Float mode confusion...

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Hello all, and if is not the correct forum I'm sorry...

To start off with I am an electrician, plumber and HVAC guy from KY with three trade degrees. My wife and I, over the last year, have built a 1300 sf house. I wired it and my brother was the lead carpenter. We've four 150-watt Kyocera panels, a Xantex 60 amp charge controller and a 1500 watt inverter powering the house, which is otherwise conventionally wired by 2008 NEC. The 12-volt battery bank is seven 120 amp-hour batteries. All battery cables are 2 awg copper welding cable, good for 100 amps, with very short runs. All the equipment is in the same room which helps keep cables short. All the dc and ac connections are TIGHT.

The house will typically go on float mode any day that there's sunlight and that's all well and good. However, I am experiencing what I consider to be serious voltage drop. As soon as the system goes off float mode it will begin to loose dc voltage. The only ac loads are generally phone chargers and CFLs. There is a laptop which is plugged in when in use and a Mac mini occasionally, which is otherwise unplugged. I thought it was a dc issue, so took five batteries offline- lo and behold, one was one volt lower than the others after several hours disconnected. Putting the system back together has not, after two float mode days, stopped the voltage drop issue.

I was under the impression that the system went into float mode once the batteries were fully charged- is this not the case? I would figure that fully charged was somewhere in the neighborhood of thirteen volts, but after coming home from work today when the house has been in float mode "pretty much all day" the battery bank voltage was 12.14... This seems low to me. Great for starting a car yes but not good for a house that's off grid. Typical use kn evening (measured with my clamp ammeter) has the house pulling 2.5 amps ac, which is of course probably a little more than thirty amps dc once line losses and equipment usage is considered. I would figure that at this load level we should have 24 hours if capacity, however, by morning the low voltage alarms will be going off if more than a couple of lights are turned on. Thus is after sitting through the night with only a few alarm clocks and chargers plugged in, and perhaps a few led night lights.

Is the wire resistance of the house a factor? There is exactly 1250 feet of 14-2 romex in the house. Since the inverter puts out split phase as opposed to typical 120 volt, neutral, ground that a normal house uses, could I be loosing power due to the fact that my light switches and breakers are only "breaking the black," as it were? Would building a step-up transformer to have a normal 240 volt center tapped neutral service be of benefit? on the dv side, I plan to add more batteries as time goes on but if there's a problem now I should solve it before I go any further.

Any ideas?

Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: Float mode confusion...

    Welcome to the forum.

    You've unknowingly answered your own question: "seven 120 amp-hour batteries".
    If you've got seven batteries connected in parallel there is no way they are sharing current evenly. What's more, that is an 840 Amp hour battery bank and 600 Watts of panel will not properly charge it. 1300 Watts would. As it is, your panels could support about 385 Amp hours if using an MPPT type controller. As it is your peak charge rate is less than the 5% minimum recommended (about 4.5%).

    The batteries need to be brought up to about 14.4 Volts at least and held there for enough time to complete the Absorb stage. Float Voltage should be around 13.8 for as long as possible.

    "the inverter puts out split phase as opposed to typical 120 volt". What does that mean? A 12 Volt 1500 Watt inverter puts out 120 VAC. The only time you have "split phase" is if you have 240 VAC with a center-tapped neutral (which should be bonded to ground). What exactly is this inverter? Is it true sine? If it is MSW do not bond neutral to ground.

    It is normal wiring practice to "break only the black" as that is all that is required to interrupt the circuit. Do not try to add a transformer to this; it won't do anything.

    Also do not add any more batteries. As it is, if your bank were viable, you'd have 5kW hours of stored capacity. To design a system right the first thing you need to know is how much power you need to supply.
  • ChrisOlson
    ChrisOlson Banned Posts: 1,807 ✭✭
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    Re: Float mode confusion...

    Seven 120ah batteries, I assume, would be marine deep cycles. They are typically lead/calcium grids and like to be charged at higher voltages.

