Confusion over using "24V" panels with a 12V battery bank

Hi Forum!
First - I've done so much lurking on this forum and really appreciate all the information people have shared. This has helped me begin my purchases and I'm now in a position to start putting everything together.

Equipment:

1-2 MPPT controllers, an Epever 30A (3215BN) and a Renogy Rover 40A Li.
2-Various "12V" Renogy/HQST/Rich solar panels - most are 100 W, a coule are 50 W. They ALL have VMPs within .2V of 18V.

I'm using a Renogy Smart Lithium battery bank that can only operate at 12 volts.

I've seen lots of conflicting information about setting up "12V" panels in series, which creates a "24V" panel, and then feeding a 12V battery bank.

I understand the Voltage issue - going off of VMP and not the nominal ratings, if I connect 2 of my 18 VMP "12V" panels in series, for a combined VMP of 36, will that be ok going to a 12V battery bank?

I know I have to stay within the voltage and amp rating of the controller - but I'm confused as to weather it's as easy as upping the voltage by putting some of my panels in series.

Here's what I'm considering: 2 x 50W "12V" panels of 18 VMP in series, 2 x 100W "12V" panels w/ 18 VMP in series, then both sets in parallel.
For the 50 watters that would be 100 W at 36 VMP, the 100 watters would be 200 W at 36 VMP, giving me a total VMP of 72.

I've been going all over the place and can't replicate this proposed scenario for a 12V battery bank.

As far as manuals - RENOGY IS THE WORST EVER, so much thanks again to all the folks who have spent time helping newbies like myself. It took me a lot of reading and cross referencing to get this far and I really appreciate it.

Thanks and happy solaring!

a MPPT SCC will take a given voltage from the PV array and "convert" it to the correct battery voltage. So long as your solar pannels are under the max allowable PV voltage and your useing the correct output for the battery you have, your fine. (some SCC are only 12v battery, some are 12/24v others are 48v or some other programable voltage)

As an FYI a PWM SCC will only take a narrow input range of solar voltage for its battery output voltage.

schmism said:

a MPPT SCC will take a given voltage from the PV array and "convert" it to the correct battery voltage. So long as your solar pannels are under the max allowable PV voltage and your useing the correct output for the battery you have, your fine. (some SCC are only 12v battery, some are 12/24v others are 48v or some other programable voltage)

As an FYI a PWM SCC will only take a narrow input range of solar voltage for its battery output voltage.

Click to expand...

Thanks!

So, just to be clear, if my controller takes 400W at 12V or 800 W at 24 V....and I'm using the 12 V battery bank, should I assume that I still have to make sure my total wattage is 400W or less? So I can increase my voltage by putting some of my 12V panels in series, but I'm still stuck with the total wattage allowed for the 12V system because of my battery. Is that correct? Put another way- the only way to take advantage of the 800W allowance on my controller is to use a 24V battery bank. Is that right?

Thanks again!

To add to schmism's post, it's worth noting that when you put different panels in series, ALL panels will be restricted to the LOWEST current panel. Given that all your panels are within 17.8-18.2V, if you wire lower power panels with higher power panels, you will cut the power of the higher power panels.

You can get around that by putting all the 50W panels in one series string and putting that string in parallel with a series string of 100W panels. The two strings must have roughly the same Vmp and Voc range.

CaliforniaSun said:

Thanks!

So, just to be clear, if my controller takes 400W at 12V or 800 W at 24 V....and I'm using the 12 V battery bank, should I assume that I still have to make sure my total wattage is 400W or less? So I can increase my voltage by putting some of my 12V panels in series, but I'm still stuck with the total wattage allowed for the 12V system because of my battery. Is that correct? Put another way- the only way to take advantage of the 800W allowance on my controller is to use a 24V battery bank. Is that right?

Thanks again!

Click to expand...

You can actually exceed that. In most cases, SCC will simply ignore any additional current available beyond what it can use. That's called over-paneling and is a somewhat common practice. Using 600W for the sake of argument, while your peak production will be clipped off, you will get the 400W for a longer period of time as long as the 600W array performs above 400W.

snoobler said:

You can actually exceed that. In most cases, SCC will simply ignore any additional current available beyond what it can use. That's called over-paneling and is a somewhat common practice. Using 600W for the sake of argument, while your peak production will be clipped off, you will get the 400W for a longer period of time as long as the 600W array performs above 400W.

