MPPT Charge Controller Sizing?

[schnitzeraffe]

So I lived through Hurricane Ian with minimal damage.

Even went through the eye of the storm!!!

So I've stepped up my generator game a little bit.

And now I want to experiment with a small off-grid back-up solar system.

We cycle the generators to minimize fuel usage.

My Kill-A-Watt meter showed that my fridge used 1.66 kWh in a 24 hour period and a small spare TV I have uses 26 watts.

My plan is to use the solar system for supplemental power during generator down time with this fridge and maybe the TV using an OTA antenna in the attic.

I may use it more depending on how things work out after experimenting.

For the panels, I bought four used 280 watt Canadian Solar residential panels model CS6K-280M.

The sticker on the back lists the Voc as 38.5 V.

My quick testing in the front yard, with less than ideal positioning, gave me 35-36 V each, so I think they should be fine.

I intend on using two to four 100 Ah LiFePO4 batteries and a 2000-3000 watt pure sine wave inverter.

My questions comes down to charge controller(s).

I've used some calculators online and it looks like I'm better off cost-wise using two smaller controllers than one large one.

I also like the idea of some redundancy, so if one controller dies I still have some power from the other and am not completely shut down.

When entering the data from the panels, most calculators give my controller output amperage as 39A (one showed 48.39A when entering temperature information as well).

If I use the Epever 40A charger mentioned on Will's web site (two of them actually - one for each set of two panels), will everything work OK?

If the amperage exceeds the 40A rating of the controller that is.

Is the excess power just wasted, going unused?

Or can the controller(s) be damaged?

 

[sunshine_eggo]

May I assume you've already conducted the energy audit?

MPPT are sized to accommodate the series Voc of the array including temperature margin and output current to the battery.

You have 4*280W = 1,120W of PV.

Your MPPT current depends on your battery voltage.

@ 12V, you'll need, 1120/14.4 = 78A
@ 24V, you'll need 1120/28.8 = 39A
@ 48V, you'll need 1120/57.6 = 19A

You'll want to create a 2S2P array with Voc around 75V (or two separate 2S arrays on 2 MPPT). This should be fine on a 100Voc controller.

Since your PV voltage is higher than battery, the PV current TO MPPT will always be less than battery current since,

Vin * Ain = Vout * Aout OR
Vmp * Imp = Battery voltage * charge current

Your current can never exceed the controller because it's a hard limit. The MPPT just can't send any more than 40A to the battery. You can hang more power than it can deliver within reason. This is referred to as over-paneling. Provided you don't exceed any input limits on the MPPT, you can put more PV power than it can deliver.

Additionally, even with the improved efficiency of MPPT, the bottom line is that it's very rare to actually get 100% rated output because solar conditions are rarely perfect.

 

[schnitzeraffe]

Thank you for the quick reply.

I had figured the 78A for a 12V battery voltage as well with my four panels set in a 2S2P configuration.

That's why I looked into using two separate controllers with two panels in parallel each at 39A.

It looked to save about $300 that way.

I just wanted to make sure there wouldn't be any damage to the components if the amperage went over 40.

I'll proceed as planned.

Thanks again!

 

[MisterSandals]

It looked to save about $300 that way.

Just out of curiosity, which 2 SCCs are $300 cheaper than 1 bigger SCC?

 

[sunshine_eggo]

Just out of curiosity, which 2 SCCs are $300 cheaper than 1 bigger SCC?

This.

 

[sunshine_eggo]

Here's an 80A notably less than 2X 40A:

SC4880 – 80A 48V, 24V and 12V battery 150 V MPPT Charge Controller only – Watts247.com

watts247.com

 

[schnitzeraffe]

I was only looking at the Renogy, Victron, and Epever controllers that Will recommended on his site.

Epever 40 amper's are $208.59 each X2 = $417.18

vs

Victron Smart Solar 150/85 = $697.00

The difference between the two is $279.82

 

[Bud Martin]

I like redundancy, especially when you live in a hurricane area.

 

[sunshine_eggo]

I was only looking at the Renogy, Victron, and Epever controllers that Will recommended on his site.

Epever 40 amper's are $208.59 each X2 = $417.18

vs

Victron Smart Solar 150/85 = $697.00

The difference between the two is $279.82

Victron is premium equipment. You're comparing apples (victron) to withering mandarins (EPever).

As you can see, the 80A I linked is > $100 cheaper than 2X 40A.

Epever also makes an 80A in a different model:

https://www.amazon.com/Controller-Negative-Charger-Temperature-Communication/dp/B07KB44GSD

There are options.

 

[sunshine_eggo]

Also, since you plan 2-4 batteries, 24V is a piece of cake and halves your MPPT current requirements. It's also way easier to expand to higher power.

At 2000-3000W, you're already maxed out at 12V.