    There are three stages to battery charging. Bulk stage is where the bank takes maximum amps up to 10% of the ah capacity of the bank and the voltage rises as they charge up. Absorb stage is where the voltage reaches the proper level to gas the batteries good. The voltage remains constant during absorb while the amperage tapers off as they continue to reach 100% SOC. Float is the final stage where the voltage is dropped to a level that maintains 100% State of Charge (SOC) but doesn't gas the batteries so they don't use too much water.

    Lead/calcium marine batteries like to be absorb charged at minimum 15.0 volts (check with your manufacturer) for at least 2.5 hours before dropping to float voltage. I would use 13.2 volts for float on those.

    The main problem, the way it sounds to me, is that you have badly sulfated batteries and they aren't getting properly charged. Finding one that is 1 volt lower than the rest indicates a problem with one cell. I would equalize charge them at about 16.0 volts for several hours, check the water in them, load test each one to weed out the bad ones, and go from there. A load test (a constant load applied at the 20 hr rate) over time, and measuring voltage drop during the test, will tell you if they're any good.

    As far as your capacity, you have 840ah, or 5 kWh to 50% SOC, assuming healthy batteries. For a bank that size you need 84 amps charge rate (12 amps per battery) to properly bulk and absorb charge them. At 84 amps it would take about 4 hours to get the bank to absorb, and then another 2.5 hours in absorb as the amperage requirement drops. You don't have even close to enough solar power to charge that bank. I'm guessing all that's happening is that you're getting them from discharged to barely into the middle of bulk before the array runs out of power for the day, and they never do reach absorb voltage.
    --
    Chris
  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
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    Re: Float mode confusion...
    Rmx256 wrote: »
    Hello all, and if is not the correct forum I'm sorry...

    To start off with I am an electrician, plumber and HVAC guy from KY with three trade degrees. My wife and I, over the last year, have built a 1300 sf house. I wired it and my brother was the lead carpenter. We've four 150-watt Kyocera panels, a Xantex 60 amp charge controller and a 1500 watt inverter powering the house, which is otherwise conventionally wired by 2008 NEC. The 12-volt battery bank is seven 120 amp-hour batteries. All battery cables are 2 awg copper welding cable, good for 100 amps, with very short runs. All the equipment is in the same room which helps keep cables short. All the dc and ac connections are TIGHT.

    The house will typically go on float mode any day that there's sunlight and that's all well and good. However, I am experiencing what I consider to be serious voltage drop. As soon as the system goes off float mode it will begin to loose dc voltage. The only ac loads are generally phone chargers and CFLs. There is a laptop which is plugged in when in use and a Mac mini occasionally, which is otherwise unplugged. I thought it was a dc issue, so took five batteries offline- lo and behold, one was one volt lower than the others after several hours disconnected. Putting the system back together has not, after two float mode days, stopped the voltage drop issue.

    I was under the impression that the system went into float mode once the batteries were fully charged- is this not the case? I would figure that fully charged was somewhere in the neighborhood of thirteen volts, but after coming home from work today when the house has been in float mode "pretty much all day" the battery bank voltage was 12.14... This seems low to me. Great for starting a car yes but not good for a house that's off grid. Typical use kn evening (measured with my clamp ammeter) has the house pulling 2.5 amps ac, which is of course probably a little more than thirty amps dc once line losses and equipment usage is considered. I would figure that at this load level we should have 24 hours if capacity, however, by morning the low voltage alarms will be going off if more than a couple of lights are turned on. Thus is after sitting through the night with only a few alarm clocks and chargers plugged in, and perhaps a few led night lights.

    Is the wire resistance of the house a factor? There is exactly 1250 feet of 14-2 romex in the house. Since the inverter puts out split phase as opposed to typical 120 volt, neutral, ground that a normal house uses, could I be loosing power due to the fact that my light switches and breakers are only "breaking the black," as it were? Would building a step-up transformer to have a normal 240 volt center tapped neutral service be of benefit? on the dv side, I plan to add more batteries as time goes on but if there's a problem now I should solve it before I go any further.

