Adding suplemental panels..

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tvengineer
tvengineer Solar Expert Posts: 31 ✭
in Solar Beginners Corner #1
Maybe this is a dumb question..


I have a system with 400watts of panes... Morningstar 30 amp controler.. 440 amphours of battery...
constant load of about 60watts 24/7

the system works fine and I monitor the battery voltage carefully.. they have never been below 50percent charged..

however.. after several cloudy days.. they do start to get low.

I have a couple of old 40 watt panels laying around ...
So.. could I connect the two 40 watt panels directly to the battery bank thru a diode with no charge controller?

Even in bright sunlight.. the 80watts of panels (minus the loss in the difference between MPP and 12 or so volts) these panels would be BARELY (if at all) supplying the load, much less be adding any real power to the battery..

so I don't think that there would be any chance of overcharging the battery bank by not having these small panels on a charge controler..

but would their output on the battery side of the Morningstar controler affect how the controller sets its output..

Yes.. I know.. I could easily get a mornistar 10 amp controller for them.. but that is not the point of my question :-) :D
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
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    Re: Adding suplemental panels..

    Probably no problem. Another 80 Watts would only increase the total available by 20% maximum. 480 Watts @ 12 V = 40 Amps max charge current - well within the limits for your 440 Amp/hrs of batteries. Since a system is typically 80% efficient, you could look at adding the two panels as bringing it up to about 96% (although not exactly).

    But is this really going to give you any advantage on cloudy days? If the clouds knock out 15%, it will make up the difference. Clouds tend to knock out a lot more than that, though. This is why systems are usually designed with over-capacity to maintain output on cloudy days. Then you have the trouble of "how many days?"

    You're right; there shouldn't be any potential for over-charging on sunny days. Since the controller will reduce output from the main panels as required and the 80 Watt panels will only achieve about 6 Amps - that's about enough to keep up with self-discharge on 440 Amp/hrs.

    It's been a long day here, so maybe I've calculated this all wrong. Wait for other responses. :p
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  • tvengineer
    tvengineer Solar Expert Posts: 31 ✭
    #3
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    Re: Adding suplemental panels..

    Thanks..

    Not exactly saying that the additional 40 watt panels would make up for cloudy days..

    more just thinking that I have them sitting there.. why not just hook them up for a little bit extra...
    the thought of using them without a charge controller... is more of a mental exercise than a practical one.

    Cariboocoot wrote: »
    Probably no problem. Another 80 Watts would only increase the total available by 20% maximum. 480 Watts @ 12 V = 40 Amps max charge current - well within the limits for your 440 Amp/hrs of batteries. Since a system is typically 80% efficient, you could look at adding the two panels as bringing it up to about 96% (although not exactly).
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,457 admin
    #4
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    Re: Adding suplemental panels..

    Just make sure they have the blocking diode and some sort of fuse (10-12 amp fuse near the battery) to protect the wiring/panels from shorts.

    Probably, connect a 10 amp DVM / current meter in the leads to see if they are providing any useful current at all before going to the trouble of mounting them.

    Given that you have a 60 watt 24/7 load--there is probably zero chance that those 80 watts of panels would do any harm at all to your batteries.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • tvengineer
    tvengineer Solar Expert Posts: 31 ✭
    #5
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    Re: Adding suplemental panels..
    BB. wrote: »
    Probably, connect a 10 amp DVM / current meter in the leads to see if they are providing any useful current at all before going to the trouble of mounting them.

    -Bill



    :blush:
    Slap forehead...

    why didn't I think of that...
    Connect them via a amp meter... and watch the amp meter from the big panels...
    see if the total current is higher with them connected .. or if the little ones make the charge controller cut back...

    duhhh...
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