Voltage at grid outage?

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stmar
stmar Solar Expert Posts: 370 ✭✭✭
in Grid Tie and Grid Interactive Systems #1
We were gone for the day and when we got back the battery bank was showing 25.1 volts and the load was 4 amps and the grid was down. My neighbor said the grid was out for a few hours. I have a Sun Frost RF-16, that runs directly off the battery bank, a full size refrigerator and a full size chest freezer, both 120 VAC running off the inverter. These are the main power loads, the rest of the loads would be clocks and other low wattage standby items, not much. I would have shed power if I had been home and would have charged the bank with a generator when it dipped below float (26.6) since it was a cloudy, stormy day, the cause of the outage, and the array was not putting out too much charge. Of course the grid came back up as soon as I started shedding load.
My question is how long can the bank be in the 25 volt range until it causes real lasting damage? Also how long should the bank maintain what voltage? I want to determine if the 25.1 volts is a problem or if it worked the way it is supposed to.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
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    Re: Voltage at grid outage?

    Do not worry. 25 Volts on a 24 Volt system is well above 50% SOC and if you recharge it in a day it isn't even worth thinking about. You do not have to maintain Float Voltage to have healthy batteries.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #3
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    Re: Voltage at grid outage?

    actually 12.5v/battery represents 75% soc on lifeline batteries. put a full charge into them as fast as you can, but they can go a few days without that full charge no problem. i say do it fast to ward off any possibility of problems by giving them a full charge for within a few days something else might crop up and demand more from your bank and you don't want to play catch up.
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  • jonr
    jonr Solar Expert Posts: 1,386 ✭✭✭✭
    #4
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    Re: Voltage at grid outage?

    It "worked the way it is supposed to". You should be able to adjust the inverter's low battery cut-out voltage to allow whatever level of discharge you want to allow.

    I am available for custom hardware/firmware development

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  • stmar
    stmar Solar Expert Posts: 370 ✭✭✭
    #5
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    Re: Voltage at grid outage?

    Right now it is set on the default value of 22 volts for low battery cut out. I have a range of 16 - 35.5 that I can set this value. Would I be asking for trouble if I set it at a higher value and be plagued by nuisance tripping? What value would be good to set this at? I was lucky that we were not gone too long and that the grid came back up and charged the bank right away. That brings me to another question: I have bulk timer set on 2 hours. If the bank gets fully charged more quickly does it override the timer and go to Float sooner? The reason I ask this is that it seems like it did not go the entire 2 hours the last couple of times I observed a grid interruption. Could this be because the batteries are new and in good shape?
    jonr wrote: »
    It "worked the way it is supposed to". You should be able to adjust the inverter's low battery cut-out voltage to allow whatever level of discharge you want to allow.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
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    Re: Voltage at grid outage?

    I recommend setting LVD at nominal system Voltage - in this case 24. That way the inverter shuts down before the batteries are drained below 50% SOC (nominal under load will be higher SOC than at resting Voltage).

    If you are experiencing "nuisance" disconnects then either your loads are too large or your battery bank too low.

    By "Bulk time" do you mean Absorb time? I know Xantrex does not follow convention in labels but the process should be the same: Bulk until Absorb Voltage is reached then maintain that Voltage until time limit is up (2 hours in this case). I don't know of any built-in chargers that have the ability to use the End Amps function on their own.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #7
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    Re: Voltage at grid outage?

    i believe it might be better at about 12.15v or 12.2v per battery or 24.3v to 24.4v total. this should keep it above 50% soc. it would switch out once reaching that point, but will preserve your batteries from going dead which could happen at 22v.
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  • Vic
    Vic Solar Expert Posts: 3,208 ✭✭✭✭
    #8
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    Re: Voltage at grid outage?

    stmar,

    Would suggest setting the LVD higher than 22 V, which usually represents a DEAD battery.

    Many Inverters have a time value associated with LVD, which would help reduce the nuisance trip-off issue. We use 46.6 V LVD on inverters here, but choosing this value depends on the nature of the battery -- Flooded, AGM, Gel, etc -- and the risks of having the inverter shut down while you are gone for an extended time, etc. Never too simple!

    The SW plus series of Inverters have this time-value setting that LVD needs to be met before shutting down the inverter, as well as an End Amps setting which will end Absorb when reached. In these inverters, then, the Absorb time is the Max Abs time -- and Abs would end at that time, regardless of the battery charge current.

    I do not know if any of these functions are built into the SW 2024 that you have. All FWIW, YMMV - Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
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  • stmar
    stmar Solar Expert Posts: 370 ✭✭✭
    #9
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    Re: Voltage at grid outage?

    Thanks for the input, I will up the LVD value above 24. The only time I have seen it below 24 was during a three day outage during a storm with not much sun. That was with my old bank and now that I have a generator I have the ability to keep it above 24 so should not have a nuisance trip problem.
    I will have to pay close attention the next time the grid goes down and see if the SW does function on battery condition and not just Bulk Timer (2 hours). I never noticed it cutting out this fast with my old bank, it seemed to take the entire 2 hours of charge, so the question did not come up. Any other SW4024 users have any input here?
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  • stmar
    stmar Solar Expert Posts: 370 ✭✭✭
    #10
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    Re: Voltage at grid outage?

    I set it at 24.4 and will see what happens, I figure being on the grid I should not have a problem and it would be an extreme situation for it to go below that so preserving the battery bank would be the goal. My LBCO Delay is 15 minutes and my Low battery cut in is 26 VDC and High battery cut out is 32 VDC. Again these are default values, are they okay or could I optimize them also?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #11
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    Re: Voltage at grid outage?

    It sounds like a good start.

    If you have two or more ways to qualify the LVD settings--I.e., below 23.0 volts for 5 minutes, below 24.4 volts for 30 minutes/etc... Will probably help you with preventing false trips from starting loads. Some LVD systems will take current into account (i.e., 24 volts for light current, 23 volt for heavy current).

    And then there is the question of battery bank temperature... I am not sure that LVD is typically temperature compensated (should really be). If your bank sits at XX Degrees most of the time, then Temperature Compensation is probably not needed. If your bank has wide temperature ranges between winter and summer--you may need a different (higher) LVD voltage for winter.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #12
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    Re: Voltage at grid outage?

    For reconnect you have to weigh the amount of charging power you have (i.e. PV Watts) against the likely loads. You want to be sure the batteries are up enough so that when those loads come on the Voltage isn't drawn down to where the inverter just disconnects again in a short time. This can be very difficult to balance, especially if you're not there to shut off 'inessentials'.

    Best to leave the high Voltage disconnect level alone, as that is meant to prevent damage to the inverter and the manufacturer knows best on that.
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  • stmar
    stmar Solar Expert Posts: 370 ✭✭✭
    #13
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    Re: Voltage at grid outage?

    Good question, according to the documentation Low Battery Cut Out, High Battery Cut Out, Low Battery Cut In use the actual battery voltage value. The temperature compensated value is used by the battery charger for its regulation settings.
    I don't know what parameters to look at to balance the reconnect voltage, I will leave it at 26 and the disconnect at 24.4 and see what happens. With a fairly reliable grid these may be okay values. I can also make sure non essential loads are disconnected when we will be gone for extended periods of time. I will definitely leave the high voltage cut out at 32 as per factory default.
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