Adding on

Desert Rat

I'm going to add 8X 370W panels (Voc 41.4, Isc 11.26, Vmp 34.5, Imp 10.72) to an existing 24V system that currently has a VFX3524 inverter, a mix of panels feeding two Morningstar charge controllers, and two EG4 200AH batteries. The panels will be split into two strings of four, one string facing east and one facing west. I know I'll need at least one additional charge controller, but which one? The options are limited due to the 24V system. Thanks for any suggestions.

JRHill

Outback FM80. You may be able to get the stuff cheap before long as everyone else jumps to a different platform. (wink)

Seriously, that FM80 is a well built and reliable tank and surprisingly, for its era, is adjustable enough to be suitable with LFP04 batteries - except for trying to use the EQ for periodically balancing. Ya can still do it but the one hour minimum is too long. I just tie a string on my finger to remember to stick around shut down the EQ manually after 10 minutes at 56.4v.

Desert Rat

Yep, the FM80 is near the top of my list. I would need two of them, though. Not a problem, as NAZ has
them at a very attractive price right now. I could buy two of them for the same price (or less) than one of some other options.

Desert Rat

I realized that a 4S string would exceed the Voc of the FM80. I could go 2S2P X 2, but that would mean a combiner box, breakers, and two runs of 6AWG wire in addition to the two FM80s. A lot to consider!

littleharbor2

Desert Rat said:

I realized that a 4S string would exceed the Voc of the FM80. I could go 2S2P X 2, but that would mean a combiner box, breakers, and two runs of 6AWG wire in addition to the two FM80s. A lot to consider!

No combiner/fusing needed for a 2p arrangement, 3 or more requires fusing. You can parallel and transition in a simple j-box. Isc. will be a simple 22.52amps.

Desert Rat

Thanks. The 8 panels will total 2960 watts, too much for a single FM80. So even without the combiner box I would still need two FM80s and two pairs of wires from the arrays to the controllers. I would like to go higher voltage and avoid the cost of two pairs of wires. It's looking like my only choice is the Schneider Conext 100-600. Thoughts?

littleharbor2

Too bad it wasn't easier to switch system voltages. If you were running 48 volts you wouldn't need a 2nd controller.

Desert Rat

I know; I've agonized over making the big switch to 48V. This system is at my GFs house, and my own system is also 24V. I have the same inverter as you, and I can't bear the thought of retiring it. Both systems were set up in the long long ago when panels were $6/watt and it made more sense to have 24V ceiling fans and water pumps.

littleharbor2

Down here in Baja there's lots of old off grid systems with Trace 12 volt inverters. They were built so well that the inverters just won't die. Consequently these people are stuck with 12 volt systems that are running lots of 12 volt panels, multiple controllers (Trace as well) and lots of big wire and fusing.

Desert Rat

Where in Baja are you?

littleharbor2

25 miles south of San Felipe.

littleharbor2

I have a SW 4048 and another SW 4024 inverter. These are back up inverters. I would have originally installed the SW 4048 except that the SW 4024 was in a factory sealed box when I opened it up and installed it in 2015.

Desert Rat

We made a trip to Baja in 2017; took the ferry from Guaymas to Santa Rosalía. The closest we got to you was Bahía de los Ángeles.

littleharbor2

I've yet to get south of LA Bay. I hear it's nice in the mid peninsula. Next spring, hopefully.

Dave Angelini

How far over the max are you for one controller? For offgrid you can go at least 20% over as you really are not doing that into the heat of the day. You mostly are charged by noon right? Not in a 100 square foot metal building...

You are stressing the controller more but buy a spare and you are good. You wiring needs to be correct to the battery with the correct breaker. 

Exquisito's in Santa Rosalia on your bucket list!

Desert Rat

Thanks Dave, I was hoping you would chime in. The new array will be 2960 watts, so way too much for a single 150V controller, right? That's why I was looking at the Schneider Conext 100-600. I see they have discontinued the Schneider SCP; can you confirm that the Xantrex SCP will work for local control and monitoring of that controller?

