Solar array charge time

Discussion in 'The Projects Forum' started by ironmike828, Jun 1, 2010.

  1. ironmike828

    Thread Starter Member

    Jan 29, 2010
    13
    0
    Hi,

    I am involved in a project that involves a BP 40 watt 12 volt solar panel. There are 2 panels making it an 80 watt 24 Volt system. The charge controller being used is a morning star SS-MPPT-15L. My question is how do I find out the time it takes to charge the batteries over the range of depletion. Any help would be appreciated I'm sure I didnt include all the information, but if anyone can point me to where to start I would be greatful.

    Thanks,

    Mike Staveskie
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Well, having knowledge of the AH rating of the batteries in the battery bank, and the number of batteries would be a helpful (and necessary) addition to the information provided thus far.

    The manual for your charge controller is here: http://www.morningstarcorp.com/en/support/library/SSMPPT.IOM.EN.011.pdf

    Looks like your solar panel array is going to be a major limiting factor. It will cause some inefficiency in the MPPT controller. Adding another pair of solar panels will help that quite a bit, but it looks like you'll be about 93% efficient from the controller standpoint.

    Charging lead-acid cells is about 70% to 80% efficient; use 75% for a nominal figure.

    With 80W in, you'll have to subtract the efficiency of the MPPT and the charge efficiency of the batteries to get an idea of the effective charge going out.

    Hope you're using deep-cycle type batteries. Automotive batteries will fall apart quickly if deep-cycled.

    Some phases of charging aren't very efficient, such as the maintenance "equalization charge" period. However, it's necessary to remove accumulated plate sulphation and to stir the electrolyte.

    You need to size the battery bank so that (most preferably) it is not discharged below 30% in order to have a good service life. Batteries that are cycled to 50% of their rated charge will have their service life cut by 2/3, which will increase your operating costs considerably.

    [eta]
    Just realized that I never really answered your question.
    You'll have roughly 70% system efficiency with the charge controller and the battery charging.
    You will also have to subtract out the power loss in your cabling from the solar cells to the MPPT, and from the MPPT to the battery bank. As you have not indicated what the distance is between these items nor the AWG size of the cable used, it is another unknown.

    If your cabling is perfect (we can dream, right?), you'll have about 55.8 Watts charging power available at the battery bank. That's roughly 2 Amperes output current for two 12v lead-acid batteries in series, while you have prime-time daylight.

    You could run quite a few 20mA LEDs with that. You definitely couldn't power a refrigerator though.
     
    Last edited: Jun 1, 2010
  3. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,172
    397
    If you have 2 each 40W 12V panels. two in parallel = 80W @ 12V or two in series = 40W @ 24V.
     
  4. Bychon

    Member

    Mar 12, 2010
    469
    41
    Pardon me, but I think your calculator slipped a digit, Bernard.
     
  5. ironmike828

    Thread Starter Member

    Jan 29, 2010
    13
    0
    thanks for the reply that put me in the right direction, I wrote this for a friend so I didnt have to much info. The 2 panels are wired in series so its a 40W 24V system. We have 2 12 volt batteries in series, with 2 strings in parallel. The setup we have only allows room for two panels because this is on top of a trailor.
    The wiring isn't more than a 4 foot distance, the size of the wire is about 12 guage so there isn't going to be a lot of power loss there.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Yes, Bernard slipped a digit.
    If you put two 40W panels together (whether series or parallel), you have 80W input available - from maybe about 10AM to perhaps 3PM. I can't give you anything approaching exact numbers on the solar panels.

    The solar panels will be more efficient if they can be kept reasonably cool. If they get hot, they will loose a good bit of efficiency, which isn't very good to begin with.

    Since you have 4 batteries in a 24v array, your effective charge current will be about 1A per string (about 1.33A actual, less the 75% efficiency of lead acid batteries). It will be about enough power to keep charged batteries charged.
     
  7. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,172
    397
    I think I should retire again, loosing too many marbles & digits.
     
  8. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,159
    As long as it's calculator digits I wouldn't worry.
     
  9. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    I was thinking the same thing... If a carpenter said he was losing digits, I would say it was time to retire too. ;)
     
  10. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,172
    397
    Calculator checks out fine, just dropped it when I slipped on some loose marbles.
     
Loading...