Looking for help understanding maximum charging current...

AsinnobooksAsinnobooks Registered Users Posts: 3
Hello all!

On the brink of setting up my first solar system as part of my van conversion. 

Looking at:

400W / 24V Panel
2 x 200Ah / 12V Gel Batteries

And am trying to work out what MPPT solar charge controller is required. The batteries say they have a maximum charging current of 37.5A, which I imagine i want to get as close to as possible in order to charge the battery as quickly as possible, but looking at descriptions of charge controllers it seems that they are rated more based on the amperage input (which i think would be 8A in my case - 400W/24V...). I also see recommendations for a 40A CC for 400W panel, but do i run the risk of damaging or destroying the battery as this amperage is great than the batteries' max. charging curren?

I'm a bit lost and hope you may be able to help me get my head round all this.

Many thanks 

Comments

  • BB.BB. Super Moderators, Administrators Posts: 31,684 admin
    Welcome to the forum Asinnobooks,

    GEL batteries are (many brands/models) are not great for solar power systems... They are great for sitting unused and during discharge. However during charging, many (some, many, most?) have issues with high charging current/higher charging voltages).

    GEL batteries have "gelled" type electrolyte (basically sulfuric acid+water+silica gel)--the downside being at high charging currents/voltage the GEL battery can gas which forms gas bubbles in the electrolyte--And this can permanently cause a loss in capacity of the GEL battery.

    AGM batteries (absorbed glass mat) batteries are very similar to GEL batteries, without the problem of forming gas bubbles in the GEL electrolyte.

    Always start with the settings in the Battery Specifications--But the numbers are roughly (12 volt bank, 24 bolt bank is 2x higher):
    • 14.75 volts set point for Flooded Cell batteries
    • 14.40 volts set point for AGM
    • 14.20 volts set point for AGM
    And dropping to around ~13.6 volts or so for "float charge" set point.

    Charging current wise--For "longest" life, around 10% to 13% rate of charge for Lead Acid type batteries is recommended. And if your controller has the option, use a remote temperature sensor to monitor battery bank temperature (higher temperature lead acid battery, lower charging voltage required).. For 13%+ rate of charge, highly recommend a RBTS (remote battery temperature sensor) to ensure a happy and healthy battery (and for RVs that run from deserts to snow country, RBTS are really helpful).

    For GEL batteries--Many recommend a 5% rate of charge (which is really too low for "optimum" off grid use--The sun is just not in the sky that many hours per day for fully recharge a 50% discharged GEL battery).

    The caveat here is that there are many different GEL battery designs/manufactures--Again fall back to the manual and make sure you meet their requirements vs the above generic recommendations.

    For MPPT controllers--The typical "max current" calculation for charging current (the most current you will see for a few hours on a cool/clear day during solar noon, a few times a year):
    • 400 Watt array * 0.77 controller+panel deratings * 1/28.4 volts battery charging voltage = 10.85 Amps usual "max" current for MPPT system
    • 200 AH * 0.05 (5%) suggested rate of charge = 10 amps "generic" max charging current
    So, your 10.85 amps is certainly close enough for "solar work"...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike_smike_s Registered Users Posts: 132 ✭✭
    It's not clear whether you're putting together a 24V system, or a 12V one with an MPPT controller.

    Some controllers (e.g. Morningstar Prostar MPPT) have a setting to limit max charging rate.
  • AsinnobooksAsinnobooks Registered Users Posts: 3
    edited May 1 #4
    Hi Bill and Mike, thank you for your responses! 

    Mike - I'm building a 12V system but want to use a 24V panel as i can afford a much better quality one if I do so.

    Bill - thank you for your explanations. I think I follow for the most part, though I'm a little lost at the end. If I understand well, your advice is firstly to use an AGM rather than a GEL?

