I have 500 watts of solar going to my 24v battery bank but im not going over 25 volts.

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nobody928
nobody928 Solar Expert Posts: 62 ✭✭✭✭
in Solar Beginners Corner #1
I have 500 watts of solar panel going in to 4x 12v 105 amp hour batteries wired in series to get to 24volts. I have 540 watts of solar also wired for 24v. My battery bank never seems to charge past 25 volts or so and mainly stays in the 24.00 to 24.9+ range. Is this normal. I have a xantrex c40 pwm and only very light loads on the battery bank.
What is a 24 volt battery bank resting voltage?

What should my bulk absorb and float be set to?
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  • jcheil
    jcheil Solar Expert Posts: 722 ✭✭✭
    #2
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    Re: I have 500 watts of solar going to my 24v battery bank but im not going over 25 volt
    nobody928 wrote: »
    I have 500 watts of solar panel going in to 4x 12v 105 amp hour batteries wired in series to get to 24volts. I have 540 watts of solar also wired for 24v.

    I am confused by this statement. Do you have 2 battery banks?
    Off-Grid in Central Florida since 2005, Full-Time since June 2014 | 12 X Sovello 205w panels, 9 X ToPoint 220w panels, 36x ToPoint 225w panels (12,525 watts total) | Custom built single-axis ground mounts | Complete FP2 Outback System: 3 x FM80, 2 x VFX3648, X240 Transformer, FLEXnet-DC, Mate-3, Hub-10, FW500 AC/DC | 24 x Trojan L16RE-B Batteries 1110ah @ 48v | Honda EU7000is Generator and a pile of "other" Generators | Home-Made PVC solar hot water collector | Custom data logging software http://www.somewhatcrookedcamp.com/monitormate.html
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #3
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    Re: I have 500 watts of solar going to my 24v battery bank but im not going over 25 volt

    Probably not normal or healthy for your battery bank.

    Do you have some sort of solar charge controller--Or are the panels just connected directly to the battery bank?

    Ideally, for a "cycling" deep cycle battery bank, you would want around 5% to 13% rate of charge. For your battery bank:
    • 4* 12 volt * 105 AH * 1/0.77 panel+controller derating * 0.05 rate of charge = 327 Watt array minimum
    • 4* 12 volt * 105 AH * 1/0.77 panel+controller derating * 0.10 rate of charge = 655 Watt array nominal
    • 4* 12 volt * 105 AH * 1/0.77 panel+controller derating * 0.13 rate of charge = 851 Watt array "cost effective" maximum

    So, your 540 watts of solar panels seem to be sufficient.

    Ideally, your "resting voltage should be around 25.4 to 26.6 volts or so if fully charged (no loads, resting for ~3+ hours). If the battery is 1/2 full (resting), around 24.0 volts... You really do not want the battery to be operating too often at lower voltages (batteries will not last as long)... And you do not want battery at less than ~75% state of charge to sit around for days/weeks/months. They will sulfate and die.

    Charging voltage, somewhere around 28.0 to 28.4 volts (if AGM) and around 29.0 to 29.5 volts (if flooded cell), usually held for ~2-6 hours (longer "absorb cycle if the batteries are more deeply discharged).

    Float voltage is usually around 26.4 to 27.6 volts or so.

    Then the questions... If this is a flooded cell type battery, have you measured (logged) the temperature corrected specific gravity of each cell? You need to understand where your batteries are at today--And (probably) get them recharged quickly.

    Next, need to figure out how much current is going from the array to the battery bank in mid-day/under full sun... A 540 watt array on a PWM controller would expect to output (typical maximum):
    • 540 watts * 1/35.0 Volt Vmp array = 15.4 amps or so (rough guess)

    If you do not have any way to measure the current flow, I would get a DC Current Clamp DMM (like this one from Sears for ~$60 is "good enough").

    Also, take an accurate volt meter and measure the voltage across each battery. Looking for differences (a battery with low voltage could have a shorted cell).

    Take a quick read through the Battery FAQ.

    -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 ✭✭✭
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    Re: I have 500 watts of solar going to my 24v battery bank but im not going over 25 volt
    nobody928 wrote: »
    I have 500 watts of solar panel going in to 4x 12v 105 amp hour batteries wired in series to get to 24volts. I have 540 watts of solar also wired for 24v. My battery bank never seems to charge past 25 volts or so and mainly stays in the 24.00 to 24.9+ range. Is this normal. I have a xantrex c40 pwm and only very light loads on the battery bank.
    What is a 24 volt battery bank resting voltage?

    What should my bulk absorb and float be set to?

    Watts isn't Volts. You need to know that the array Vmp is actually 35 +/- a Volt. If the panels are only 30 Vmp and wired in parallel there is not sufficient Voltage to charge a 24 Volt system.

    Likewise a PWM type controller can not make use of higher Voltages to produce more current. As such you could have insufficient current to charge 210 Amp hours of battery. (Although this is least likely.)

    With a PWM controller the '540 Watts' of panel would typically be configured as:
    Four 135 Watt panels with a Vmp of 17.5 and Imp of 7.7, two parallel strings of two in series to provide an array of Vmp 35 Imp 15.4.

    A 24 Volt system typically charges around 28.8 to 29.6 Volts. The 15 Amps of peak current would be about 7% charge rate with no loads. Resting would be about 25.5 Volts.

    If the batteries are ruined they may not charge no matter what you try.
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  • SolInvictus
    SolInvictus Solar Expert Posts: 138 ✭
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    Re: I have 500 watts of solar going to my 24v battery bank but im not going over 25 volt

    From information in other posts, nobody928 has:

    4 105 Ah, 12 V AGM East Pen Batteries wired in a 24 V array.
    4 140 W PV panels.
    Xantrex (Schneider Electric, Trace Engineering) C40 PWM charge controller.

    140 W PV panels usually have 36 cells in series to make a voltage at maximum power suitable for charging 12 V systems. However, he had to manually attach MC4 connectors to his wiring which makes me wonder about the type of his PV panels.

    nobody928, what is the manufacturer, model number and electrical characteristics of your PV panels?

    nobody928, when you rewired your battery array to make it 24 V, did you correctly rewire your PV panels to create two series strings? Did you check the voltage output from your PV array because your battery voltage never going above 25 V sounds like your PV panels are not sending any power into your batteries? It could be caused by the PV panels not being wired correctly, improper settings on the charge controller or something malfunctioning.
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