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spyonabird
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Join Date: Apr 2008
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Solar panel and battery size

04/28/2008 10:24 AM

I put together a system and I'm not sure if it is adequate. I have a load that is on 24/7, draws 150ma. 12-volts. I have a 7 ah battery, 5-watt solar panel and a charge regulator.

Do I need a regulator? The load will run on 9-12 volts. The regulator cuts out at 11-volts and back on at 12.6-volts. How much direct sun is really needed to charge the battery from 11 to 12.6?
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jack of all trades
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#1

Re: Solar panel and battery size

04/28/2008 3:03 PM

Well your load is 1.8W = 43.2Wh/day. Given standard safety factors (and average equivalent sun hours) when estimating solar and battery sizes, you will need 5x5W panels to recharge the 7Ah battery, with the 7Ah battery giving you about 1-1.5 days of reserve power in the event you don't have enough sun (eg-at night, overcast day, etc).

You will need more solar panels and I would also increase your battery capacity if possible. Ideally a 5 day battery reserve (or more) should be used, giving you about 27Ah of batteries. You will quickly find that skimping on battery's will result in you simply running out of stored power when the solar panels cannot refill them if you get a few cloudy days in a row.

Your charge regulator is your regulator if you are connecting your load directly across the battery. It is also there to disconnect the solar panels from the battery at night. You would only need a separate regulator if your load couldn't handle the fluctuating battery voltage or peak charging voltage of (12.6V).

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Milo
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#2
In reply to #1

Re: Solar panel and battery size

04/28/2008 11:10 PM

Good advice here. I gave it a GA.

jack, would n't a diode between the panels and the battery also prevent the reversal of current at night?

milo

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#6
In reply to #2

Re: Solar panel and battery size

04/29/2008 3:14 PM

Yes it would, but remember the solar charger regulator also does this and acts as a voltage regulator. You could just use a diode but the charging voltage to the battery would vary wildly depending on solar conditions (also don't forget about the voltage drop lost across the diode). Using a diode is useful if you are trying to implement a simple solar + battery system where the solar panels were powering the load only and the batteries were non-rechargeable and only used when there wasn't enough sun.

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#3
In reply to #1

Re: Solar panel and battery size

04/29/2008 12:11 AM

Good clarification. I vote you as good answer.

Can you please add some diagram for total system?
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jack of all trades
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#7
In reply to #3

Re: Solar panel and battery size

04/29/2008 3:20 PM

There really isn't much to a stand-alone (non-grid connected) solar system. Battery bank on one input to the solar regulator, solar panels on another input, load connected to the output of the solar regulator.

Google "solar regulator" for circuit diagrams and overall system drawings.

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spyonabird
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#4
In reply to #1

Re: Solar panel and battery size

04/29/2008 8:47 AM

Thanks very much for your explanation. I see where I miss calculated. I also do not quite understand where you get the 25-watt solar panel size. Time for some more homework. Thanks for getting me on the right track.
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#8
In reply to #4

Re: Solar panel and battery size

04/29/2008 3:25 PM

I also do not quite understand where you get the 25-watt solar panel size.

Effectively it comes down to the fact that you never get 5W continuously out of a 5W solar panel. You need to take into account various de-rating factors and system losses otherwise your overall system will not work in the real world.

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#5

Re: Solar panel and battery size

04/29/2008 9:06 AM

Power is One unit collected and stored to one unit used, the batterry is the collection point, The formula for that is p= e*i. If the solar panel is collecting 5 P watts at 12 E then then the collction is 3.3^ AH. Now take the average hours you can collect from the area you are in and that gives the result but you must take into consideration a 10 % loss for the equiptment. Lets pretend you get 5 hours per day of charging time then the result would be 40/12=3.3^ * 5 = 16.6^ - 10% ah per day of charging. Which is double what the battery can store.The drain is much less than that so your system should be more than able to handle the load. 24 hours * 150ma = 3600 MA which is not even one close to your storage capacity.Temperature and no sun can affect this but your battery should be able to carry you during these times with its extra storage. However if your load is ac then the ouput becomes 3.3^ ah /120 v = 0.0275 P or W which is not enough to keep the system charged. Hope i helped.
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#9
In reply to #5

Re: Solar panel and battery size

04/29/2008 11:35 PM

Confusions of Units.

Please correct all units. Besides,it is not clear from where these figures arrive
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Taganan
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#10

Re: Solar panel and battery size

05/02/2008 12:47 AM

Will read up later.

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Transcendian
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#11

Re: Solar panel and battery size

05/04/2008 11:06 PM

It would be nice if you listed the brand names of the batteries and other components involved. What exactly are you running on the circuit? Do you mean Controller, or regulator? Are you interfaced with any grid power? Again, the load is listed as Volts, and operating. Are you putting that to an inverter that makes it AC?

Forgive me. I compute simply from demands. A thousand watt demand means I have to give 10 amps. Forgive me I struggle to reconcile DC and AC nomenclature and C as same as Amps.

In all cases I figure in 20 percent failure.

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Anonymous Poster (2); gsuhas (1); jack of all trades (4); Milo (1); spyonabird (1); Taganan (1); Transcendian (1)

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