    Thereafter, I am slightly confused (sorry). Here's what I've understood:
    -The battery charing voltage (28.4V) comes from two 12V, 200Ah batteries wired in parallel and their volt set points.
    -The 0.77 is the effiiency of the 400W solar array.
    -The 5% rate of charge is the rate at which a GEL battery can safely be charged at (I'm seeing that figure at 20% for AGMs?)

    What i dont understand is the "max current" (10.85A) for the MPPT system as in my (clueless) mind, the calculation seems to use the oanel details and then the battery details. Unless this means that the maximum current that will ever need to go through the MPPT is 10.85A?

    Furthermore, I dont understand the discrepancy between the max charging current calculation (10A) and the specified max charging current on the battery spec (37.5A). 

    Bearing all that in mind, if I go for 400W, 12V panel and 2x 155Ah, 12V AGM Batteries. What MPPT CC would I need? 

    Sorry for my rudimentary ignorance. 
  • mcgivormcgivor Solar Expert Posts: 3,815 ✭✭✭✭✭✭
    @Asinnobooks

    To do a rough calculation take the panel wattage and divide by the nominal voltage, 400W ÷12V = 33.3A, in reality it will likely be around 25A therefore a 40A controller would be a good fit, but pay attention to the final paragraph.

    An MPPT controller will accept a higher voltage than the battery and convert the excess voltage to increase charging current at the lower  voltage the battery requires, the power output of the panel won't be increased by the MPPT controller. In most cases panel output will be less than the Standard Test Conditions  (STC) usually in the 70-80% range. This link explains in detail how MPPT controllers work https://www.solar-electric.com/learning-center/mppt-solar-charge-controllers.html/

    There are batteries which are referred to as GEL in Europe that are deep cycle, the term differs in North America where GEL refers a standby type battery not designed for regular cyclical use, consult the manufacturer for details.

    Two 12V  200Ah batteries in parallel with a maximum charging current of 37.5A each current would be doubled to 75A  or roughly 18% of total Ah capacity, using the 25A value from above the charging rate with a single 400W panel would be 6.3% which is pretty low, however to build a ballanced system one would begin with loads, then the battery capacity needed to support them and finally the PV required to replenish the withdrawals.

    It would appear that you're actually starting midstream, which is a fundemental mistake many make, doing things right the first time will save you the headache and expense of loosing the batteries prematurely due to cronic undercharging. My suggestion is to rewind to the beginning and design something that will work, perhaps with a new post to eliminate confusion.








    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
  • BB.BB. Super Moderators, Administrators Posts: 31,684 admin
    Thereafter, I am slightly confused (sorry). Here's what I've understood:
    -The battery charging voltage (28.4V) comes from two 12V, 200Ah batteries wired in parallel and their volt set points.
    -The 0.77 is the efficiency of the 400W solar array.
    -The 5% rate of charge is the rate at which a GEL battery can safely be charged at (I'm seeing that figure at 20% for AGMs?)

    I had guessed you were doing a 24 volt battery bank (2x batteries in series). In fact, you are doing 2x batteries in parallel. So the correct equation for a 12 volt battery bank would be:
    • 400 Watt array * 0.77 controller+panel deratings * 1/14.2 volts battery charging voltage = 21.7 Amps usual "max" current for MPPT system
    • 400 AH * 0.05 (5%) suggested rate of charge = 20 amps "generic" max charging current (for US GEL batteries)
    Mcgivor does a good job of explaining... 2x batteries in series, the voltage adds (12+12=24volt). 2x batteries in parallel, the AH capacity adds (200+200=400AH).

    Note that the energy stored from two batteries series vs parallel connected store the same amount of energy. Just deliver it at different battery bus voltages:
    • 2 * 12 volt series batteries * 200 AH capacity = 24 volts * 200 AH = 4,800 Watt*Hours (stored energy)
    • 12 volt battery * 2 parallel * 200 AH capacity = 12 volts * 400 AH = 4,800 Watt*Hours (same stored energy)

    The ~77% derating is because "Hot" solar panels have lower Vmp (voltage maximum power). And MPPT controllers are "constant power devices (ignoring losses):
    • Parray = Vmp * Imp (solar array voltages) = Vbatt * Ibatt (battery side of controller)
    Since Vmp falls almost 19% (or ~79% of rated Vmp under "standard test conditions" with "cool" solar panels/cells), that means the harvest from the array also falls almost 19% too (the other major losses are the 5% electrical losses of the MPPT controller).

