Off grid Thin Film panel tracks battery voltage

Hi Folks

I'm new to this solar world but I'm doing a bit of a project ona Thin film solar panel for off grid functionality.
I have a 130 Watt Mitsubishi panel connected to a flexcharge NC25A 24 v charge controller charging 2 12 v Trojan 24TMX 85ah batteries connected in seris for a 24 v configuration.
I have a chart recorder tracking the voltage of the battery and the panel.
The open circuit voltage of the panel is about 120 v max.
The problem seems to be that when I connect the panel , via a blocking diode and fuse to the battery , the panel voltage seems to track the battery both day and night.
I expected the panel voltage to drop off to close to 0 volts at night and the battery to drop slowly from max voltage. But even at night the panel voltage and battery voltage are tracking each other. My batteries don't seem to be charging fully. I think I have the circuit wired correctly. Does anybody think I have over looked something here - incorrect charge controller possibly.

Comments

  • WindsunWindsun Solar Expert Posts: 1,164 ✭✭
    Re: Off grid Thin Film panel tracks battery voltage

    Where are you actually measuring the panel voltage at?

    Unless that is an MPPT controller, if you have a 130 watt panel putting out 120 volts, you will only get around 1 amp or so into the battery.
  • System2System2 Posts: 6,290 admin
    Re: Off grid Thin Film panel tracks battery voltage

    I'm measuring the panel voltage at the input to the charge controller. I measured the current and I was getting about 1.3 Amps.
    I can't measure before the blocking diode as its inside the junction box.
    So you saying the charge controller is not good enough - it was only 50 dollors.
    I should most like buy a bettter charge controller.
  • Solar GuppySolar Guppy Solar Expert Posts: 1,962 ✭✭✭
    Re: Off grid Thin Film panel tracks battery voltage

    with a VOC of 120V, you probably blew up the 50 dollar controller, the cheap ones are usually maximum input of 20V
  • BB.BB. Super Moderators, Administrators Posts: 29,981 admin
    Re: Off grid Thin Film panel tracks battery voltage

    What is the specifications of your 130 watt solar panel (Voc, Vmp, Isc, Imp, etc.)...

    There are two major classes of charge controllers--one is the PWM type where there is a electronic switch inside the controller that simply turns and off the connection from the panel to the battery. If you have 1.1 Amps in from the solar panel, you will have 1.1 Amps out to the battery bank--regardless of the panel's Vmp ratings. With this type controller, when the switch is 100% on, the battery voltage and the panel voltage will be almost exactly the same. At night, the panel voltage may be anywhere from zero to the battery bank voltage--depending on the design of the controller and if there is a blocking diode in the solar panel or not.

    The other type is a MPPT solar charge controller... This type is capable of taking 120 volts at 1.1 amps and down converting it to 12 volts at 11 amps (less conversion losses of 5-10% or so).

    All charge controllers have minimum and maximum Volt, Amp, and Power ratings. If this solar panel is as you describe (120 VDC)--it is possible that you have blown the 24 volt charge controller (most do not like voltages this high).

    Vmp of the panel should be roughly equal to Vbatt+2 volts or so for efficient use with a PWM type charge controller.

    Where Vmp>>Vbatt--then typically a MPPT type controller should be used to efficiently down converter the high voltage/low current of the panel into low voltage/high current at the battery.

    Here are a couple charge controller FAQ's that may help:

    All About Charge Controllers
    Read this page about power tracking controllers

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Off grid Thin Film panel tracks battery voltage

    that pv is a 12v pv rated at 17.4v for its vmp and an imp of 7.47a. you're doing something wrong either in wiring it or measuring it.
    https://www.mitsubishielectricsolar.com/products/specifications/
  • System2System2 Posts: 6,290 admin
    Re: Off grid Thin Film panel tracks battery voltage

