Can a good MPPT charge controller handle 1800w of solar for with just 2 Lead batteries?

OffLineOffLine Registered Users Posts: 4
edited September 2021 in Solar Beginners Corner #1
I really only want to run loads off the panels, but most inverters require a battery.

if i have a six 310watt (VOC 40V) panels . I also have two 12v (80ah) lead acid batteries and connect them in serial for 24v.

I don't have a MPPT charge controller or a pure sine inverter yet. I would like to get a 2000w psw inverter.

My loads would be about 800 - 1200w.

I was reading another Post/Thread that lead me to believe i can do this:
What happens to excess power when batteries are full? — northernarizona-windandsun (

Someone told me I have to much solar for the batteries I intend to use and "it would end up frying my battery over time"



  • petertearaipetertearai Solar Expert Posts: 469 ✭✭✭✭
    Best to stay with convention and  get a good charge controller.  
    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 
  • BB.BB. Super Moderators, Administrators Posts: 32,597 admin
    Just running solar panel power into an AC inverter--It can be done, but it raises lots of questions/issues....

    Solar panels output only the Current (at ~80% of Vmp on a warm day) based on the actual sun hitting the panels. And solar panels have zero surge current (power) capabilities... If you try to pull more power (current) than there is sun on panels, the solar panel voltage will collapse (until the inverter fails to run). Then everything needs to restart (inverter may need a reset, your AC loads will see brown out/loss of AC voltage and need to restart, etc.)--How your AC loads react to restarting depends on what they are (a computer would need to reboot, a TV will need to restart, a laptop with batteries will "ride through" the loss of AC power, an electric motor will need to restart and draw starting current, etc.).

    While running AC from solar panels can be done.... Usually there are better ways of doing this. For example, if you have a water/well pump to cistern/pond, you can get a "solar ready" VFD (variable frequency drive) which will vary the AC voltage and frequency based on the solar energy available (i.e. run at 60+ Hz if lots of solar, down to 10 or 20 Hz, and 1/6th motor RPM when less sun... And a VFD will offer "soft start"--No starting surge current--Usually just slowly increasing current to "full current" as motor comes up to speed).

    You can uses a website like NREL"s PVWatt program which models the amount of harvest from your solar array down to the date and hour by hour level (based on something like 20 year history):

    For example, here is the 24 hours for April 12th, for an 1,860 Watt array, tilted to 23 from horizontal, facing south:

    "PVWatts: Hourly PV Performance Data"
    Requested Location: west palm beach
    Location: Lat, Lon: 26.73, -80.06
    Lat (deg N): 26.73
    Long (deg W): 80.06
    Elev (m): 5.473684311
    DC System Size (kW): 1.86
    Module Type: Standard
    Array Type: Fixed (open rack)
    Array Tilt (deg): 23
    Array Azimuth (deg): 180
    System Losses: 0.77
    Invert Efficiency: 96
    DC to AC Size Ratio: 1.2
    Average Cost of Electricity Purchased from Utility ($/kWh): 0.104
    Capacity Factor (%) 20.5
    Month Day Hour Beam Irradiance (W/m^2) Diffuse Irradiance (W/m^2) Ambient Temperature (C) Wind Speed (m/s) Plane of Array Irradiance (W/m^2) Cell Temperature (C) DC Array Output (W) AC System Output (W)
    4 12 0 0 0 21 3 0 21 0 0
    4 12 1 0 0 21 3 0 21 0 0
    4 12 2 0 0 21 3 0 21 0 0
    4 12 3 0 0 21 3 0 21 0 0
    4 12 4 0 0 21 3 0 21 0 0
    4 12 5 0 0 21 3 0 21 0 0
    4 12 6 262 34 22 3 46.397 21.582 67.082 56.341
    4 12 7 622 76 23 4 269.213 27.603 430.576 411.49
    4 12 8 777 98 24 4 526.483 34.572 896.782 863.126
    4 12 9 857 111 25 4 753.432 40.834 1276.969 1228.215
    4 12 10 898 122 26 5 928.887 44.022 1558.988 1497.168
    4 12 11 913 131 26 5 1032.198 46.26 1714.509 1550
    4 12 12 918 132 26 5 1059.225 46.933 1753.427 1550
    4 12 13 911 127 26 5 1007.58 45.996 1675.645 1550
    4 12 14 882 121 26 5 878.435 43.458 1477.006 1419.148
    4 12 15 828 111 25 5 687.356 38.592 1172.799 1128.469
    4 12 16 731 95 24 5 446.349 32.628 754.149 725.412
    4 12 17 533 69 23 4 192.1 26.755 291.741 276.154
    4 12 18 69 14 23 4 12.75 22.31 21.745 11.859
    4 12 19 0 0 22 4 0 22 0 0
    4 12 20 0 0 22 4 0 22 0 0
    4 12 21 0 0 22 4 0 22 0 0
    4 12 22 0 0 22 4 0 22 0 0
    Just looking at the last column, your hourly harvest averages around 1,550 Watts in the middle of the day (11am to 2pm)--Which is would just run your 1,200 Watt loads for a few 3 hours a day (all things being equal--Which they rarely are).