    We use rules of thumbs here a lot... This is to make the design process much simplier and quicker to a reasonable design without going through each of the details.

    If you wish to better understand the details, we can certainly go into them.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • AsinnobooksAsinnobooks Registered Users Posts: 3
    @mcgivor - thank you for your response. Believe it or not i actually thought i was starting at the beginning here. I have identified that I need about 120-130Ah per day maximum and after looking around online, came to the conclusion that approx. 300Ah of AGM or GEL (I'm in europe) batteries would do the job and that 400W of solar power would be sufficient for charging. (I will also connect the batteries to the van battery's alternator). That's been my thinking hitherto, what would you advise is order to "rewind"? I definitely don't want to make any mistakes in planning this and am keen to do it the right way.

    @BB. - thanks again. Some of those calculations are making more sense now, though I you lost me with this:
    • Parray = Vmp * Imp (solar array voltages) = Vbatt * Ibatt (battery side of controller)
    Since Vmp falls almost 19% (or ~79% of rated Vmp under "standard test conditions" with "cool" solar panels/cells), that means the harvest from the array also falls almost 19% too (the other major losses are the 5% electrical losses of the MPPT controller).

    I guess my questions concern, in essence, what the next steps are and ultimately what i need to purchase if the 400W 24V panel array and 2x 155Ah, 12V AGM batteries (or perhaps GEL if the fact that I am in Europe means that they are actually suitable [they are less expensive!]) work together. 

    I'd definitely like to understand better the details, as you suggested, as I don't want to go into this without a full understanding. I'm just really starting with no technical knowledge and all the acronyms etc are still extremely confusing and overwhelming. Thank you so much for your help!

  • BB.BB. Super Moderators, Administrators Posts: 31,684 admin
    Starting with the "technical questions".... Solar cells are NOT solar batteries.

    We are all pretty familiar with what a battery is... More or less a battery holds 12 volts from zero to maximum current (Amperes). A "constant voltage" source.

    Solar panels, for the most part, are "constant current sources". If you have an Imp = 10 amps solar panel (current maximum power)... Under full sun the panel will (for a first approximation) output 10 amps from zero volts to Vmp (voltage maximum power).

    Say you have a Vmp=35 VDC for your panel (we need to know the details, Vmp and Imp at least for your panel(s)). Then we look at the equation for power:
    • Power = Voltage * Current
    If your load is at 35 volts, then the panel will output (under full noontime sun on a subfreezing day):
    • Power panel = Vmp * Imp = 35 VDC * 10 Amp = 350 Watt "rated panel"
    HOWEVER, if your load is a "dead short" or a 12 volt battery (no MPPT charge controller, just wired to battery or using a PWM type controller), the power equation looks like this:
    • Power from panel = Vmeasured * Imeasured = 12 volt battery * 10 amps = 120 Watts
    • Power from panel = 0 volts (dead short) * 10 amps = 0 Watts from panel
    Panel temperatures can get very hot on a nice sunny summer / windless day... Upwards of 80C... At that point, Vmp (standard temperature rating and sun @ 25C) falls by 80% or so:
    • Panel output hot summer day = Vmp * temp derating * Imp (Imp does not change much with temperature
    • Panel output hot summer day = 35 Vmp(std) * 0.80 temperature derating * 10 amps = 280 Watts from panel to MPPT controller (derated for "normal operation" conditions)
    Think of an MPPT (maximum power point tracking) Solar charge controller as being similar to the automatic transmission on a car... It takes the V and I of the solar panel and "matches" the conditions of the load V and I of battery bank (like matching the engine Torque and RPM to wheel RPM and Torque required).