    HI Bill
    The Solar Panel is mitsubishi tandum PV module MT130
    The spec is as follows - according to the spec in front of me
    [B]Max Power 130 watts
    max power voltage - 101 V
    max

    max powe Current - 1.29A
    Open circuit Voltage 131 V
    short circuit current 1.53 A[/B]

    the charge controller spec is
    Max charge input voltage = 140 V dc ( so should be ok for my panel)

    Thanks for the link. I will need to find a MPPT controller with an Input voltage capability of greater than 131V
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Off grid Thin Film panel tracks battery voltage

    sorry as i stand corrected on the pv.
    http://www.greenworks-energy.co.uk/brochure/Solar/Kingspan-solamax.pdf

    i concur the original controller probably blew out by this high voltage adding that an mppt controller with a high voltage input rating that can down-convert is the best option to fully utilize the pv.
  • BB.BB. Super Moderators, Administrators Posts: 29,981 admin
    Re: Off grid Thin Film panel tracks battery voltage

    Archie,

    I guess you are somewhere around Dublin Ireland?

    Anyway--I like Xantrex as they have a couple "string calculators" for their off-grid solar charge controllers and their Grid Tied inverters.

    Using their off-grid charge controller site, you can select your solar panels and plug in your typical min/max temperatures to see what sting configurations/voltages are supported by the XW 60 MPPT controller.

    Your Vmp min is around 82 volts and Voc max is ~146 volts (14F - 95F)--your min/max temp range may not be quite this wide.

    Many of the older/smaller solar MPPT charge controllers do not have a high voltage of 145 vdc (operational) or 150 vdc (max-non-operational)...

    The Xantrex XW is probably one of the best MPPT controllers out there at this time... Morning Star makes a nice small MPPT controller -- but its Voc is not near this high.

    You are kind of in a tough situation right now... The larger MPPT controllers have their best efficiencies at 400 watts and larger. The Morning Star is better in the range of 100-400 watts (200 watts at 12 volts, 400 watts at 24 volts maximum power).

    The solar panel you have is a very high voltage unit that is probably intended for larger Grid Tied installations (whose operational voltage range is typically around 200-600 VDC) and not really a good choice for a "small" off-grid system.

    The NC25A is not a MPPT type charge controller and will not "efficiently" charge a battery where the Vmp of the panel is so much higher than the bank voltage... For example, using your panels on a 24 volt battery bank. Assuming 28 volt charging voltage and:
    Pmax @ STC 130 W
    Pmax @ PTC 123 W
    Vmp at Pmax 101 V
    Imp at Pmax 1.29 A
    Voc @ STC 131 V
    Voltage change -419 mV/C

    So, if you are using a MPPT controller (assuming 100% conversion efficiency):
    • P=Vmp*Imp= 101v * 1.29a =130 watts ( MPPT to Battery power)
    But, using a PWM or On/Off controller (like the NC25A):
    • P=Vmp*Imp= 28v * 1.29a = 36 watts (PWM to Battery power)
    At this point, even if everything is working 100% correctly, the NC25A setup will only charge 36 watts into your battery bank and toss the other 94 watts away.

    Your current battery bank is a 84 amp*hour 24 volt bank... Typically, we recommend a rule of thumb of 5%-13% of the bank rating for solar panel sizing (assuming 30 VDC maximum charging voltage). Approximate Min/Max solar panel sizes:
    • Power=84 AH * 30 volts * 0.05 * 1/0.95 charger eff = 133 watts (minmum solar panel)
    • Power=84 AH * 30 volts * 0.13 * 1/0.95 charger eff = 345 watts (maximum solar panel)
    Choose to get rid of the 130 watt panel and get some 24 vdc rated panels. Or get a real MPPT controller (probably in the XW 60 class) and buy 1-2 more 130 watt panels and add them in parallel to the original (note, you will need a combiner box and/or series protection fuses/breakers to reduce fire hazards).

    Anyway--I hope this explains to you why your current setup is not really capable of charging your battery bank as configured (you are charging with ~1% charge rate instead of the 5% you thought you were).

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