diy solar

diy solar

What am I missing?

gac1218

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Jun 9, 2022
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I have an off grid cabin. I have recently upgraded to a 24 volt 3000 watt inverter charger and added 4 more for a total of six 345 watt solar panels 10.34 amps, 41/ 33.37 volts. I have the panels wired in 3 sets of 2 in series and paralleled in the combiner box with three 15 amp breakers. That goes to the midnite solar 45 amp charge controller to the four 6 volt 235 amp hr fla batteries wired to 24 volts and to the inverter.

Why do my batteries lose power in full sun when I max the system out at 1400 watts when I have like 1900 available with the panels?

I assume the power has to go through the batteries and they just can't absorb power fast enough???

Do I need more batteries?
 
45amp out of the charger at 25v is only 1125watts. Get a bigger charger.
 
Makes sense. You are drawing about 65 amps from your batteries under a 1400W load and your SCC can only replenish 45 amps under perfect conditions. Therefor the batteries will lose power.
 
As others have said you are using nearly 400 watts more than your solar controller can supply. It is very difficult to get fla batteries fully charged on an active solar system. If the have not been fully charged very often their performance and efficiency will drop causing even a very modest load to flatten them quickly especially as they get close to 50% state of charge. Make sure your batteries get fully charged at least several times a week every day is better.
 
45amp out of the charger at 25v is only 1125watts. Get a bigger charger.
I guess I am confused. If each set of panels wired in series put out 10.34 amps x 3 sets=31 amps. Wouldn't that be the maximum amperage the 45 amp charge controller could use?
 
I guess I am confused. If each set of panels wired in series put out 10.34 amps x 3 sets=31 amps. Wouldn't that be the maximum amperage the 45 amp charge controller could use?
No an MPPT charge controller changes the incoming power to match your batteries. That results in significantly more amps coming out of the controller than went in but at lower voltage. Watts divided by volts = amps.
 
You have 6 x 345W panels which is a total of 2070W. 2070W / 25V battery charge voltage is up to 82.8A of battery charge current. But your controller only supports 45A. You need an 80A (or 75A) charge controller to make full use of your panels.
 
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The 45 amp maximum is on the output. The input is always less current.
I guess I am confused. If each set of panels wired in series put out 10.34 amps x 3 sets=31 amps. Wouldn't that be the maximum amperage the 45 amp charge controller could use?
 
Makes sense. You are drawing about 65 amps from your batteries under a 1400W load and your SCC can only replenish 45 amps under perfect conditions. Therefor the batteries will lose power.
So, I don't think it is even getting 45 amps from the panels as wired.
As others have said you are using nearly 400 watts more than your solar controller can supply. It is very difficult to get fla batteries fully charged on an active solar system. If the have not been fully charged very often their performance and efficiency will drop causing even a very modest load to flatten them quickly especially as they get close to 50% state of charge. Make sure your batteries get fully charged at least several times a week every day is better.
Thanks'
 
You have 6 x 345W panels which is a total of 2070W. 2070W / 25V battery charge voltage is up to 82.8A of battery charge current. But your controller only supports 45A. You need an 80A (or 75A) charge controller to make full use of your panels.
Thanks. So, if I get a new 100 amp mppt charge controller, am I good on the 100-150 volt limitation with my configuration? Or does that voltage address the battery bank and not the panel output?
 
So, if I get a new 100 amp mppt charge controller
A 100A controller would be good for up to about 2500W on a 24V system.

am I good on the 100-150 volt limitation with my configuration? Or does that voltage address the battery bank and not the panel output?
It's neither. The controller's max PV input voltage is based on the temperature adjust Voc of your solar panel array. I'm guessing the Voc of your panels is about 45V. With them arranged in 2S3P you have an array Voc of 90V. You'll be over 100V at roughly 11ºF/-12ºC. Most likely a 100A controller will support at least 150V of max PV input voltage so you should have no problem with a 2S3P arrangement.
 
A 100A controller would be good for up to about 2500W on a 24V system.


It's neither. The controller's max PV input voltage is based on the temperature adjust Voc of your solar panel array. I'm guessing the Voc of your panels is about 45V. With them arranged in 2S3P you have an array Voc of 90V. You'll be over 100V at roughly 11ºF/-12ºC. Most likely a 100A controller will support at least 150V of max PV input voltage so you should have no problem with a 2S3P arrangement.
It's 41, so double that in series and I'm 82, so all is well. Thanks again.
 
With a Voc of 82V you would have no trouble with a 100V SCC. But I'd be really surprised if you find 100A SCC that only supports 100V max input voltage.
 
It is only a 12/24 I would be inclined to get 150 volt in case I expand.
The 12V/24V is the battery voltage. That's fine unless you think you might grow to a 48V system in the future. The 150V would be the max PV input voltage. You can add more panels in parallel to avoid needing 100V versus 150V. You plan on 2S3P which works with a 100V 100A SCC. You go to 2S4P on the same controller. Going with a 150V controller does allow for 3S2P or 3S3P if desired.

Either way though I'd stay away from that ultra cheap SCC. If you want a top-of-the-line SCC that supports 150V and 100A, the Victron SmartSolar 150/100 is $800 and supports 12/24/36/48V. If that's not in your budget then find one more in the middle of the two. Just remember that such high amperage isn't cheap. Maybe use two smaller SCCs each with half of your panels. In some cases that can save you some money.
 
This might be clear in your mind already but for the sake of anyone yet to understand. Suppose......

1100w of solar at 70v (two 60cell panels in series, making 1100w/70v=15.7amps from the panels. Say we loose 100w when we take the 70v down to 14v to charge our 12v battery, ending up with 1000w to the battery at 14v is 1000w/14v=71.4amps. So you need a 70amp or larger charger to deliver the 1000w to the battery. The chargers are rated at the amps they can output.
 
This might be clear in your mind already but for the sake of anyone yet to understand. Suppose......
I showed the simple calculation back in post #7. It's basically panel wattage divided by battery charge voltage. That's the max charge current your SCC could possibly ever need to support. Typically you choose a smaller controller because you rarely get 100% power from the panels due to factors such as panel angle, shading, and the real world not matching the ideal STC conditions of the panel ratings.

I'm not sure why you say you will lose 10% panel power due to the voltage conversion. Most SCCs have a maximum efficiency of in the high 90s. My Victron is at 98%.
 
I showed the simple calculation back in post #7. It's basically panel wattage divided by battery charge voltage. That's the max charge current your SCC could possibly ever need to support. Typically you choose a smaller controller because you rarely get 100% power from the panels due to factors such as panel angle, shading, and the real world not matching the ideal STC conditions of the panel ratings.

I'm not sure why you say you will lose 10% panel power due to the voltage conversion. Most SCCs have a maximum efficiency of in the high 90s. My Victron is at 98%.
10% was a "for instance', not a fact nor a predication. There are wire losses in addition to charger losses. I was just showing a system with losses.
 
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