Is a 40a mppt controller going to send too much current to a 230ah battery?

SupraLanceSupraLance Registered Users Posts: 21 ✭✭
edited February 2018 in Solar Beginners Corner #1
This is a newbie question, and I suspect I know the answer but would like confirmation...
I've read that a battery should be charged at between 5-13% of it's ah capacity, so a 230ah battery should be charged with 12-30a with about 23a being optimum to keep the acid stirred and avoid sulfation.
I know a 40a mppt controller will charge the battery in stages, and control the voltage to the battery throughout these stages.  But what about the amps?  If the controller is actually gathering 520w from the pv modules and sending 13.8v to the battery with no other loads, will it be sending 520w/13.8v=38a to the battery?  Will this hurt my battery, since it is charging at faster than 13% of it's ah capacity?  Or is it just that there are 38a available to the battery but it will only pull what it needs (I think that is actually the answer, but then why the 13% listed as a max in the recommended 5-13%)? 


  • mcgivormcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    edited February 2018 #2
    Bare in mind that the 5 &13% values are recommended minimum values, 5% being for occasional use, weekend use whereas 13% is for full time off grid, a lead acid  battery can be charged at higher values within reason, 20% is not unreasonable which would be ~46A, so below the output capabilities of the PV outlined in the example above. Check with the battery manufacturers recommendations if available. Although the current will be high initially, it will drop as the battery approaches full charge in the absorption stage. Some controllers allow a current limit to be programmed, which will prevent overcharging when the PV has the capacity to exceed maximum recommended values, usually in the higher end units, however if the PV can't exceed this it would be intrinsically self limiting. 
    The controller will pass all available current within its design specifications, 40A in your case, if the 520w stated is the rated output of the panels combined STC values, the most you would actually get is around 420W or 80%, unless extremely cold, and or at very high altitude.
    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.
  • EstragonEstragon Registered Users Posts: 4,495 ✭✭✭✭✭
    The 13% high end of the recommended range is a cost-effective limit. Going higher, within reason, can be useful and is now more affordable with the lower cost of pv in recent years.

    The main advantage to going higher is you're more likely to get to fully charged on lightly overcast days, or in climates with regular afternoon cloud/squalls, for example.

    As you suggest, batteries will only accept charge at their own rate, and pv will only produce what the loads pull.
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • jonrjonr Solar Expert Posts: 1,386 ✭✭✭✭
    edited February 2018 #4
    Max charge rate varies significantly with the type of battery and the state of charge.   You cannot count on a battery limiting itself - but as mcgivor says, the controller (or the max panel output) can.

    I am available for custom hardware/firmware development

  • mike95490mike95490 Solar Expert Posts: 9,465 ✭✭✭✭✭
    The 38A you speak of, would be the rare occasion when the battery is really low, it's cloudy all morning and the the sky clears right at noontime. Boom - 38A.   A low battery can safely absorb a lot of amps.  But as the battery charges up (and you see this with any automotive battery charger with a meter) the charge amps taper off. Nothing to do with the charger, its fixed voltage, but as the battery comes up in voltage to the charger voltage, the current tapers off. No harm done.  On an average day, sun comes up, battery starts to charge with a gradually increasing solar harvest, and by noon, you should be will within safe limits. 

    Here's an example of my generator charging batteries, steady amps till a certain voltage is reached, then amps start to taper off (and I shut down the genset to not waste fuel. And maybe the sun comes out later and tops off the battery with free solar)

    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: ,

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