    And if your loads draw more than ~1,550 Watts, the solar output will collapse and the AC inverter output will shutdown and force a restart (which must remain under 1,550 Watts surge).

    More or less, with solar I would suggest a minimum of 2x-4x "safety/fudge" factor (larger array). Otherwise, you are left with issues of afternoon clouds, birds flying over, loss of output due to dust/bird droppings on panels, etc....

    A 2,000 Watt AC inverter will draw >4,000 Watts--If the AC loads (starting current for motors/etc.) are large enough...

    Your 24 volt @ 80 AH battery bank (flooded cell lead acid)--If it was running the AC loads (just for example--Because that is how we normally design an off grid solar system)--The maximum suggested draw from a good quality, fully charged bank, would be around C/5 hour discharge rate maximum:

    => 24 volts * 80 AH * 1/5 hour discharge rate * 0.85 AC inverter eff = 326 Watts (with support for ~2x that or 652 Watt starting surge for a few seconds/minutes)

    For a 24 volt FLA battery bank, I would normally be suggesting a 400 AH battery bank to run a 2,000 Watt inverter minimum (even 2x larger at 800 AH is better fit)--Based on 500 Watt inverter per 100 AH @ 24 volt battery bank capacity.

    Before I do a whole bunch more typing and making wild guesses about you AC power needs--It would be better for you to tell us about your energy needs--And help design a system that better meets those needs.

    Your thoughts?

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • OffLineOffLine Registered Users Posts: 4
    edited September 2021 #4
    My thoughts?

    Is my idea going to "fry my battery" as someone suggested to me?
    Or will a good mppt solar charger ( ie Victron) protect it?

    My energy needs while the sun is up is 800 - 1200w. If it's an hour or two or three a day, so be it.
  • BB.BB. Super Moderators, Administrators Posts: 32,597 admin
    More or less, a 10-13% rate of charge for lead acid it "optimum"... They can take upwards of 20% to 25%--But you want to make sure you have a remote battery temperature sensor on the battery bank to solar charge controller ("hot" batteries need lower charging voltage).
    • 80 AH * 10% = 8 amps charging current
    • 8 amps * 29 volts charging = 232 Watts charging power
    • 80 AH * 20% current = 16 amps charging
    • 16 amps * 29 volts = 464 Watts charging power
    So--Limiting the output charging current to 8 or 16 amps to "protect" the battery is not going to provide enough current for your 800-1,200 Watt loads.

    A "good" MPPT solar charge controller can be programmed to limit its output current... Protects battery bank, but also limits the current available to the battery bus.

    Midnite Solar has a current shunt setup called the Whizbangjr in series with the battery bank: ("electronics"--Need a current shunt too) (current shunt) (solar charge controller--Larger current output)

    It is used by the solar charge controller to measure/control the charging current to the battery bank. It may have the capability to limit the charging current to the battery bank (or it may not--Need to study the specs for the Midnite Classic and WhizbangJr to figure this out)--While letting the controller supply enough current to run your AC inverter:
    • 1,200 Watts * 1/0.85 inverter eff * 1/24 volts nominal bus voltage = 58.8 amps
    • 1,860 Watt solar array * 0.77 panel+controller derating * 1/24 VDC nominal bus voltage = 59.7 Amps typical max (few times a year, under cool clear sun @ solar noon)...
    Typical max current/power from array--I would plan on getting 1/2 typical (not noon, less than ideal weather, etc.)... If you want "1,200 Watts" from the solar...

    As long as you don't significantly discharge the battery bank (say below 80% SoC)--You probably will not "fry" the 80 AH battery bank with the ~60 Amp max charging current (so any good MPPT controller >=60 Amps) will probably work for you--And you can skip the WbangJr and Midnite controllers...

    Roughly, Lead Acid batteries limit their charging current at 80% and higher state of charge (self limiting current acceptance).