    Any ideas?

    Welcome to the Forum, and if the location is too far off one of the Moderators can move it. :-)

    The length of the AC wiring will have no effect at all on the batteries or the output of the inverter for that matter.

    When you interrupt one side of the circuit, that is all it takes to keep the load from using energy, whether it is AC or DC.

    Seven 120 AH batteries in parallel is a very poor way to build a battery bank, and could be part of your problem.

    However, there are some more serious indications is what you have described:

    1. The battery wiring is 2 AWG, but if you pull full load from the inverter, you will be drawing about 160 amps. Any motor starting surge could take it higher, and if the voltage at the inverter terminals goes below 12 volts under load, then the current will go even higher.

    2. It sounds like your batteries have either been overdischarged or chronically undercharged and now have very little capacity left.
    This could be because the settings on the Xantrex are wrong, or it could be because the panels are not giving you enough power.
    Normally the batteries will go much higher than 13 volts during the Absorb stage of charging, and the Float should be higher than the 12.14 which you report.
    Either your voltage settings are wrong or you Absorb time is set far too short, or there are just not enough hours of sun for your 600 watts of panels.
    (A rule of thumb is that you should have 1 watt of panel for each Amp-hour at 12 volts in your battery bank. That would mean 840 watts of panel or more.)

    The very first thing I would suggest is that you get and use a hydrometer to check the specific gravity of your batteries.
    Second, look at the Bulk, Absorb and Float voltage settings on the Xantrex and tell us what they are.
    Third, run a generator to charge the batteries for at least 6 hours before the sun comes up on the panels. (If the generator driven charging also goes into Float, you can stop then.)
    Fourth, start planning to buy new batteries, as it is likely that your current set is on its last legs. Don't buy any new batteries yet though. You need to get some advice on how to configure you battery bank first.
    SMA SB 3000, old BP panels.
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
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    Re: Float mode confusion...

    RMX, I agree with the others that a massively parallel battery bank is trouble.
    Please read this: http://forum.solar-electric.com/showthread.php?14674

    Also, you should have a fuse in series with each of your seven batteries. What you were seeing at the end of float was probably several batteries discharging into another. That can make for some dangerously hot batteries and wires within the battery bank.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • petertearai
    petertearai Solar Expert Posts: 471 ✭✭✭✭
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    Re: Float mode confusion...

    A battery moniter will help keep in eye on the state of charge. I have a Victron and find it very good. There are 3 stages in the battery charging each day. 1st is bulk, this where the panels will use all power available to try to get the voltage up to the absorb voltage you have set. Once at this voltage the controller will try to maintain this voltage for the period of time that you have the absorb time set to. Once this time is up then the float voltage setting comes into play.
    Certainly I would think you need to double the amount of pv you have . Sulpation and deficit charging is my diagnosis. And is others have said too many parallel strings
    A hydrometer will help give you idea of the battery state of charge. What are your charger settings?
    Regards Peter
    2225 wattts pv . Outback 2kw  fxr pure sine inverter . fm80 charge controller . Mate 3. victron battery monitor . 24 volts  in 2 volt Shoto lead carbon extreme batterys. off grid  holiday home 
  • Rmx256
    Rmx256 Registered Users Posts: 3
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    Re: Float mode confusion...

    Thanks for the replies..

    Yes they are marine deep cycle.
    The inverter never pulls full load. I've never seen the house pull more than four amps.
    When I say split phase I mean that I do not have hot-neutral-ground. I have 120 volt split phase as if I had 240 volt split phase: line line ground.

    So I will this evening check the batteries and remove a few from the system and see how that works.
  • ChrisOlson
    ChrisOlson Banned Posts: 1,807 ✭✭
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    Re: Float mode confusion...
    Rmx256 wrote: »
    I have 120 volt split phase as if I had 240 volt split phase: line line ground.

