Too many solar watts for battery bank?

rrroae
rrroae Solar Expert Posts: 46 ✭✭
in Solar Beginners Corner #1
We're looking to move our solar from our house to our cabin for power.  Our batteries were shot so we purchased 4 Trojan T-105s.  We have 1440 watts of solar panels and our system is 24 volt(Magnum inverter, MPPT controller).     So our question is do we have to many panels for our 225 amp/battery bank?  Any chance of frying our new batteries

One additional question.  What should we set the charge rate percentage to?
· Share on Twitter

Comments

  • littleharbor2
    littleharbor2 Solar Expert Posts: 2,113 ✭✭✭✭✭
    #2
    Part time use, 5 - 10%  Full time 10 - 13%. 4 Trojan t-105 @24 volts,  11 amps to 25 amps.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric,  460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.

    · Share on Twitter
  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    #3
    The T105s are likely around 225ah, so a limit of around 40a max charging seems about right to me. At 28v charging, the array would put out ~50a under standard test conditions, likely closer to 40a real world, so I can't see it frying your new batteries.

    Within reason, having more panel capacity is good, as you can still get some charge on lightly overcast days. The batteries generally won't take too much current to fry them anyway, but using a remote battery temp sensor would be a good idea, so as the bank warms, the controller will dial back the charging voltage to compensate. Just setting the current limit really conservatively can be a problem on days where you need the bank to take all the cuuernt it can (like a generally gloomy day with sunny breaks).
    Off-grid.  
    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
    · Share on Twitter
  • Photowhit
    Photowhit Solar Expert Posts: 6,006 ✭✭✭✭✭
    edited July 2017 #4
    Yeah, I think Littleharbour2's math is a bit off. In general you won't have a problem. You will normally come up slowly in the morning as you have better and better exposure tot he sun and often be in Absorb with the charge controller limiting the voltage and the battery accepting less and less current. You should be fine. Particularly if this is a household with some minor loads going...

    Also remember that at the height of charging at 29 volts your available amps from the array will be less...


    The math would look something like this;

    1440 watt array, producing @ 75% (NOCT vale) = 1080 watts

    1080 watts ÷ 29 = 37 amps potential, less any running loads and loss in charge controller (1-4%) and inverter use (10-25 watts)

    37amps ÷ 225 = @16.5% before any additional loads and losses.
    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.
    · Share on Twitter
  • littleharbor2
    littleharbor2 Solar Expert Posts: 2,113 ✭✭✭✭✭
    #5
    Photowhit said:
    Yeah, I think Littleharbour2's math is a bit off. In general you won't have a problem. You will normally come up slowly in the morning as you have better and better exposure tot he sun and often be in Absorb with the charge controller limiting the voltage and the battery accepting less and less current. You should be fine. Particularly if this is a household with some minor loads going...

    Also remember that at the height of charging at 29 volts your available amps from the array will be less...


    The math would look something like this;

    1440 watt array, producing @ 75% (NOCT vale) = 1080 watts

    1080 watts ÷ 29 = 37 amps potential, less any running loads and loss in charge controller (1-4%) and inverter use (10-25 watts)

    37amps ÷ 225 = @16.5% before any additional loads and losses.

    True, my numbers are a bit conservative. 225 amps / .13 is a hair over 29 amps.  While we haven't heard about the actual usage of this system other than it is going into a cabin, I merely gave them a safe charging rate. I made no comment as to whether their array was oversized.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric,  460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.

    · Share on Twitter
  • jonr
    jonr Solar Expert Posts: 1,386 ✭✭✭✭
    #6
    Trojan says "a battery with a 20-hour capacity rating of 225 AH will use a charger rated between approximately 23 and 30 amps".

    But note that you can have extra panels and limit the charge output to 13% of the C20 AHs.

    I am available for custom hardware/firmware development

    · Share on Twitter

Categories