Impulse bought two 100w 24V panels... Now what?

Robw_zRobw_z Registered Users Posts: 6
Hello,

First post. I have a Volkswagen Vanagon in which I would like to run an electric compressor fridge rated at 40watts consumption an hour, 12-24volts, 60 watt nominal input. I have an Optima yellow top battery in addition to my starting battery.

Two AM100 100watt panels came up on Craigslist, $200 for the pair. Apparently the AM100's are made by BP solar and are 3-4 years old. Couldn't pass it up, thought I got a great deal until I realized they were 24V panels(rated 21-27 volts), and this apparently means I need an expensive charge controller with MPPT, and even the cheaper MPPT controllers I've read are suspect(all my solar knowledge comes from a single day of research).

So, I'm asking if I should start from scratch, sell these and get a 12V panel, or if there is a charge controller out there with MPPT that can make this a reasonable project cost wise and make me feel like these panels were a good deal. All I want is a single 100W panel charging a single battery(I plan on selling 1 of the panels at least). Thanks so much for any input and for patience in answering for what all I know is a very common question.

-Rob

Comments

  • icarusicarus Solar Expert Posts: 5,433 ✭✭✭✭
    Re: Impulse bought two 100w 24V panels... Now what?

    The small Morningstar MPPT controller would Be a good choice.

    That said, you are right, many cheap MPPT controllers are junk, (see also BZ controllers!)
    You might be better off selling what you hve, and find a native 12 vdc panel.

    Welcome to the forum, good luck and keep in touch,

    Tony

    PS. Power is measured in "watt/hour" or Amp/hours, not watts per hour. It may seem like a semantic diffence, but in doing calcs, one has to measure the watts (or amps) used, and the duration on those loads to determine the energy used,,ie watt/hours.

    T
  • Volvo FarmerVolvo Farmer Solar Expert Posts: 209 ✭✭✭✭✭
    Re: Impulse bought two 100w 24V panels... Now what?

    That panel is not really a 24V panel either. a true 72 cell 24 volt panel would have a VMP in the 34-36 volt range. Yours have a Vmp of 21.5V.

    I think you could use those panels to charge a 12V battery through a regular old PWM controller, you just might not see the full 100 watts out of them because your battery voltage is quite a bit lower than the Vmp of the panels.
  • WindsunWindsun Solar Expert Posts: 1,164 ✭✭
    Re: Impulse bought two 100w 24V panels... Now what?

    The AMI panels are oddballs - they are neither 12 nor 24 volt, more like 16 volts. They can be used fine with an MPPT controller - in fact that is what they were designed for. I don't recall the entire history of them, but I think they were originally made mainly for RV use.
  • Robw_zRobw_z Registered Users Posts: 6
    Re: Impulse bought two 100w 24V panels... Now what?

    Thank you for the replies. So how many effective watts will a 16V 100W panel put out through a non MPPT CC? On the back my panel says Vpm: 21.5V, Voc: 27V, Ipm: 4.5A, Isc: 4.8A. I am putting that down just to make sure we're all on the same page.

    Through further reading it seems these are referred to as "18V panels" even though that isn't 100% exactly their voltage. Why were these made and what are their advantages, and are they only realized through MPPT?

    I think I could probably get away with 75 Watts to run the 12V fridge, though it is disappointing to run a panel with low efficiency. What specs should I look for in a non MPPT CC so that I don't blow it? Thanks again,

    -Rob
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Impulse bought two 100w 24V panels... Now what?

    rob,
    if using a pwm cc the current max will be the imp of the pvs and it will be rare occasions you'd ever see that. to run a refrig 24hrs a day will probably not be possible with limited charging power and you need the batteries to hold enough capacity as well to last all day without depleting the capacity farther than 50%. going by 60w/hr over 24hrs this is 1440wh (watt hours). if you reap 75% of the pvs output you'd have 150w and if you are lucky you may see 5hrs like this so 5 x 150w = 750wh or roughly half of what you'd need to run it for a day given 5hrs of full sun.

    look for the max input voltage specification of the pv (vpm 21.5v) and add another 15% for cold days even if you don't get too cold just to buffer. now if the controller can take that as it will specify that in its specs then you are good to go with that requirement aspect. also be sure the cc will pass all of the current both pvs will possibly generate. 4.8a isc x 2 = 9.6a. i'd advise against a 7a cc, but a 10a will work.

    edit to add- a dollar per watt for used pvs is not a great deal to me as the warranty no longer applies, you don't know if the pvs are outputting their stated wattage or for how long they will, and there's brand new pvs out there pretty close to that area of cost.
  • Volvo FarmerVolvo Farmer Solar Expert Posts: 209 ✭✭✭✭✭
    Re: Impulse bought two 100w 24V panels... Now what?

    I think if you had a low 12 volt battery, you could hook the solar panels up directly and measure how much current you get out of them before investing in a CC. I would guess somewhere south of four amps, but I can not find the volt/amp curve for your particular panel.
  • WindsunWindsun Solar Expert Posts: 1,164 ✭✭
    Re: Impulse bought two 100w 24V panels... Now what?

    What you will actually get in watts is basically Imp x battery voltage, so at 4.5 amps x 12 volts = 54 watts
  • PhotowhitPhotowhit Solar Expert Posts: 5,650 ✭✭✭✭✭
    Re: Impulse bought two 100w 24V panels... Now what?
    Windsun wrote: »
    What you will actually get in watts is basically Imp x battery voltage, so at 4.5 amps x 12 volts = 54 watts

    This isn't as bad as it looks , if your unfamiliar with solar. There will be losses in any system. A true 12 volt nominal panel will have a VMP of 17-18 volts, so a VMP of 21V is only 20% greater. 12 volt batteries actually charge from 12 -14.5 volts.

    I'd say use what you got...

    Now about your fridge. 40watts is a very small fridge, is this a compressor thermostat type or a thermoelectric? I think the thermoelectric run continuously so might not work at all with out a huge battery. I have a small compressor fridge and I think it draws over 80watts, but cycles on and off.
    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.
  • Robw_zRobw_z Registered Users Posts: 6
    Re: Impulse bought two 100w 24V panels... Now what?

    You guys are great thanks.

    The fridge is dorm fridge sized(65 liters) and has a highly efficient danfoss compressor. It is called a Truckfridge: http://www.truckfridge.com/tf65.html

    It runs 50% of the time at worst. Some of the VW guys claim to run them 24/7 on 80W 12V panels.

    I found a used HPV-22b CC for $150: http://www.heliotrope-pv.com/hpv22b.html

    Good deal?

    Basically Looks like I'm either gonna go high quality PWM, or used/low quality MPPT.
  • WindsunWindsun Solar Expert Posts: 1,164 ✭✭
    Re: Impulse bought two 100w 24V panels... Now what?
    Robw_z wrote: »

    I found a used HPV-22b CC for $150: http://www.heliotrope-pv.com/hpv22b.html

    Good deal?

    Basically Looks like I'm either gonna go high quality PWM, or used/low quality MPPT.
    For not that much more, the Morningstar MPPT 15 is a far better controller http://www.solar-electric.com/mosumpsochco.html

    The main reason that Heliotrope pretty much went out of business is because they were not very good MPPT controllers, and though they called them MPPT, it was a stretch. We tested the HPV-22 a few years back, and they are an analog unit, and track the max power point very poorly, if at all under most conditions. What we noticed happening quite often was that if a cloud or other shadow passed over for a short time, it would get "stuck" on the low end of the power curve when the sun came back out.
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