What battery voltage should I have my system shutdown at with a 24v system??

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90cummins
90cummins Solar Expert Posts: 57 ✭✭
in Solar Beginners Corner #1
What voltage should I set the low volt warning to on my Xantrex XW4024 inverter?
I have 4 (NEW) L16RE-B batteries rated at 370ah each.
I checked the specific gravity of all cells and it was in speck and I also did an equalization charge as Trojan recommended for new batteries.
If I set the low voltage warning to 24v it goes off within minutes with a 1000 watt load.
Being an auto mechanic I realize that the voltage will droop more as the load is increased and is not a true indication of battery state of charge.
Using the Trojan calculator which includes inverter inefficiency's I should be able to get 14 hours at 1000 watts for a 30% DOD.
My Xantrex controller allows me to set the low battery warning as low as 20v.
I do not want to damage my batteries so I need experienced advice.

Thanks 90cummins
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
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    Re: What battery voltage should I have my system shutdown at with a 24v system??

    I think you should check just how much V-drop you're getting with that 1kW load. About 42 Amps on 24 Volts, and that should not be too much for 370 Amp hour batteries at all (about 11%). If the wire size is too small for the length you'll get more drop than you should. Normally nominal Voltage under load is right for LVD as it prevents batteries going below 50% SOC. If you have particularly heavy loads, check how much drop you get and if that can't be improved on with cable change adjust the LVD downward by that much. In other words if you get 0.5 Volt drop then set the LVD down to 23.5 instead of 24. 1 Volt drop: 23. Et cetera. Just don't go down any lower than 21.
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  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #3
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    Re: What battery voltage should I have my system shutdown at with a 24v system??
    Using the Trojan calculator which includes inverter inefficiency's I should be able to get 14 hours at 1000 watts for a 30% DOD.
    I think you better rework the math on this one. 370 amp hrs * 30% = 111 amp hrs / 42 amps = 2.6 hrs. This also depends on the Battery temperature.
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  • 90cummins
    90cummins Solar Expert Posts: 57 ✭✭
    #4
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    Re: What battery voltage should I have my system shutdown at with a 24v system??

    All the low voltage cabling is 4/0 with properly crimped ends and the cables are as short as possible with the batteries next to the inverter.
    I did check the resistance with my voltmeter and it was essentially zero!
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  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #5
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    Re: What battery voltage should I have my system shutdown at with a 24v system??

    Your Batteries are new, they will take 30-100 cycles to get the plates formed before they reach their full capacity, until then you'll get more voltage drop and erratic capacity. You'll also get weird SG levels and charging. Right now they could be at say 75% capacity and after you apply their temperature they could be at a true capacity of 50% +/-. A lot of people would never notice this, but it really shows when you have new batteries with a winter start up.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
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    Re: What battery voltage should I have my system shutdown at with a 24v system??

    Note: BC04 is right about the calculated run time and the need for break-in cycles. Measuring wire resistance with an Ohmmeter will only tell you there's a piece of wire there; it won't give you any useful information regarding potential Voltage drop. Measure it under operating conditions and adjust accordingly.
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  • 90cummins
    90cummins Solar Expert Posts: 57 ✭✭
    #7
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    Re: What battery voltage should I have my system shutdown at with a 24v system??

    I used a volt meter to measure voltage potential across each positive & negative cable with the inverter under load.
    Going from the Battery + post to the Inverter + post there was no voltage indication, same was done on the negative cable.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #8
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    Re: What battery voltage should I have my system shutdown at with a 24v system??
    90cummins wrote: »
    I used a volt meter to measure voltage potential across each positive & negative cable with the inverter under load.
    Going from the Battery + post to the Inverter + post there was no voltage indication, same was done on the negative cable.

    Which still is not what you need to do.
    Under load measure the Voltage between (-) and (+) at the inverter input and at the battery connection.
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  • 90cummins
    90cummins Solar Expert Posts: 57 ✭✭
    #9
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    Re: What battery voltage should I have my system shutdown at with a 24v system??

    I did a load test today with the load between 1 & 2.5kw and measured the voltage drop between the batteries and inverter terminals.
    The voltage drop fluctuated between .0 & .1 volts.
    I'll perform another load test tomorrow to see if the capacity improves.
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  • CDN_VT
    CDN_VT Solar Expert Posts: 492 ✭✭✭
    #10
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    Re: What battery voltage should I have my system shutdown at with a 24v system??

    As I was taught 90 Cummings , Your do it correct . That IS the way for voltage drop , & then reverse the fluke leads to read any negative with the dvm from stray gauze (if your in a heavy mag area)

    I set my 24v LVD at 22.9 V

    VT
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