6 volt vs 12 volt storage advice

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I have 2500 watt total of panels, two Outback MPPT (1 60 and 1 80) set for 48, xantrec 24 volt inverter, and 12 SunExtender 2120 batteries (12 volt). We also have a TLG-500 wind generator with which we feel there is more hype than fact.

Our problem is that there does not appear to be enough storage. There are times our standby generator will go on twice in one night. Our float usually starts at about 10 a.m. and goes through sundown. In spite of living at the mouth of a very windy canyon, the TLG does not keep us going through the night.

We are considering changing the batteries of our current battery bank for 4 SunExtender 4050HT (6 volt). Would this solve the storage problem? As our 2120's are more than 18 months old so we do not feel that adding new 2120's would be advantageous.

Any advice would be greatly appreciated. (We are completely off grid.)

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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: 6 volt vs 12 volt storage advice

    Merry Christmas!

    Now for the presents, which may include a lump of coal. :p

    You appear to have a couple of problems. For one thing, two Outback controllers? And what does "set for 48" mean? If your inverter is 24 Volt, the whole system should be 24 Volt. Let's look at it from that perspective.

    2500 Watts of panel is a lot for off-grid. Roughly it would perform like this:

    2500 * 77% efficiency = 1925 Watts / 28.4 charging Volts = 67.7 Amps peak current.
    The FM80 alone is capable of handling that; no need for the MX/FM60.
    It could charge up to 1340 Amp hours of battery, but would probably perform best at 677 Amp hours. That at 24 Volts nominal gives you up to 8 kW hours of stored capacity. The panels would "harvest" at best 7.7 kW hours during typical day, so that would be in the ball park.

    Your Sun 2120's are approximately 200 Amp hours each @ the "20 hour" rate. You have twelve. That's six parallel strings of two each for 1200 Amp hours. This is getting towards the upper limit of capacity that the panels can charge, especially in poor sun. It is also a "big no-no" to try and connect so many parallel banks. Current flow becomes problematic, and bus bar connections are a must.

    All things considered, you've probably not been completely charging the batteries you have meaning they're now losing capacity that can't be recovered. Adding more battery capacity will only make it worse. Unfortunately you can' take Specific Gravity readings on AGM's. I suggest you disconnect each string, charge the batteries separate from the system, then let them sit and take a resting Voltage reading. Look for the best ones, and put the bank back together using the best half. See if the smaller bank can maintain the charge over night.

    Which leads us to the unanswered question: loads? Get a handle on how much power you use and reduced it as much as possible. That is key to a successful off-grid system. Adding a battery monitor is always a good idea, especially with AGM's. One example is the Trimetric: http://www.solar-electric.com/tm2020.html

    Don't knock the TLG500. Among small wind units it's one of the best. The usual problem is that there simply isn't really enough wind available at most sites to get any real power from.

    Hope this helps. Or at least starts to. :D
  • BB.
    BB. Super Moderators, Administrators Posts: 33,477 admin
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    Re: 6 volt vs 12 volt storage advice
    tomalidav wrote: »
    I have 2500 watt total of panels, two Outback MPPT (1 60 and 1 80) set for 48, xantrec 24 volt inverter, and 12 SunExtender 2120 batteries (12 volt). We also have a TLG-500 wind generator with which we feel there is more hype than fact.
    First question--48 volt battery bank and 24 volt inverter and (I guess) TLG connected to 24 volt bank?
    Our problem is that there does not appear to be enough storage. There are times our standby generator will go on twice in one night. Our float usually starts at about 10 a.m. and goes through sundown. In spite of living at the mouth of a very windy canyon, the TLG does not keep us going through the night.
    Many Wind Turbines are spec'ed at 12 and 24 volt (or even higher)... In general, running such a turbine at higher voltages (12 volt @ 24 or 48 volt battery bank) can dramatically reduce their total output power--Such a configuration requires significantly higher wind speeds to generate the higher voltage before the first watt is generated.

    Also, is the tower at least 30' tall (and preferably 60'+ tall) and no up wind obstructions within 300-600 feet (turbine should be 30'+ above any obstructions with 300-600 foot radius).

