how many amps am I pulling?

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johnelarue
johnelarue Solar Expert Posts: 33 ✭
in Solar Beginners Corner #1
Hello again,

As previously mentioned on other question threads, I have 3- 50watt , 12V panels. 2- 105AH RV batteries. The batts are not true deep cycle so they will be discharged to 70% of full only, hopefully.


The loads were 5 - CFL's. The specs read: 0.20Amps/12Watt/100Volts

I want to replace with 5 - LED's. The specs read: 0.11Amps/6.4Watt/100Volts

So I have 30 Amp/hours to use at the 30% discharge rate. Say I round up the optimal 0.11Amps to 0.20 for inefficiencies etc...(sorry this is where I am obviously very poor at electronics) Can I assume I can run these lights for

30Amp Hours X 0.20amps = 150 hours????

How many amps am I really drawing here?

Thanks,

John
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  • rama
    rama Solar Expert Posts: 36 ✭
    #2
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    Re: how many amps am I pulling?

    This is the first time I answer someone else's questions, so feel free to correct me if I'm obviously way off base... but here it goes.

    To determine how many hours you can run the load (5 led, 5*6.4 = 32W) you'll have to first find out how much Wh your array can produce. So 150W array*3.0 sun hr (estimated for JPN)*0.7 (PV derate) = 315 Wh. So if your area is able to produce 3.0 sun hours in a day, theoretically you can produce 315 wh in one day. 315/32w = about 10 hours you can run these LEDs.

    This is about as far as I get... :p
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,448 admin
    #3
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    Re: how many amps am I pulling?

    Rama, good answer...

    I would only add that there are inverter losses (typically around 85% efficient) and battery charging/discharging losses (worst case around 80% for flooded cell batteries)...

    So, I like to use for an overall system derating of:
    • 0.77 (warm/dirty panels+charge controller losses) * 0.85 inverter * 0.80 battery = 0.52 (52%) end to end efficiency
    So for a 150 watt array and your numbers:
    • 150 watts * 3 hours of full sun * 0.52 = 234 WH of 100 VAC Inverter Power from off-grid battery PV system.
    That would be the typical maximum power that one would expect to use from such a system (charge battery during the day, discharge into loads at night).

    It is "ugly"--but 50% of rated power for an off-grid solar system is accurate.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #4
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    Re: how many amps am I pulling?

    they are addressing not so much what 30% of your battery can deliver at 30% dod (70% soc) as they are thinking in terms of what can reliably be supplied to the batteries typically on a daily basis from the pvs. i believe you were asking only of 30% capacity of the batteries though.
    it will be difficult to say exactly how long you can do this as the efficiency of most inverters fall way off on small loads. the inverter will have a minimum amount of power it needs to operate itself independent of the load power needed and as such the efficiency near the inverter's full load rating will be better than what it is with low or no loads with variances in between. even this statement by me may vary or be null and void for some inverters. if one would arbitrarily pick an efficiency of the inverter at say 70% (we don't know for sure what it is for yours) then you can figure the math more readily for yourself. maybe you should try forgetting the guesswork and measure the actual draw of the current at the battery. from this you can just divide it into your prearranged 30a max you wish to keep and that would result in how many hours you can run it at.
    make sense to you?
    now if you don't want the batteries to possibly be slowly depleted you then need to account for what the pvs can supply every day as your maximum depleted amount and you can then take your measured draw and divide that into your actual max for depletion. i hope you understand where it is i'm going with this and i know what i'm trying to explain to you, but i think it may be a tad confusing to you.:confused::roll:
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  • johnelarue
    johnelarue Solar Expert Posts: 33 ✭
    #5
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    Re: how many amps am I pulling?

    Thank you much Ramaand as always BB

    neil Thanks, I understand what you are saying. One question, how would I measure the draw off the batteries while in operation?

    john
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,448 admin
    #6
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    Re: how many amps am I pulling?

    John,

    There are several answers to your question...

    One is a "simple amp meter"--It is like a car's speedometer, it just reads the current draw at a moment of time... Either a direct meter (break the "cable" and insert the meter)... Or a shunt based meter (place a large precision resistor in the battery wiring, then use a remote meter to measure the voltage drop across the shunt).

    Or you can go with a Amp*Hour/Watt*Hour meter and totalize the current*time...

    A Battery Monitor (Victron is another brand with a couple good references here) is designed to measure power into and out of the battery bank. As close as you can get to the equivalent to a computerized gas tank gauge for your battery bank. Not-cheap unfortunately.

    There are Amp*Hour / Watt*Hour meters designed for RC (radio controller) hobbyists--Unfortunately only measure current / Amp*Hour in one direction.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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