Mix and match panels on a SB 2000e

westbranch

I read a thread over on the Outback site about Crewzer's solar menagerie. My question is about the advisability/performance of putting together the following:

the SB 2000 has a 20 amp input rating / max 25 amps.

I have 2 Mitsub 120 w panels Isc 7.75 A, Imp 6.69 A total Imp = 13.38 A, Voc 22.0, Vmp 17.6
I want to add another panel to get closer to the 3%-of-capacity target for charging.

3 panels will give me 20.46 A Imp. Over the 20 amp design spec. so no go...

My local shop has a 110 w Mitsub panel Isc 7.16, Imp 6.43 A, Voc 21.2, Vmp 17.1.

Using the 110 w panel would get me just under 20 amps (19.81) but what would the effect of the mix/match be? would I still get the 19.81 A I am after?

TIA

Eric

mike95490

Re: Mix and match panels on a SB 2000e

westbranch wrote: »

Using the 110 w panel would get me just under 20 amps (19.81) but what would the effect of the mix/match be? would I still get the 19.81 A I am after?

Maybe. So we are on the ragged edge. Do you currently see the full power from your panels, or are you shy the normal 15% ? Any idea what cloud edge effect has on your system - will it exceed the 25A ? (only lasts a few seconds, not minutes)

westbranch

Re: Mix and match panels on a SB 2000e

Mike, not sure about reaching max panel output, only there on weekends and in the summer, if it will happen it is now, spring time, cool clear skies. No batt monitor as yet, XBM on order.

No idea as to edge effect, we get lots of potential as in the mountains.

Eric

crewzer

Re: Mix and match panels on a SB 2000e

Eric,

This post may set off a howl of protests. I urge you to read the post in its entirety before drawing any conclusions or making any decisions.

My interpretation of the NEC is that the SB2000E’s input current limit is 20 A Isc STC, and not 20 A Imp STC. This issue doesn’t seem to be addressed particularly well in the 2000E’s manual, but it is fairly well covered in the manual for the similar 2512 controller.

Since the total Isc of your two present modules in parallel is 15.5 A, you’ve only got 4.5 A of Isc NEC-compliant-current-headroom remaining. That translates into a "12 V" module rated at ~70 W STC or so.

One issue is whether you want to follow the NEC requirements or not. Since the SB2000E’s output current limit is 25 A, you also need to consider the cold-weather MPPT effect on array voltage to the controller and charge current from the controller. Or, you can design for a 95% “safe” solution and hope that the controller’s thermal management and output current limiting features do their jobs.

In that case, you could hook up an array rated at (14.4 V x 25 A) / 90% = 400 W STC and hope for the best. My gut feel is that the 350 W array you’re considering would probably work just fine >99% of the time, but I don’t know enough about your array’s environment.

Another possibility might be to place a switch in series with the 110 W module and disconnect that module from the array when ambient temps are low, PV array voltage is high, battery voltage is low, and charge current is high.

HTH,
Jim / crewzer

BB.

Re: Mix and match panels on a SB 2000e

Eric,

I would contact Blue Sky Tech support and ask them about the input/output current and if the unit has internal current limits or not...

Personally, I am of the opinion that a well designed MPPT controller can easily withstand being connected to panels that output upwards of 25% (or more--depending on wiring and equipment connected) without any issue at all.

A MPPT controller should have internal limits on power conversion and output current--just like virtually every computer switch mode power supply made in the world today does.

You are looking at STC rated panels, and I find (in the SF Bay Area which does not get very cold) that my Grid Tie system rarely (maybe once or twice a year) gets close to the PTC ratings (~85% or so of the STC rating) of the system...

-Bill

westbranch

Re: Mix and match panels on a SB 2000e

Thanks Jim and Bill, I do have an inquiry off to BlueSky on the units limits, still waiting for a reply.

I expect (hope) their answer will clarify the Isc vs Imp issue.

Eric

westbranch

Re: Mix and match panels on a SB 2000e

Jim, Bill, I found the reference to 20 A Isc MAX .
It is buried in the fine print in installation instructions... rather than specs...
So that puts an end to that plan, taking into account edge effects, safety margins, etc.

Hope the Classic get released soon... or an MX60 will be needed... jump in with both feet eh?

thanks again for the help.
Eric

niel

Re: Mix and match panels on a SB 2000e

to add a footnote here, out of curiousity, i looked at my sb50 specs and it also mentions isc and not imp. now that's an interesting way of upping the appearance of one's specs to be slightly better than they are.