    Did you check code? You got a floating neutral. That's not even legal in a residence in the US.
    --
    Chris
  • Rmx256
    Rmx256 Registered Users Posts: 3
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    Re: Float mode confusion...
    ChrisOlson wrote: »
    Did you check code? You got a floating neutral. That's not even legal in a residence in the US.
    --
    Chris
    This is the output of the inverter. I was not impressed.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,479 admin
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    Re: Float mode confusion...

    It sounds like you have a MSW (modified square wave) inverter. They cannot support a ground referenced AC output (if you try a grounded neutral plus have the battery negative ground referenced--it creates a dead short through the inverter and will let out the magic smoke from the inverter).

    Most TSW (true sine wave) inverters can support a grounded neutral configuration.

    MSW inverters are much less expensive and will power about 80% of the AC devices out there OK (10% may over heat and fail, and another 10% may fail over time--And it is hard to know ahead of time which will work OK and which won't).

    TSW inverters are "utility" grade power and, for a remote home/critical application, usually worth the cost.

    Also, remember that an inverter may burn 6-20 watts or more (depending on inverter size/type/etc.). So a large inverter with small loads can be a significant amount of your total system power usage...

    Many times, it is a good idea to get a smaller TSW inverter to power your 24x7 loads. And some inverters have low power options ("search mode" where inverter "sleeps" until >6 watt load is turned on; a remote on/off switch so you can turn it on when needed). And a second, larger inverter that can be turned on when needed (running tools, pumping water, etc.).

    By the time you add a refrigerator to the system--You are moving up in "class" for your power system. You need more panels, larger battery bank, and larger inverter to run the fridge. And most (all?) of the modern energy star fridges need power 24x7 to keep the defrost timers sane.

    You can do things to reduce power usage--A chest freezer converted to a fridge (new/alternate thermostat) can use 1/4 the power and avoid the defrost timer issues--But is not the most convenient appliance (have to move baskets to access food below, etc.).

    It is always back to knowing your loads and conservation--Then design a system that supports them.

    -Bill

    All About Inverters
    Choosing an Inverter - Home Power Magazine
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ChrisOlson
    ChrisOlson Banned Posts: 1,807 ✭✭
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    Re: Float mode confusion...
    Rmx256 wrote: »
    This is the output of the inverter. I was not impressed.

    That inverter is for a mobile application with a floating neutral. RV's, boats, an portable applications require the floating neutral because the ground is usually to the vehicle frame and battery ground, and not earth ground. So you can come in contact with the hot leg and not get a shock to ground with the floating neutral.

    Inverters legal to use in a residence require a bonded neutral, either at the inverter panel if it has a AC distribution panel, or at the service panel if the inverter is feeding a service.

    A residence with floating neutral will not pass inspection anywhere in the US or Canada, and is quite dangerous because you have no ground fault protection.
    --
    Chris
  • Photowhit
    Photowhit Solar Expert Posts: 6,003 ✭✭✭✭✭
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    Re: Float mode confusion...

    I guess others have made most of the viable points, Your 600 watts of array will provide roughly 1/2 of the array x the solar insolation(usually about 3.5-4 hours depending on where your located) in watt hours of usable electric, so @1200 watt hours. Your demand of 2.5 amps AC X1.2 (for inverter ineffiecencies) x 110(?) volts is about a 330 watt load...

    I've used a marine battery over the winter and it made it about 4 months, so my guess is your batteries are dieing... a much better battery for storage would be a golf cart battery. These may last 3-5 years if not discharged more than 20% or so. They are available at Sams Club and Costco for @$80 each for 6V 215Amp hour battery. Used in strings of 2 they will also help keep your nuber of strings down to a reasonable number (2 or 3) I would NOT buy another marine battery unless you have a boat!

    As others have said all things start with loads though, so how did you figure your designed array and battery bank?
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.