Click to expand...

Hey Thanks, snoobler!

The two 50W in 2S and the two 100W in series, with both series sets in parallel to the controller. The 2 100W panels are 17.9 VMP, the 2 50W panels are 18.1 VMP.

So, from my original post, I think my plan is lining up with what you're suggesting?

This is what I wrote as my initial thinking-

Here's what I'm considering: 2 x 50W "12V" panels of 18 VMP in series, 2 x 100W "12V" panels w/ 18 VMP in series, then both sets in parallel. For the 50 watters that would be 100 W at 36 VMP, the 100 watters would be 200 W at 36 VMP, giving me a total VMP of 72V.

Thanks again!

snoobler said:

You can actually exceed that. In most cases, SCC will simply ignore any additional current available beyond what it can use. That's called over-paneling and is a somewhat common practice. Using 600W for the sake of argument, while your peak production will be clipped off, you will get the 400W for a longer period of time as long as the 600W array performs above 400W.

Click to expand...

Ok gotcha, yeah that makes sense from what I've seen as far as what I'll actually pull from the panels (given the losses that occur in the chain).

But, it sounds like you are saying that yes, as long as my battery bank is 12V, regardless of whether or not I wire some panels in series to make them "24V", I still have to stick by the 12V wattage rating of the controller...correct?

Thanks again!

Regardless of what you have going on on the input side, a charge controller will always be limited to its maximum output current. Since Watts = Volts x Amps, and the max amps are fixed, the max power output in practice is determined by the battery bank voltage.

For example with a 30A controller and battery voltage of...
...12v x 30A = 360W max power output
...24v x 30A = 720W max power output
...48V x 30A = 1440W max power output

Technically the max output will change a bit based on state of charge, but the principle is the same.

This article from Victron has some useful info. Its written with relation to their controllers but the general points are broadly relevant

So yes, you are partially correct here:

But, it sounds like you are saying that yes, as long as my battery bank is 12V, regardless of whether or not I wire some panels in series to make them "24V", I still have to stick by the 12V wattage rating of the controller...correct?

Click to expand...

Its not that you "have to stick by" the 12v watt rating, its that you will be limited to that output rating regardless of what you do on the input side. As Snoobler said, you can 'overpanel' within the limits defined by your controller, which will allow you to maximize your controllers output for more hours, but you can't output more than your controllers max output even if your array theoretically could.

CaliforniaSun said:

Hey Thanks, snoobler!

The two 50W in 2S and the two 100W in series, with both series sets in parallel to the controller. The 2 100W panels are 17.9 VMP, the 2 50W panels are 18.1 VMP.

So, from my original post, I think my plan is lining up with what you're suggesting?

This is what I wrote as my initial thinking-

Here's what I'm considering: 2 x 50W "12V" panels of 18 VMP in series, 2 x 100W "12V" panels w/ 18 VMP in series, then both sets in parallel. For the 50 watters that would be 100 W at 36 VMP, the 100 watters would be 200 W at 36 VMP, giving me a total VMP of 72V.

Thanks again!

Click to expand...

Dzl said:

Regardless of what you have going on on the input side, a charge controller will always be limited to its maximum output current. Since Watts = Volts x Amps, and the max amps are fixed, the max power output in practice is determined by the battery bank voltage.

For example with a 30A controller and battery voltage of...
...12v x 30A = 360W max power output
...24v x 30A = 720W max power output
...48V x 30A = 1440W max power output

Technically the max output will change a bit based on state of charge, but the principle is the same.

This article from Victron has some useful info. Its written with relation to their controllers but the general points are broadly relevant

So yes, you are partially correct here:

Its not that you "have to stick by" the 12v watt rating, its that you will be limited to that output rating regardless of what you do on the input side. As Snoobler said, you can 'overpanel' within the limits defined by your controller, which will allow you to maximize your controllers output for more hours, but you can't output more than your controllers max output even if your array theoretically could.

Click to expand...

Hey thanks so much Dzl! Super helpful and clear, much appreciated.

And in my original calculation it looks like I made a mistake.
2 x 50W in series - 36V
2 x 100 W in series - 36 V

putting them both in parallel, I don't add the voltages to get 72V, rather, the voltage stays constant at 36V and the amps get added. Which is great.

Thanks again for taking the time.