    Just make sure the battery bank is installed in some place where if the batteries overheat (or worse)--Outside in a concrete block area--Not in a wooden floored outbuilding) and you have a RBTS (remote battery temperature sensor)--You probably would be OK... For example, if you set your AC loads/Inverter to shut down at ~26 VDC for over 1-2 minutes--to allow for surge current (i.e. equal to 13.0 volts on a 12 volt battery bank)--It looks like it could work. Basically, you are not discharging the batteries if they stay above ~26 VDC... And the 2 minute surge timer--Just keeps the inverter happy long enough to ride through starting surges without taking a whole lot of stored energy/AH from the battery bank (i.e, 20% * 80 AH = 16 AH of "buffer" between 80% and 100% SoC).

    However I would suggest a somewhat larger array (if you want 1,200+ Watts)... And I would not "fall in love" with your battery bank... The first year you use it, the bank may fail (over charging current--especially if your 26 VDC inverter shutdown fails)... And I would want to make sure that the battery box is fire resistant and not installed near anything "you care about"--Again just in case. 

    If you get 3-5 years of service life from the battery bank (warm/hot temperatures are very hard on Lead Acid battery banks--More or less, running the battery at 77F+18F=95F -- The battery bank will age 2x faster (1/2 the life) vs one kept at 77F or lower.

    Keep wiring for battery bank to AC inverter short and heavy (to supply inverter surge current if/when needed)... Hopefully the batteries will give you enough surge current to "ride through" short cloud/bird flyovers/etc... type events. If you could justify 4x 6 volt @ 200 AH "golf cart" batteries--They might do better for you (and hopefully not too much more expensive--GC batteries are high volume and relatively cheap. Roughly $100 each.

    You would be pushing the envelope--Hope for the best, and plan for the worse type situation here.

    Sounds like it may be doable... You may need/want a timer (for example) or a pilot solar cell to measure actual available solar radiation so  you can ensure that you have >800 (or >1,200) Watts available before starting your AC inverter+Loads... Don't want start/shutdown as sun rises (may be hard on batteries and AC loads). More or less, your pilot solar cell, shorted (i.e., a low value resistor) and measure the voltage (0.05 to 0.10 VDC or so across shunt resistor) calibrated to (for example) 50%-80% of Isc rating of pilot cell--To ensure you have enough sun to run your loads (solar charge controllers generally have no idea of how much "available" sun there is... Just there is >zero Watts available to start charging the battery bank).

    Make sense?

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490mike95490 Solar Expert Posts: 9,571 ✭✭✭✭✭
    All the parts work together.   Size of Loads,  Battery, Solar, Charge controller

    What can damage a charge controller is being asked to limit charge to the battery, from an array that is too large.   Even the very best controllers have a "suggested" max solar input.   The actual max varys with the ratio of the battery voltage to the panel voltage.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    gen: ,

  • PhotowhitPhotowhit Solar Expert Posts: 5,804 ✭✭✭✭✭
    I don't know what type of batteries you are running AGM or flooded Lead acid. I'd think you are close to the minimum I would try.

    I don't know if the Victron will limit charging current, If it will, for a 24 volt battery bank, limit it to 10-13% of your battery capacity. So 2 - 12 volt 80ah batteries in series would be a 24 volt 80ah battery bank, so limit the current to 8-10 amps.

    I ran a system with 15-1600 watt array into a 220 ah 24 volt flooded lead acid battery bank. Without limiting the current, just allowing the batteries too limit their intake. It work pretty well, but I was depleting the batteries at night and they had a huge change in state of capacity. Not sure this adds much to the discussion, but more current was available than the batteries could accept.

    I recall replying to a similar question recently, have you posted before under another ID?
    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, 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.
  • OffLineOffLine Registered Users Posts: 4
    edited September 2021 #8
    Thanks for your replies. 

    Photowhit - No I have not posted before under another ID. This is my first post and I just joined the forum

    Mike95490 - I haven't purchased a SCC yet, but when i do i will make sure it meets the specs of my input "max solar input". I'm still trying to figure out what's best for me. Thanks

    BB - thanks for your information. You responses are very detailed.

    I currently have a 10kw grid tied system on my roof. I'm sure most of you are aware that grid-tied systems do not generate any power if the grid goes down, unless you have batteries or certain systems like the Sunny Boy, which i do not have one of these. I will not be adding a battery bank for my current system or modifying what if have, that is not an option.

    This is my initial idea: I have a boat with two Lead Acid batteries. In a pinch, i would like to be able to use some of my panels (removing the Optimizers) with my two boat batteries to get me a little power if the grid is down, DURING THE DAY.

    If I purchased a MPP LV2424 (all-in-one) that seem to be fairly popular, wouldn't that work with the minimal battery setup I want to use for now? I believe this has a max solar input of 2400w and requires a 24v setup.
  • BB.BB. Super Moderators, Administrators Posts: 32,597 admin
    This it the unit you are looking at(?):

    They do not have the manual online--So cannot look at any needs they may have for minimum battery bank and such...

