Dead batteries mean no power?

stockoneder
stockoneder Registered Users Posts: 7 ✭
in General Solar Power Topics #1
I have a question regarding how a grid-tied hybrid system with a battery bank would work if the batteries were basically dead.
Would the system still provide power to connected loads if the batteries
were so dead they barely held a charge during the day, while the solar panels were making electricity?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #2
    Re: Dead batteries mean no power?

    Solar Guppy would have to give you the exact answer...

    But my guess is that there still is a 120 Hz current ripple in the battery bank as the inverter "inverts".

    As the battery fails, it will supply less of the peak current and absorb less of the peak charge current--basically turning into a giant, poorly operating, capacitor.

    I would guess that the Vmax/Vmin of the ripple will become worse and worse as the battery fails--and that both the charge controller (may mess up MPPT function) and the inverter (Vmin may cause inverter to stop GT feed) will have problems with the large voltage swings. Plus the battery may begin to overheat as its internal resistance rises.

    You may also have other failing battery issues... Shorted or open cells--which will have their own secondary effects (open output, overheating cell(s), excessive hydrogen outgassing etc.).

    Lastly, a dying battery bank may have losses of 2% per day (and probably worse as the battery approaches dead)--so you are throwing away more or more energy into battery losses (and the GT Inverter may end up trying to "charge" the battery at night to keep it alive enough for the solar charge controller to work--charge controllers typically won't start charging if the battery voltage is too low).

    And I would guess that you may have to keep adding water as the battery just starts doing electrolysis instead of actual charging.

    Why the question?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • RCinFLA
    RCinFLA Solar Expert Posts: 1,485 ✭✭✭✭
    #3
    Re: Dead batteries mean no power?

    A grid tied battery hybrid inverter is just like a power plant generator tied in parallel with the grid. The inverter is syncronized (frequency, phase, and voltage) to the grid before the relay is engaged to tie it in parallel with the grid.

    Most all these inverters use the battery supply to run the electronics that generate the waveforms and syncronization necessary to connect to the grid. So without battery capable of at least running the electronics within the inverter they won't even power up.

    Once the inverter is syncronized to the grid and tied in parallel with it, it can push to grid (sell from battery or PV pushing at battery) or push to battery (charging) with just slight adjustments to inverter output voltage relative to grid voltage. Inverter a little higher voltage pushes to grid, a little lower voltage and it sucks from grid and pushes to batteries.

    A grid tie battery-less inverter electronics are powered from grid to do the initial syncronization. It acts more like a current source then a voltage source of a battery equipped inverter. Because the PV panels are a variable current source they are unreliable as a reference voltage like a battery. Without the resevoir tank of a battery, the battery-less grid tie inverter must shut down if grid A.C. supply is lost.
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #4
    Re: Dead batteries mean no power?

    wouldn't battery based GT shut off the inverter till the batteries come up to some sort of charge before re-enableing the inverter? Just a WAG.
    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

    solar: http://tinyurl.com/LMR-Solar
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  • RCinFLA
    RCinFLA Solar Expert Posts: 1,485 ✭✭✭✭
    #5
    Re: Dead batteries mean no power?

    Inverter is the charger. These battery based, grid tied inverters are bi-directional design. The switching FET's allow the power to flow in or out of the inverter that is syncronized to the AC line.
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  • mikeo
    mikeo Solar Expert Posts: 386 ✭✭✭
    #6
    Re: Dead batteries mean no power?

    I
    have a question regarding how a grid-tied hybrid system with a battery bank would work if the batteries were basically dead.
    Would the system still provide power to connected loads if the batteries
    were so dead they barely held a charge during the day, while the solar panels were making electricity?
    Yes the inverter will continue to push power into the grid as long as the charge controller can maintain the battery bank above the sell voltage. The inverter itself pulls very little DC from the bank and will shut down when the battery voltage gets below the low voltage cutoff setpoint value when the solar quits producing. In the morining when the solar pulls the battery backup above the sell voltage, one has to manually restart the inverter. At least that is the way my SW series inverter works as I ran it for almost a year on shot batteries until I found a deal on better ones. The draw back, is the loss of any emergency back up power.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #7
    Re: Dead batteries mean no power?

    How did the batteries get dead?

    The inverter should keep them charged as long as there is grid power or sufficient solar. It should only draw off them when the grid is down and there's not enough sunlight to counter load demands. Unfortunately the low voltage shut-down is usually too late to prevent severe depletion, but even so good batteries should charge back up once power is restored.
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  • mikeo
    mikeo Solar Expert Posts: 386 ✭✭✭
    #8
    Re: Dead batteries mean no power?
    How did the batteries get dead?
    In my case they died of old age, they had a 1982 date stamp on them and were telco batteries that were given to me when they were taken out of service. Whether you use the inverter to charge the batteries is a matter of system design and programming. My batteries are strictly for backup, in fact I may have to go turn them on in backup mode as the grid has dropped out here on a perfectly beautiful day, I will give them a couple of hours to restore power. My system design turns off the inverter after the solar drops below 1 amp in the evenings and does not turn it back on until the charge controller is supplying greater then 6 amps in the mornings. Till the inverter get the turn on signal, the batteries are in bulk mode for about 30 minutes then drops to float mode at 28 volts for the rest of the day. The only draw on the batteries at night are the few milliamps the inverter needs to keep it settings and the self discharge of the battery. According to the Trimetric, the lowest voltage the on the batteries is 25.7 volts. On the average the grid power is off where I live about 3 weeks a year, the longest so far this year was 12 days after the ice storms in the spring.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #9
    Re: Dead batteries mean no power?

    "Yes the inverter will continue to push power into the grid as long as the charge controller can maintain the battery bank above the sell voltage."

    don't forget that the controllers themselves need battery voltage to operate and if it goes too low it will not allow the controller to function and prevent the power from the pvs from being passed on to these doa batteries. the only hope of functionality in that case is if the built-in charger keeps the voltage on the batteries high enough to allow controller functionality. the batteries that failed will boil water (even if agm as the vent will be wide open) like there's no tommorrow and the good ones will soon fail from feeding their power into the bad battery(s).
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