    It sounds like you are in a windy area--Have you measured average wind speeds at your location? Trees "Flagging" from prevailing winds?
    We are considering changing the batteries of our current battery bank for 4 SunExtender 4050HT (6 volt). Would this solve the storage problem? As our 2120's are more than 18 months old so we do not feel that adding new 2120's would be advantageous.

    Do you know the typical daily Watt*Hour / kWH / Amp*Hours @ XX volts load of your system (day/night, mostly night, etc.)?

    In general, extra battery capacity only helps if you have sufficient generation capacity to get the batteries 100% full...

    What is your daily charging profile like?
    • Resting voltage before sunrise (no wind)... This gives you an estimate of bank state of charge (sealed batteries, cannot measure specific gravity).
    • What is the charging voltage that you bank reaches during the middle of the charging (solar and wind if you have separate readings)... Looking for a battery to start >12.0 volts (50% state of charge--resting voltage--no charge/discharge current for several hours). Look for battery to reach ~14.2 volts when charging (>~80% state of charge), and hold 14.2 volts for 2-6 hours (absorb state to reach 100% state of charge). At that point, a charge controller may drop voltage to "float" ~13.2-13.7 volts...
    • How often do you take the batteries to "full" (genset, solar/wind RE)?
    • What is the programmed start conditions (voltage?) for the genset?
    • How much load (watts / Amp / *Hours @ xx volts) do you use at night?
    What kind of measuring equipment do you have?
    How are the batteries wired in parallel--Are the electrical runs to each string balanced?

    Checked all the electrical connections? Checked the voltage across each battery in the system (while under heavy load)? Any batteries that are at High or Low voltage?

    If you do not have a DC Clamp meter to measure the charge/discharge current of each series battery string--You can take a DMM set for 1.xxx volts full scale. Identify an equal length of wire in each series string to use as a "shunt" (power resistor). With the bank under heavy load (charging or discharging) measure the voltage drop across the links for each series string. If the voltage drops are similar--the battery strings are probably sharing current ~equally. If one string has a much higher voltage drop or much lower, then the banks are not sharing current properly.

    I am not a big fan of small wind--So, I would first focus on your solar array and make sure that it is operating at 100% (connections good, controllers programed correctly, voltage at controller output is very close to battery bank voltage--appropriate sized wiring between controller battery bank).

    When the bank needs charging, the solar array is pumping the expected current into the battery bank at ~14.2+ volts? Basically, when in "Bulk mode", the controllers should be outputting 100% available current to bank until batteries reach set point of 14.2-14.5 volts--When voltage is reached, then the charge controller start backing off on current to maintain 14.x volts for 2-6 hours to fully charge the battery bank (refer to battery manual for proper charging voltage/profile).

    Do you have a remote battery temperature sensor for the solar charge controllers? Sometimes hot charge controllers reduce output voltage when the battery bank is quite a bit cooler--causes batteries to recharge slower and you don't get 100% power from solar array.

    In general, I wonder if you are deficit charging your battery bank (simply not replacing all of the energy used every day + 10-20% for losses--AGMs are closer to the 5-10% losses).

    That may be causing your genset to start at night (running the batteries from 20-50% state of charge instead of 50%-90+% state of charge). What is the specifications of your AC / Genset Battery Charger (current / voltage output ratings of charger; and your genset ratings)?

    How do you have your genset programmed to start/stop. How long does it run while charging?

    Batteries do not like deep discharging (cycling below 50% state of charge can reduce cycle life; discharging below 20% State of Charge can drain an individual cell to zero volts and even begin to reverse charge--pretty much killing that battery). And the batteries do not like to set for days/weeks/months below ~75% state of charge--that will lead to sulphation and reduced bank capacity.

    Regarding the Wind Turbine--How do you control the battery bank state of charge (shunt/dump regulator?). If you have a shunt regulator, what is the programmed set point voltage for that?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2
    System2 Posts: 6,290 admin
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    Re: 6 volt vs 12 volt storage advice

    Thanks, Gentlemen.

    My mistake the panels are set at 48v. the batteries and inverter at 24. I will recheck everything tomorrow and get back.

    We really appreciate the help.