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MtnDon
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# Posted: 16 Jul 2012 09:18pm
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In the PV industry . . . STC (standard test conditions) specifies a temperature of 25°C and an irradiance of 1000 W/m2 with an air mass 1.5 (AM1.5) spectrum. These correspond to the irradiance and spectrum of sunlight incident on a clear day upon a sun-facing 37°-tilted surface with the sun at an angle of 41.81° above the horizon. This condition approximately represents solar noon near the spring and autumn equinoxes in the continental United States with surface of the cell aimed directly at the sun.
The battery will charge with those inputs, but it would charge faster if the volts were higher even if the amps were correspondingly lower. The higher voltage pushed the current into the battery more effectively. However if the controller can not be user programmed you are stuck with that.
A premium controller is user programmable for the point where the controller switches from bulk charge to absorption charge. The absorb time should also be user programmable as should be the voltage used for the float charge.
If 13.34 or thereabouts is the highest voltage reading you get while the system is charging (and with no loads or only light loads at that time) it will be difficult to reach a true fully charged point. With a flooded battery the sp gr should be checked; that is the true indication of charge... remember to temperature correct. AGM batteries can only be voltage tested; not as accurate.
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TomChum
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# Posted: 16 Jul 2012 11:32pm
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In the morning my 250 AH battery is at 12.4 or 12.5v. At about 10 AM 170watts of panels shows 1-2 amps on the meter, and voltage might be 12.7. When the sun hits the panels I get about 12 amps. Voltage rises too. 12.9, 13.1, 13.3. It usually gets to 13.7 by the end of the day. If I'm away from the cabin for a few days the midday voltage will be at 14.4v ( elect fridge is turned off while I'm away) when I arrive.
I'm sure there is a point where my voltage passes thru 13.34 too.
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MtnDon
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# Posted: 17 Jul 2012 12:28am - Edited by: MtnDon
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Quoting: TomChum At about 10 AM 170watts of panels shows 1-2 amps on the meter, and voltage might be 12.7
12.7 is fully charged when the battery is at rest, and has been at rest for several hours. When the system is charging the 12.7 doesn't mean a whole lot when it comes to knowing what the state of charge of the battery is. Even a battery that is nearly dead will show the higher voltages when the panels are charging the battery.
It is good to see the reading hit 14.4 on some days. That is telling you that on those days the battery probably truly reaches a full charge. The last 10% or so, going from 90% full to 100% full takes a longer time than one would think it should.
12.4 is almost 80% full, 12.5 is about 90% full (after a rest period.)
On days when the voltage reading gets to 13.7 as the highest figure the battery may not really be full. It's hard to say. But if it hits 14.4 on days when the use has been low or non existent, that is an indicator that when the cabin is occupied the battery is not reaching 100%. That's not too bad unless it is the normal thing rather than the exception. At least the morning reading isn't down to below 12 VDC like some folks do. That leads to very much shorter life. So you're likely not doing too badly all in all.
If no power is used for three hours after the charge stops the voltage reading at that point is going to be pretty close to being correct. Loads and inputs cause the voltage reading to be lower or higher than what the true state of charge would indicate.
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TomChum
Member
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# Posted: 17 Jul 2012 01:28am
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Those were my conclusions too. I'm pretty sure that I need to add another panel, to arrive @ appx 250 watts (to feed my electric fridge) and then in the winter will have the extra capacity too.
But anyway back to the original subject where the battery was getting 7.37 Amps at 13.34v. These amps are still going into the battery? If you could push a steady 7 amps (assuming) into a battery for 6 hours, wouldn't that completely charge a 42AH battery regardless of the voltage?
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sparky1
Member
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# Posted: 17 Jul 2012 09:06am
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I had charged the battery (Day Before) I put the panel to it--It's a brand new battery-a Battery can only Take what it Needs there fore---The Panel CAN"T put out 145 watts when the battery doesn't Need it.
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MtnDon
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# Posted: 17 Jul 2012 09:14am - Edited by: MtnDon
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Quoting: TomChum If you could push a steady 7 amps (assuming) into a battery for 6 hours, wouldn't that completely charge a 42AH battery regardless of the voltage?
The thing is that as a battery reaches full charge it gets more difficult to push the amps into the battery. So in the last few hours the number of amps being accepted will taper down. That means more time is needed. So the straight math ( 6x7 = 42 ) does not compute in the real battery charging world. Having a higher voltage like 14.8, makes it easier to push the amps into the battery. Think of it like a balloon or tire with no air in it. As you blown more and more air into it you need more push, more pressure to overcome what is already there.
Also remember that batteries are not 100% efficient so it takes more amp hours going in to fully charge a battery than its rated capacity. So, IF the actual 7 amp rate could be continued for 6 hours (from dead state) the battery still would not be full. Batteries warm up as they charge, that's elecytical power being wasted. That has to be accounted for.
(heading away for a week so no more commentary for a while....)
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