How does power pass through when batteries are at 100%?

Valentin
Valentin Registered Users Posts: 2 ✭
edited April 2017 in Solar Beginners Corner #1
Hi all,

In a DC-coupled system, say batteries are at 100%. PV continues to produce more than the loads are consuming. How does that power go from the PV to the loads? Is it somehow passed through directly from the PV through the controller to the inverter -- and in the process skipping the batteries? Or does the inverter draw from the batteries while the controller keeps charging them -- in effecting doing micro cycles of discharge/recharge back to 100%?

Thanks.

--Valentin
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,622 admin
    #2
    Lead Acid batteries will take a bit of current (typically somewhere around 0.01% to 1.0% of the battery capacity rating) in current if you are floating them.

    When you have daytime loads, the charge controller holds them at ~13.6 volts or so (12 volt float value). And the inverter draws a 120 Hz sine squared waveform (basically, the inverter draws current for each + and - cycle of your 60 Hz AC power) from the battery bank.

    If the controller holds the voltage at 13.6 to 13.8 volts (float), the battery acts like a giant electrolytic capacitor. It does not really micro cycle because the voltage cycles around 13.6 to >12.7 volts.

    If the charge controller was set to (for example) 12.7 volts float, then the battery would micro cycle down to 12.4 and and back to 12.7 volts (micro cycling--numbers are sort of made up--I have not put a volt meter on a large DC battery bank while running heavy AC loads)...

    Most AC inverters do not have very much power/energy storage on the front end (big capacitors are expensive)--They use the battery bank to supply all "meaningful" current (including the 120 Hz pulses).

    Anyway, that is the expectations.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Valentin
    Valentin Registered Users Posts: 2 ✭
    #3
    BB. said:
    Lead Acid batteries will take a bit of current (typically somewhere around 0.01% to 1.0% of the battery capacity rating) in current if you are floating them.

    When you have daytime loads, the charge controller holds them at ~13.6 volts or so (12 volt float value). And the inverter draws a 120 Hz sine squared waveform (basically, the inverter draws current for each + and - cycle of your 60 Hz AC power) from the battery bank.

    If the controller holds the voltage at 13.6 to 13.8 volts (float), the battery acts like a giant electrolytic capacitor. It does not really micro cycle because the voltage cycles around 13.6 to >12.7 volts.

    If the charge controller was set to (for example) 12.7 volts float, then the battery would micro cycle down to 12.4 and and back to 12.7 volts (micro cycling--numbers are sort of made up--I have not put a volt meter on a large DC battery bank while running heavy AC loads)...

    Most AC inverters do not have very much power/energy storage on the front end (big capacitors are expensive)--They use the battery bank to supply all "meaningful" current (including the 120 Hz pulses).

    Anyway, that is the expectations.

    -Bill
    Thanks, Bill. Does it work (broadly) like that for AGM batteries?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,622 admin
    #4
    Agm and flooded cell are the same chemistry.

    Agm are sealed (with pressure relief) and have a catalyst like platinum or other metal to recombine hydrogen and oxygen back to water. Agm have lower electrical resistance and higher surge capacity. And lower discharge / higher electrical efficiency.

    But because they are sealed, they are more sensitive to overcharging. Catalyst could overheat and they can vent electrolyte (which is not normally replaceable / refillable).

    - Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • bsolar
    bsolar Solar Expert Posts: 103 ✭✭✭
    edited April 2017 #5
    for all intents and purposes, its going to take a couple of amps to hold a good size charged bank in float .. the rest of the amps your array can supply can be directed to the inverters via the charger(s) . .exactly what its doing at the 'micro' level doesnt matter, if its bumping on the batteries and the chargers are counter bumping ect .. the battery bank is an integral part of the system for sure and needs to be there, in good shape, and hooked up ..  if your loads over-draw on what the array is putting out, a cloud goes over or the load is just larger than the array can produce, then you dip into the battery ..
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