    So... when in "bulk charge" mode (the MPPT controller is outputting maximum power to a discharged battery), the Power into the MPPT controller is about the same as the power out... Say 280 Watts is available from my imaginary solar panel, then charging a 12 volt battery bank:
    • Power = Voltage * Current
    • Current = Power / Voltage
    • Current into battery = Psolar / Vbatt
    • Ibatt = 280 Watts (hot panel) / 12 volts charging "very discharged" battery = 23.3 amps into battery (less than 50% charged)
    • Ibatt = 280 Watts (hot panel) / 14.2 volts charging "80% full" battery = 19.73 amps into 80% charged battery
    Above numbers are just rough examples--Exact numbers will vary.

    Here is a thread that discusses about understanding solar panels:

    https://forum.solar-electric.com/discussion/5458/two-strings-in-parallel-with-unequal-string-voltages

    And, an FAQ that has lots of links to various solar/battery/conservation discussions. See if there is anything of interest:

    https://forum.solar-electric.com/discussion/4426/working-thread-for-solar-beginner-post-faq/p1

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Super Moderators, Administrators Posts: 31,684 admin
    Document your system design:
    • 2x 12 volt @ 200 AH batteries for a 12 volt @ 400 AH battery bank
    • 400 Watt @ "24 volt" solar panel (that could be Vmp from 24 volts to 36 volts or so
    • 130 AH @ 12 volt day "max loads"
    • 40 Amps (*) MPPT charge controller
    • Location--Perhaps somewhere around Lyon France(?)
    We very much like to design the system for your loads and location. Normally design for 1-3 days of storage (bad weather) and planed discharge to 50% for longer battery life (for lead acid batteries).

    1 day discharge and 50% max discharge or 2x daily load storage can work OK for weekend/sunny weather use... For full time off grid, suggest 2 day storage + 50% discharge or 4x daily load (lead acid batteries take several days to fully recharge if discharged deeply).
    • 400ah/130ah= 3.1 x storage capacity
    Would like to see more battery capacity if this was a full time off grid system used year round... If 3 season weekend/trailer/small weekend cabin... Can work OK.

    Next, charging... Normally suggest a minimum of 5% rate of charge for weekend/sunny weather system. 10-13% minimum for full time off grid daily use/year round system:
    • 400 AH * 14.2 volts charging * 1/0.77 solar panel+controller deratings * 0.05 rate of charge = 369 Watt minimum array
    • 400 AH * 14.2 volts charging * 1/0.77 solar panel+controller deratings * 0.10 rate of charge = 738 Watt array nominal
    • 400 AH * 14.2 volts charging * 1/0.77 solar panel+controller deratings * 0.13 rate of charge = 959 Watt array "typical" cost effective maximum
    And sizing the array based on your daily loads, where you live (amount of sun) and seasonal usage (and/or genset winter charging):

    For an array facing south in Lyon:
    http://www.solarelectricityhandbook.com/solar-irradiance.html

    Lyon
    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 44° angle:
    (For best year-round performance)

    JanFebMarAprMayJun
    2.18
     
    3.01
     
    4.06
     
    4.39
     
    4.74
     
    5.13
     
    JulAugSepOctNovDec
    5.52
     
    5.23
     
    4.45
     
    3.07
     
    2.20
     
    1.74
     
    If you "toss" the bottom three months (winter) and only use the system 9 months a year:
    • 130 AH * 12 volts = 1,560 Watt*Hours per day
    • 1,560 WH per day * 1/0.61 DC off grid system eff * 1/3.01 February "break even" = 850 Watt array for February "break even" operation
    Anyway--Some basic information/math... Seasonal usage, "base loads" that must run all the time (refrigerator, LED lights) vs optional loads (irrigation pump, TV, etc.) vs genset usage (more panel, less fuel needed--especially in winter)...

    Your thoughts?

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