  • oil pan 4
    oil pan 4 Solar Expert Posts: 767 ✭✭✭✭
    #6
    12v system plus air conditioner, don't even bother with it.

    Solar hybrid gasoline generator, 7kw gas, 180 watts of solar, Morningstar 15 amp MPPT, group 31 AGM, 900 watt kisae inverter.

    Solar roof top GMC suburban, a normal 3/4 ton suburban with 180 watts of panels on the roof and 10 amp genasun MPPT, 2000w samlex pure sine wave inverter, 12v gast and ARB air compressors.

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  • bill von novak
    bill von novak Solar Expert Posts: 891 ✭✭✭✭
    #7
    hmong2017 said:
    I have a small 12V system setup. 

    4 of 100Watt 5A Mighty Max brand going into a MPPT Charge Controller 30A, charging my (2) 100Ah Acid Sealed Battery in parallel unbranded. The inverter is 3000Watt 12V output 110V. 

    My batteries are 100Ah each. I paralleled them, so I should have 200Ah. The Batteries are 2 years old. 
    My Air Conditioner is 8000BTU and use or draw 7A and uses 120V. 
    The inverter output is only 110V but it works and powers my AC.

    So... I have been using it but I only get an hour MAX of runtime for my air conditioner. Is something wrong or is it really just an hour of runtime for real?  

    Can someone give me a math to explain? Thank you
    At high rates a 200ah battery becomes a 160ah battery.

    At 2 years old probably a 140ah battery.

    140ah*12V =  1680 watt-hours, 840 watt-hours usable (50% max drawdown)

    7a*120V= 840 watts, 85% efficient inverter = 1000 watts

    I'd say an hour if you are lucky.
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