PV Wattage and Battery Amp Hours

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stmar
stmar Solar Expert Posts: 370 ✭✭✭
in Grid Tie and Grid Interactive Systems #1
This may be elementary but I am having a hard time getting my head wrapped around it.
When you put batteries in Series you increase voltage and decrease amp hours. i.e. 4 X 6volt (400 ah) @ 24 volt = 400 amp hours
When you put PV panels in Series you increase voltage but wattage stays the same. i.e. 2 X 12 volt 100 watt panels @ 24 volt = 200 watts

Did I understand this correctly and can someone give me the simple explanation as to the difference in calculating?
What kind of in line fusing do you use for your array? I assume it is on each string and would be at the array and would be in addition to your breaker and controller at the battery bank.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
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    Re: PV Wattage and Battery Amp Hours
    stmar wrote: »
    This may be elementary but I am having a hard time getting my head wrapped around it.
    When you put batteries in Series you increase voltage and decrease amp hours. i.e. 4 X 6volt (400 ah) @ 24 volt = 400 amp hours
    When you put PV panels in Series you increase voltage but wattage stays the same. i.e. 2 X 12 volt 100 watt panels @ 24 volt = 200 watts

    Did I understand this correctly and can someone give me the simple explanation as to the difference in calculating?

    Nope.

    When you put batteries in series you increase Voltage but Amp hours stays the same:
    Two 200 Amp hour 6 Volt batteries in series = 200 Amp hours @ 12 Volts.

    When you put batteries in parallel you increase Amp hours but Voltage stays the same:
    Two 100 Amp hour 12 Volt batteries in parallel = 200 Amp hours @ 12 Volts.

    The amount of power in Watt hours is identical in either case: 200 Amp hours * 12 Volts = 2400 Watt hours.

    It is the same with PV. Series connection increases Voltage but not current, parallel connection increases current but not Voltage. Watts remains the same as it is V*A.
    What kind of in line fusing do you use for your array? I assume it is on each string and would be at the array and would be in addition to your breaker and controller at the battery bank.

    Here you are mixing PV configuration with battery bank configuration.
    A PV array requires no fuses or breakers unless there are more than two parallel connections. Then each string (even if it is a string of one) of PV needs over-current protection so that the combined Isc of the other panels does not exceed the maximum current rating of a shorted one. Otherwise PV is self-limiting in current so if the wiring is sized right it can theoretically dump Isc into a short indefinitely. The collective array does not need circuit protection as it will do nothing in the event of a short.

    Batteries are not self-limiting and can put out a huge amount of current in an instant. As such they should always have a fuse or breaker on both the input circuit (from charge controller) and output circuit (to inverter and/or other DC loads).
    Whereas it is not vital to fuse individual battery strings (especially if there are only two) it is an extra measure of precaution against shorts within the bank wiring. The more parallel strings the safer you are with individual fuses.

    Clearer? Or muddier?
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  • stmar
    stmar Solar Expert Posts: 370 ✭✭✭
    #3
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    Re: PV Wattage and Battery Amp Hours

    A little of both, now I understand that the PV does not necessarily need to be fused.
    I am still muddy about the PV Watts. If I have 2 X 12 volt 120 watt panels and wire them in series to get 24 volts would I have 240 watts?
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  • Mountain Don
    Mountain Don Solar Expert Posts: 494 ✭✭✭
    #4
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    Re: PV Wattage and Battery Amp Hours

    Yes, 240 watts if the sun hits them right.
    Northern NM, 624 watts PV, The Kid CC, GC-2 batteries @ 24 VDC, Outback VFX3524M
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #5
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    Re: PV Wattage and Battery Amp Hours
    stmar wrote: »
    A little of both, now I understand that the PV does not necessarily need to be fused.
    I am still muddy about the PV Watts. If I have 2 X 12 volt 120 watt panels and wire them in series to get 24 volts would I have 240 watts?
    Yes, and if you wire them in parallel you would get 240W at 12V nominal. Which would actually be about 240V/18V = 13.3A.
    In the series configuration you would get 2x18V (using the Vmp value rather than the nominal 12V value) and 6.67A.

    But how much of those watts actually got to the panels will depend on they type of charge controller you use.
    With a PWM controller and the parallel setup you will actually get roughly 13V x 13.3A = 178W.
    With a PWM CC and the series setup you would get 13V x 6.7A = 88W. Not a good configuration to use.

    With an MPPT CC, on the other hand, and conservatively assuming an efficiency of 94%, you would get 226W from either configuration, and power delivered would not be dependent on the battery voltage at the time.
    SMA SB 3000, old BP panels.
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  • boomadge
    boomadge Solar Expert Posts: 25 ✭✭
    #6
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    Re: PV Wattage and Battery Amp Hours
    stmar wrote: »
    If I have 2 X 12 volt 120 watt panels and wire them in series to get 24 volts would I have 240 watts?

    Yes series 2x12v for 24v @ 240 watts, (volts and watts double but amps do not)
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  • stmar
    stmar Solar Expert Posts: 370 ✭✭✭
    #7
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    Re: PV Wattage and Battery Amp Hours

    I will probably go with another PS-30M since that is what I have already. Thanks for all the input I think I have an idea of what to do.
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