Over current with SB2000

System

The SB2000 charge controller was advertised as a 25A controller.

Great, I thought, I've got a little less than 24 A out of my pv array and this controller will be just right.

But when I got the controller I saw that it is rated at only 20A input.

This makes it useless to me unless it is capable of running with a little overcurrent.

Can it?

niel

Re: Over current with SB2000

it would be wasted as the controller is limited. if it's not too late you should ask who you bought it from if you could send it back for credit towards another controller. also, it is possible to get another mppt controller and just parallel to that one, but the sb2000 is 12v only if that's ok with you to continue with 12v inputs.

crewzer

Re: Over current with SB2000

Can you tell us more about the source of the 20 A spec? Blue Sky's online spec sheets and the SB2000 manual clearly state a 25 A spec.

It's possible that the requirements NEC 690.8 were applied to arrive at the 20 A spec. Specifically, 690.8 requires that an electronically-limited controller be specified for 125% of an array's short-circuit current spec. However, this would also mean that the controller's recommended application would be for an array rated at 18 A Imp, since Imp is ~90% of Isc.

Note that you may run into similar issues with other controllers. For example, my MX60 is rated for 60 A of output current, but the NEC calculation limits the input current to 48 A Isc, and this latter value is specifically cited in the owner's manual.

HTH,
Jim / crewzer

System2

Re: Over current with SB2000

True Crewser, the manual states 25A output. But the input is limited to 20A. So sayeth the manual that came with my controller.

Maybe I can assume that my panels will produce max I for so short a time that I can run the controller a little over current.

My array changes as friends lend me their unused PV panels and then take them back as they rearrange their systems. But I will be using 2 KC130's and one KC120 as the basis for my system and adding whatever temporary panels I get.

Right now my 220AH bank is charged by 10AM (on a normal day) but I want to install electric refigeration so my load will increase.

I'm not sure about the value of an MPPT controller for a 12V system that won't allow you to wire your PV panels in series.

westbranch

Re: Over current with SB2000

Gentlemen, THANK YOU all for this [b]very important[/b] piece of information..

I have a SB 2000e and was toying with adding 3 more Mitsubishi 120w panels, rates at 6.8 A, total of 4, thinking that the slight overload would not be an issue to the 25A controller. (ie 125 % of 25 A) Now I see that I am limited to 3 panels max!..

thanks again

Eric

crewzer

Re: Over current with SB2000

OK… I think I understand the problem: The original SB2000 is rated for 20 A. The SB2000E -- a later model -- is rated for 25 A.

See: http://www.blueskyenergyinc.com/pdf/SB2000ManualE.pdf
and: http://www.blueskyenergyinc.com/pdf/ManualRVPPSB2KErevE.pdf

MPPT can still be useful without DC-DC down conversion. The ability of the SB2000 / 2000E to operate a PV module at/near its MPPT voltage instead of at just the battery voltage may allow it to deliver some additional charging current to the batteries. The benefit of this feature is most evident when the battery voltage is low and/or the PV module is cold, i.e., in the winter.

However, when using low STC Vmp modules (i.e., the KC120 at 16.9 Vmp STC) in a warm/hot environment (low operational Vmp) and the batteries are not significantly discharged (bulk charge voltage is fairly high), there won’t be much “left over” voltage to convert to extra charge current.

HTH,
Jim / crewzer

westbranch

Re: Over current with SB2000

Jim, I just reviewed the manual for the 2000 E and near the bottom of page 3. item 3 i , it also says not to exceed 20 A @ 25* so , as I calc., I am limited to 3 panels under 6.7 A of short circuit current (3 x 6.7 = 20.1)... correct?

Eric

crewzer

Re: Over current with SB2000

Eric,

Good catch! That instruction is probably related to the NEC issue I mentioned earlier. Curiously, The "old" SB2000 (20 A) contains the same 20 A guidance (pg.2, para. 6.g) :?

It's my understanding that the original reason for the NEC's 125% multiplier was to allow for unusually high insolation excursions that could produce higher than rated output current. These conditions include exceptionally bright mid-day Sun, high-altitude applications, reflections off snow and ice, and so-called "edge of cloud" events.

MPPT technology is now another contributing factor. For example, an MPPT controller could supply almost 25 A at 14 V on a cold day from an array rated at 17.6 Vmp x 20 A Imp. So, yes, you are technically limited to 20 A Isc from your PV array.

As a practical matter, my controller rarely delivers more than 22 A when charging my "24 V" battery bank from my 725 W STC array. However, I've seen it briefly deliver >30 A in the winter.

HTH,
Jim / crewzer

westbranch

Re: Over current with SB2000

Right, I have the potential for one of those 'odd' situations, the cabin is at 3000 ft, we get - 30* C (-20F) frequently in the winter with lots of snow... seldom above 5* F.

As well I count on that increased input in the winter due to low sunlight hours. I even go so far as to orient the panel to vertical, cant get to negative angles, to max out the interception... :evil: Our old radio tech tweaked things one winter and said - 5 degrees is the best for intercepting snow bounce on a solar panel.

good to know these things !

thanks,
Eric

crewzer

Re: Over current with SB2000

Eric,

You'll definitely see some MPPT benefit in you location in the winter. However, you'll have to manually adjust the controller's MPPT potentiometer to realize the most benefit (highest output current in Bulk mode) during the year. For example, you'll probably find you need a "low" setting in the summer and a "high" setting in the winter.

HTH,
Jim / crewzer