Help sizing my panel array to current mppt and charging system.

[brianjennings]

Hi. First post here. I have a question or two concerning sizing new panels to a pre existing charging system. I have a renogy 30 amp rover mppt controller, that charges a 300 ah li 12 volt battery. I want to get a few panels, probably used, to power this system, as close to the 30 amp limit as possible. My rover Manuel states that 12 volt panels have a range of 400 watts, and at 24 volts, that doubles to 800. I am assuming that the math here is proportional, so that at 36 volts, it would be 1200. And so on. My mppt is rated to go as high as 100 volts. I am only interested in connecting my panels in parallel. But im looking at panels within a 24 - 45 volt range.

First, how do i calculate my acceptable range for new panels, by the VOC value, or the Vmpp value, so that i can scale them to my mppt? For example. I am looking at some 275 watt lg panels, that have a 31.70 vmmp, but a 38.70 Voc. Im using the 31.70 to scale this to the controller to approximately 1050 total watts. Is this correct? Cause it sounds like i would be able to get away with 4 of these panels for a total combined wattage of 1100. Going just 50 watts over. However, these panels would be lying flat on my rv roof, and never see 100% of their max power. Im just wondering if im doing my math right, and using the right values to calculate my range.

The amperage of panels in question are rated at 8.68 immp and 9.26 isc.

My other question is, my mppt is going to step everything down to 14.4 to charge this li batt. But not the amps, with the new panels in parallel, right? They should stay compounded at (8.68 * 4 = 34.72), but at the 31.70 volt range? or would they be stepped down at the 14 volt range? And, how does my mppt convert excess voltage into amps with the arrangement I am considering (4, 275 watt panels @ 31.70 volts)? Or would 3 panels be a better fit for my system? Thank you in advance for all feedback.

 

[yabert]

At 14.4V the 30A charge will give 432W at the battery.
Despite if you put 10 000W of solar panel to the MPPT, the max charging power will be 432W.

MPPT max voltage is based on Voc. If you plan 39Voc panel, a maximum of 2 panels can be in series (78V).

the arrangement I am considering (4, 275 watt panels @ 31.70 volts)

You can also put 4 panel in parallel, but that 1100W will still be limited at 30A on the battery side (so around 14A at 31.7V solar side).

 

[mikefitz]

states that 12 volt panels have a range of 400 watts, and at 24 volts, that doubles to 800. I am assuming that the math here is proportional, so that at 36 volts, it would be 1200. And so on.

I think you will find the instructions are saying that with a 12v battery system the controller can process 400 watts of panels, with a 24v battery , 800 watts and so on.
The controller can deliver a maximum of 30 amps into a 12v battery regardless of panel power.
The MPPT controller is a power converter, with your unit limited to a 30 amp output limit. To get full power out of the 4 panels you need either an additional controller, 2 panels into each, or a controller suitable for the converted amps , 1100 watts/ 14v battery volts = 78 amps. Since its rare to get full panel power a 60 amp rated controller would be OK, but you loose some peak power with good solar conditions.

 

[brianjennings]

Thank you. I have posted the instructions. Just trying understand how it all works so i can choose the right panel array.