    With solar--You are your own "power company"--So you are the designer, money person, installer, and maintenance in general... And when you are trying for something "different"--The system operational characteristics (to support non-standard configuration) and your money is is even more at risk.

    With any complex system such as an All-in-One--You can get tripped up buy system behavior that you have not foreseen with a "vanilla" installation. And the chances of having issues with non-standard design--The risk is even higher--And the seller/factory may not even talk with you about those issues (there goes your support and warranty). And most companies will not ever let you talk with one of their engineers about such non-standard configurations (many/most will not ever let you talk to one of their design engineers ever).

    I am all for experimentation--See if you can get a manual from MPP and figure out any additional control inputs (such as the ability to turn the inverter on and off--With your home-made solar energy sensor. Some inverters have a "simple" remote on/off sense line--Which could be setup with the solar pryanometer for this function. Don't know if this MPP system has such an input/control. I would probably simply use a small (weather protected) solar cell, load resistor/shunt, and a voltage compactor (or a small Raspberry Pi, Arduino, or similar computer/controller) for the measurement function.

    Some of these inverter "systems" have some sort of Digital/CAN communications bus (this model has USB and RS 232 comm connections--Don't know if for monitoring or more detailed control--again see if you can get more details)--If you have experience with programming/digital communications systems--That may make your task much easier to get the system behavior you need.

    I think that getting the ability to cycle the inverter on/off with a solar energy sensor is critical to the successful operation of your system design. Get as much information about the major components and do your paper design(s) first before purchasing any hardware.

    And be prepared to "walk away" from your investment if it does not work (or convert to a standard/larger battery bank for normal off grid operation). Also see if you work a fail safe (i.e., micro computer crash, no USB or RS 232 communications with inverter--What happens then when sun goes down and 1,200 Watt load on 80 AH battery bank--Will system "kill" the batteries/etc.?).

    Note: I have seen that this MPP family of All-in-One systems needs an inverter-generator for stable 120 VAC frequency for proper operation--It may not qualify a typical genset (non-inverter type)--If you ever plan to use genset backup with this system (in one of their glossy fliers). phase LV2424.pdf genset requirement.pdf (not sure applies to this model of inverter system)

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Super Moderators, Administrators Posts: 32,597 admin
    One other observation--Typical Boat batteries are either standard "car batteries" or "marine batteries"--And will not last long if deep cycled.

    There are a few Grid Tied AC inverters that have (what one brand calls "secure power")--The ability to supply 120 VAC @ up to 15 amps (for one brand) when AC mains fail. And when you have utility power, you can add it to other GT inverters for normal operation (the Secure Power option may need manual intervention for you to "turn on" secure power--Possibly use a simple transfer relay to switch protected loads to S.P. connection):
    4.8.1 Enable secure power supply operation If an outlet and a switch for secure power supply operation are connected to the inverter, you can supply a load with current from the PV system in case of a grid failure during the day. If you enable the secure power supply operation, the inverter supplies loads that are connected to the outlet for secure power supply operation. In case of overload, underload or insufficient solar irradiation, the voltage supply of the outlet is briefly interrupted. The inverter automatically attempts to reestablish the voltage supply 20 seconds after the interruption. This can lead to inadvertent starting of the load that is connected to the outlet. Ensure that the load connected to the outlet does not consume too much power. If necessary, reduce the power consumption of the load.
    There are other brands out there that may offer such off grid 120 VAC function--But this is out of my knowledge range... Calling our host (Northern Arizona Wind and Sun) -- Their engineers may have more options available.

    And there are MUST brand Off Grid (hybrid) AC inverters--Some which are designed to run without a battery bank at all (I think--A couple years ago I tripped onto one of their zero battery option products--But I do not have time to look through their products to find which units support the zero battery bank option--A quick look at a few of their productions... I did not find it)...

    I know nothing about their product line--But they offer an online chat where you can ask questions about which model(s) may offer off grid AC power without using a battery bank--They have quite a large offering and finding that zero battery option product is not obvious.

    Using a designed for purpose inverter/system (like the SMA with Secure Power) seems to be a good option. Gives you GT Solar when all is well (no "wasted" solar panels/energy when grid is up) and is designed for what you want (AC power, no battery). You could always use an external transfer relay and a qualifier (like sun sensor, or simply a timer that waits 15 minutes to "retry" attaching loads)--Or if SMA or other brand(s) include some sort of power qualification programming--To prevent crash/restart AC power cycles.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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