Solar array size/ battery charging formulas

larrybc1
larrybc1 Solar Expert Posts: 44 ✭
in Off Grid Solar & Battery Systems #1
I have a question about how to figure out how many AMPS are being produced by a solar array.....and then to determine max charging current for a 900AH/48 volt battery bank..

I have seen a couple of formulas and dont know which one is right...assuming the solar array is 7200 watts is the correct formula:

7200 x 0.77 (efficiency factor). divided by 48 ?...so would be 115 AMPs...or is it:

7200 divided by 58 (voltage charging current from CC)...or is it:

7200 x0.77 (eff factor) divided by 58 ?

On the battery bank side 900 AH bank the max charging would be 13%.. So
900 x .13 = 117 AMPs..

The reason I am asking is that I want to increase my solar array because I have a lot of loads during the day and my current charge rate is a little less than 9% and I am having a hard time getting batteries fully charged because of the loads..so i want to get as close to max charging rate as possible...I have 2 refrigerators and 2 freezers plus lights that run during the day...Also during cloudy days I want to be able to harvest more energy that what I have at present.

Thanks for the help

Thks for the help
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Solar array size/ battery charging formulas

    The first one is most likely to give you the maximum current available:

    Watts of array * typical efficiency (77%) / nominal system Voltage (lowest Voltage you'd want to be at which is when you'll need the most charge current). So yes the 7200 Watt array on a 48 Volt system should produce about 115 Amps.

    If you're trying to compensate for loads it becomes more difficult as loads aren't on all the time. You need to factor their average draw in.

    As for getting more charge on less-than-sunny days you will need to add PV and limit the output current.

    All things considered you may be looking at adding a FlexNet DC from Outback. That way the total current isn't limited by controller settings to prevent over-current to the batteries.
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  • larrybc1
    larrybc1 Solar Expert Posts: 44 ✭
    #3
    Re: Solar array size/ battery charging formulas
    Cariboocoot wrote: »
    The first one is most likely to give you the maximum current available:

    Watts of array * typical efficiency (77%) / nominal system Voltage (lowest Voltage you'd want to be at which is when you'll need the most charge current). So yes the 7200 Watt array on a 48 Volt system should produce about 115 Amps.

    If you're trying to compensate for loads it becomes more difficult as loads aren't on all the time. You need to factor their average draw in.

    As for getting more charge on less-than-sunny days you will need to add PV and limit the output current.

    All things considered you may be looking at adding a FlexNet DC from Outback. That way the total current isn't limited by controller settings to prevent over-current to the batteries.

    Thks for the info..So my battery bank --Trojan Industrial 6 volt batteries at 900AH/48volts...So is the max charging current 117 AMPS---(13% of 900AH)?If so then my 7200 Watt array world be supplying 115 AMPS and would be at the high range but would be ok as long as I have a Remote Battery Temperature sensor from outback...also what would the Flexnet DC do for me?? thks for the info
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
    Re: Solar array size/ battery charging formulas
    larrybc1 wrote: »
    Thks for the info..So my battery bank --Trojan Industrial 6 volt batteries at 900AH/48volts...So is the max charging current 117 AMPS---(13% of 900AH)?If so then my 7200 Watt array world be supplying 115 AMPS and would be at the high range but would be ok as long as I have a Remote Battery Temperature sensor from outback...also what would the Flexnet DC do for me?? thks for the info

    Depending on what Trojan recommends you may be able to go higher than 13% for peak current, but you won't necessarily want to or want to sustain it at higher level. The RTS is a must.

    Info on the FNDC: http://www.solar-electric.com/batteries-meters-accessories/metersmonitors/oupoflsymo.html and http://www.solar-electric.com/lib/wind-sun/flexnet-dc.pdf
    For the best understanding of it read the manual: http://www.solar-electric.com/lib/wind-sun/FlexnetDC-manual.pdf

    The trick to using it for this situation would be utilizing the AUX function to switch on/off additional PV capacity as load demands. There are limitations to this and it may not give you the results you want so study its operation carefully before you buy one.
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