Solar Project - Phase II & Batteries Help

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ki7fx
ki7fx Registered Users Posts: 10 ✭
in Solar Beginners Corner #1
Greetings again all...
I have completed the first part of my solar hobby project and have learned a lot in the process. Specifically, there is no way in the world that I am saving any money by going solar :-) I have a couple of thousand into a project that will save me about 3 bucks a month on my energy bill... It is, however a _lot_ of fun and very educational, so it's worth it!

I am running two 250w panels (series) in the back yard, securely mounted to a homemade frame, with 120 ft of 6g cable running into the Morningstar MPPT 60 controller. On a sunny day, the controller is reporting ~70v @ 16 amps when cranking full power into charging batteries. So far, so good...

Inside, 12v DC loads only for a security system/cameras (IR illuminators at night), internet switches, routers, radio, etc. I am currently using 5 (daytime) to 6 (nighttime) amps of current 24 hours a day (5.5 x 24 = 134 Ahrs). This should be my baseline load calculation.

Battery bank is two 12v 75Ah AGM batteries in parallel (150Ah @ 12v). Batteries run down to about 11.90 volts at night (early AM) until the sun comes up. On a sunny day, the controller charges the batteries to 100% in about 5 to 6 hours and then goes into "float". Winter is coming, so this will get much worse. I have only run this configuration for a week of sunny weather to get an idea of the performance.

BUT: if it's cloudy in the morning, I have to run into the basement and turn on the charger. I know the batteries are the weak link here. Keeping with a 12v system, can someone tell me how to configure a BETTER battery bank with more reserve?
If I double the battery capacity, it will discharge less, but take twice as long to recharge the larger size. This is where I'm lost. Will it work to increase my capacity to around 300 or 400 AH battery without adding more PV panels?

I have been keeping the charger on the batteries 24x7 the last few cloudy days because I don't want to cause any premature battery problems. However, the sun is shining today, and the charger is only consuming 7 watts of power because the Solar controller is taking care of business. Should I just leave this configuration? These batteries are about a year old, and have been on the charger/power supply constantly during this time as a DC UPS. Would this indicate that they are good enough to just add two more exact match batteries to the bank, or start over with four (or more) new batteries?

As always, I really appreicate the help from you guys.. This forum is a great resource, but the math does get overwhelming at times... :-)

Mike

Thanks, Mike
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
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    Re: Solar Project - Phase II & Batteries Help

    You've got the answer in your own post: your loads amount to 134 Amp hours @ 12 Volts. Your batteries amount to 150 Amp hours @ 12 Volts. That means your loads will discharge your batteries 90%. Your batteries are toast.

    If you want to keep the batteries functioning you have to limit the depth of discharge to 50% for standard deep cycle batteries. 25% is even better. This means your minimum battery bank size needs to be 134 * 2 or 268 Amp hours.

    Then there's the recharging issue:
    500 Watts of panel in five hours will produce about 1925 Watt hours. Your loads amount to 1608 Watt hours. Pretty dicey; cloudy weather being exactly what will imbalance that in a hurry.
    That much panel should be able to support 320 Amp hours of 12 Volt battery. If you want to limit the DOD to 25% you'd need 536 Amp hours of battery and 842 Watts of panel, roughly speaking.

    At the very least I'd up the battery capacity to a couple of 220 Amp hour 6 Volt golf cart units in series. It won't eliminate the need to recharge from mains during cloudy weather, but it will reduce it. If you could afford to invest in some L16 size 320 Amp hour 6 Volts you'd maximize the use of the panels you have now, reduce the cloudy weather charging problems, and have your daily Watt hour needs covered.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
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    Re: Solar Project - Phase II & Batteries Help

    "I am running two 250w panels (series)............ the controller is reporting ~70v @ 16 amps when cranking full power into charging batteries."

    this does not add up properly. if the pvs are at a vmp point of 70v, it is impossible to have 16a flowing from the pv wattage you indicate. 16a x 70v = 1120w. this will never happen from 2 250w pvs.
    if the 16a is on the output of the cc and at 12v this is doable being around 200w, but i would consider this to be on the low side for 2 250w pvs. now maybe your batteries are at a high state of charge too that has caused a foldback on the power being delivered.

    i also agree with coot that you are taking the batteries down too low as they really should not be drawn down past 50%. it would seem you need both more battery capacity for your loads and more in pv to keep your batteries charged with.
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  • ki7fx
    ki7fx Registered Users Posts: 10 ✭
    #4
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    Re: Solar Project - Phase II & Batteries Help

    correct... I was typing too fast.. that was the current on the output of the cc. I have only run this config for a few days and was trying to remember what I saw. I did some shopping around for batteries today and will have to come up with something.. most likely some GC batteries from Sams I think..
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