Dave Angelini

Yes the SCP is fine and the store sells it or can get it. The old Schneider SCP is now a Xantrex SCP with the correct firmware for Schneider devices.  

I do not have the specs on the outback and got to go.

BB.

There is the "useful", "cost effective", and the "safe" specs for over paneling...

Useful over paneling... A larger array generates more power in poor weather/shading conditions. Extends "hours of harvest" into earlier morning and evening times. And during winter, especially useful for Li Ion batteries which can charge at very high rates (typically 50% to 100% of AH capacity--I.e., in 2 or even 1 hour). Charging Lead Acid batteries, typically 20%-25% charging capacity (high charging current is "not good" for Lead Acid battery cycle life... Again, always refer to mfg. specifications for charging information.

Cost Effective Over Paneling... Solar panels "lose" (loose?) Vmp (voltage maximum power) at they get hot in the sun (vs Mfg. Specifications)... On a hot/windless day, panels can drop Vmp-array by almost 20%--And Power=Vmp*Imp "lost" power. And another 5% of so loss in controller+wiring. For a "maximum" cost effective array, you can use 77% or 75% solar+controller derating--I.e., more panels will not pump any more power through the charge controller. For Example, a 24 Volt 80 Amp controller and battery bank:

80 Amps * 29.5 volts battery charging * 1/0.77 array+controller deratings = 3,065 Watt array max "cost effective"

Solar "math" is approximate... You can go over (or under) the 3,065 Watt array without any big issues. And you can see going to 48 volts battery bank, the controller can manage a 2x larger array (controllers are "output current limited", not Watt limited).

You could always (for example) 1/2 the array facing south east and the other 1/2 facing south west--This is "great" for lead acid batteries. "Virtual Tracking" and more hours per day of charging (lead acid batteries "love" more time in absorb.

Li Ion batteries are very happy with a hour or two of "high current charging"--This charge at nearly 100% efficiency. Great for winter days with short window for "full sun".

You can over panel to your wallet's ability. If more than the 3,065 Watt array from above calculations, then the controller will "safely" and reliably "clip" the output to the Controller's maximum rate current (and can reduce current if controller starts to run hot).

Lastly is Safety, many controllers will list a maximum array current (or over current). Too much "available" current can cause safety issues in the controller (over heat internal controller wiring if there is a short circuit or otherwise).

-Bill

Dave Angelini

Agree with Bill as usual. The Schneider high voltage MPPT does not like much more than 25% over paneling for grid tie. As Bill said the voltages, heat, and current are a moving feast. For Offgrid it still is a failure mode that is logged for warranty. I remember during testing the subject was brought up in the circuit evaluation testing in 2010. 

I have a client or 2 up in Alaska the severely over panel mppt 600v for winter and the shut down strings
the other months. You are the boss and it is, as Bill said, your wallet✔👍

Desert Rat

Thanks, guys.
Bill, this is a new way of thinking for me. Outback says 2000W maximum array @ 24V for the FM80. I guess that's what you're calling the "safe" specs. I realize there is a big difference between STC and real world conditions. As I understand it, the 150V maximum for the FM 80 et. al. is a hard and fast rule. So if I used the FM80 I would have to go 2S4P and deal with the resulting bigger gauge wire from the array. 

Dave, have you actually used the Xantrex SCP with the 100-600 controller? I had another "engineer" tell me it wouldn't work I believe you, though, especially if you've seen it yourself.

Dave Angelini

He may be assuming you have an XWP and you need the Insight device for that.  XW, XW+ and SCP are out of memory and do not have adjustments for new features/FW. You will be able to adjust the basics with SCP and I have them running.

For the cost difference insight home will allow you to see the solar from anywhere,
 and even without an internet connection at your home on a puter, tablet, or S. phone.