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dongringo

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Posts: 3
Reply with quote  #1 

System - less than 1 year old
Location: Hicktown in Mexico

Panels 2x200W = 400W
Controller = MPPT 50A
Inverter = 1500W MSW
Batteries 12V - 6x105A = 630A
Average sun hours = 5
Average daily W/h = less than 800W per KILLAWATT meter

Multimeter reading of batteries after full charge, disconnected
and after 24 hr rest = 12.65V

IN USE on full charge with panels disconnected
from sundown to sunup using about 300W
batteries read 12.28v +/-
app. 32% discharge

Calculating battery capacity
630A = 7800 W
less 50% maximum draw = 3900W
less 20% inefficiency = 3150W
Should be enough to last 3 days.

WHERE DOES MY WATTAGE DISAPPEAR TO?
Does anyone have an answer?
Thanks

 

jjackstone

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Posts: 47
Reply with quote  #2 
Probably should get the terminology right to start with. Watts is more or less instantaneous power being used. Watt-hours (or kilowatt hours) is the amount of energy being used over a period of time. So if you use 300 watts of power for two straight hours then you have used 600 watt hours of energy.

From your information it looks as though you are putting 800 watt hours of energy into your battery on a standard day. Then you are using 300 watt hours overnight. The batteries should easily charge back to full capacity the next day.

Batteries are generally rated by Amp hours. It appears that your pack is rated at 630 amp hours. Or as you state around 7800 watt hours. It looks like you are calculating the amount of energy that should be available to you by derating for useable energy and charging inefficiencies to get 3150 watt hours available to you. 

So what wattage or energy is it you are trying to determine that has disappeared? Are your batteries not recharging properly? Or do you think that if 300 watt hours is 32% discharged, that you only have about 900 watt hours available to you instead of the 3150?

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JJ
dongringo

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Posts: 3
Reply with quote  #3 

Thanks JJ

If my batteries capacity, fully charged, is 7800w, 3150 watts net
and I use 800 watts in 24 hours, without panels
my system should be funcional for up to 4 days


Instead, by the next morning, having used app 800 w, my batteries are depleted to 11.95V,
less than 50% capacity !!
Instead my batteries dropped the entire net 3150 w

As I said, the batteries are a year old, and when disconnected 24 hours, they hold charge.

Something is wrong.

stmbtwle

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Posts: 2,745
Reply with quote  #4 
Battery voltage drops under load. Is your second reading while the load is still on or after the batteries have had a "rest" to "recover"?

You might do better to check the specific gravity rather than the voltage, I believe it is a better method (though admittedly not as convenient). http://support.rollsbattery.com/support/solutions/articles/4347-measuring-specific-gravity


Also check your wiring between the batteries and the voltmeter. Undersized wire or a corroded connection can give you erroneous readings.

Finances notwithstanding, I'd consider adding another 200w panel. Your controller can handle it, and it should give you better overall performance, especially in the winter.

__________________
Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
jjackstone

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Posts: 47
Reply with quote  #5 
Makes more sense to me. Agree with checking all wiring and specific gravity. It's possible you have one bad battery loading down the system. If you have a way to check each battery individually with a load tester that would be good.

Looks like you are averaging a power draw of around 33 watts so it doesn't appear that the batteries are being pushed that hard so that's a good thing.

Couple other questions.

Has the system always performed this way?

Does your controller allow you to verify the actual amount of energy transferred to the battery pack?

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JJ
stmbtwle

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Posts: 2,745
Reply with quote  #6 
JJ has a point.

I've found the hard way that a solar system can "hide" a bad battery. If you have one 12v battery with a bad cell, that battery would normally only read around 10-11v. However it's in parallel with the good batteries and THEY are at 12v, so they will "overcharge" the bad battery till it reads the same as the others. This could be why your voltage is dropping more than you expect. Are any of the batteries or cells using more water than the others? If so they are suspect.

Agree this should NOT be the case with one-year-old batteries.

Sealed batteries are difficult as you can't check the water or specific gravity. The only way I know to check them is to break the bank down to individual batteries, let them "rest" a few hours and then check them separately.

As you're probably trying to use the system, you might try disconnecting and checking ONE battery at a time, while the other 5 remain on line.

A pain in the @$$, I know.

__________________
Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
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