Mppt switches to float charge too soon

CraigmarcCraigmarc Registered Users Posts: 17 ✭✭
I have an mppt charge controller that switches to the float charge voltage while the batteries are only 35-60 percent charged. Shouldn't it only be switching to the float charge voltage when the batteries are more fully charged? This greatly slows down the charging rate. Is there an adjustment on the controller that can be made?


  • BB.BB. Super Moderators, Administrators Posts: 32,748 admin
    Brand/model of MPPT charge controller? MorningStar tristar mppt 45

    And--I guess--You are focusing what the controller reports as State of Charge--I had asked you to measure the charging voltage at the battery terminals and how long (i.e., 14.4 volts for 4 hours, or what)?

    More background information in this earlier discussion of yours?

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CraigmarcCraigmarc Registered Users Posts: 17 ✭✭
    It is an mppt 45. The charging voltage is 13.7. I'm using the indicator lights to to determine the state of charge when the batteries aren't being charged. Is there a way to determine the batteries state of charge with a multimeter to check if the indicator lights are accurate?
  • BB.BB. Super Moderators, Administrators Posts: 32,748 admin
    I am a little confused, from your other thread:
    No the charge controller is set to 14.3. 7.1-7.2 are the batteries specs.

    Your controller should hold 14.3 volts for 2-6 hours (the lower the state of charge, the longer the controller should "hold" the absorb set point). Once the battery is "charged", then the controller should drop back to "float" (assuming the charge is completed before the sun sets).

    13.7 is pretty close to "float" set point. Which is used to "maintain" the present battery state of charge (i.e., make up for self discharge), and not intended to charge the battery to 100% full.

    The two above voltages are only "useful" to talk about when the sun is up and the MPPT controller working correctly. When the sun is set (and/or if you have other methods of charging--That is a different part of the discussion).


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • VicVic Solar Expert Posts: 3,164 ✭✭✭✭
    edited June 2016 #5

    Hi Craig..,

    Are you using a Remote Temperature Sensor (RTS/BTS)  with the Charge Controller (CC)?

    For your Sun Extender AGM batteries,  the very best measure of the SOC,  is to Rest the battery for about 6 or so hours.  Resting is when the battery has had NO charge or NO discharge for some fairly long period of time.  Then you would use a good Multi meter (DMM),  to measure the battery voltage.   THEN,  you would want to Compensate this reading for the actual battery temperature,  at about -- 3 milliVolts per battery cell for every degree C change from the Reference temperature (usually 25 C,  77F).  This compensation value is common for AGM batteries,  check the data for your exact batteries to confirm that the compensation is -- 3 mV/cell/C.   Since a 12 V Lead Acid battery has six cells,  the total compensation would be about -- 18 mV per degree C change from 77 F for the BATTERY's temperature.

    Since you do have two strings of batteries in parallel for your 12 V system,  this Resting would not be too difficult,  just,  inconvenient.  Of course you would also need that DMM.  Some suggest that on 12 and 24 V battery systems,  that a good quality  battery selector switch could be used  --  just switch Both,  to string 1,  Rest string 2,  and measure its voltage after a number of hours,  and so on.

    FWIW,  perhaps more detail than you wanted.   Good Luck,   Vic


    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • CraigmarcCraigmarc Registered Users Posts: 17 ✭✭
    It's set to 14.3 absorption and
    13.7 float, I'm saying that it is dropping to the float charge voltage too soon for some reason.

    I'll try testing the battery tonight after it has rested but I don't have a temp sensor. Also aren't you supposed to test the battery when it has a slight load on it?
  • vtmapsvtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    Craigmarc said:
     Also aren't you supposed to test the battery when it has a slight load on it?
    Depends... what type of test are you talking about?  If you are trying to measure its SOC by measuring voltage, then (as has been explained) there should be no charge and no load for several hours.

    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • t00lst00ls Solar Expert Posts: 239 ✭✭✭
    battery state of charge is done after charging  and has had a full hour or more rest.....each battery needs disconnected from each other to do this

    load testing is also done with each battery disconnected .....but in this case, you measure the voltage as it is being can buy load testers at your local automotive parts store

    the lights on the charge controller.....I would pay any mind to need to know what the real voltage is, get a digital voltmeter

    if you dont have any loads running, then more than likely your batteries will go to float after a few minutes of charge.....if a battery is already charged this will happen....but you dont know what the actual state of charge is or what the voltage reading how in the world could you know it is going to float to soon
  • BB.BB. Super Moderators, Administrators Posts: 32,748 admin
    How soon is too soon? Minutes, 2 hours, ?

    And is the battery voltage the same as the battery terminal voltage on the charge controller (full sun, 14.3 volts on battery)?

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