Charge controller for uneven array of panels

Registered Users Posts: 27
I'm trying to figure out the best way to charge my 48v 450ah battery bank with some new solar panels I have. I ended up with an odd amount of them and on top of that I would like to use my old panels as well.

I have 11 250w panels and 2 150w panels.
The 250w panels are 30.4v Vmp, 37.5v Voc at 8.22a Imp
The 150w panels are 17.5v Vmp, 21.5v Voc at 8.56a Imp

With the Xantrex XW 60 mppt CC I can series the 250w panels in strings of 3 max. That would leave me with 2 left over and the 150w panels. Can these 4 be wired in series for a fourth string on the same CC without lowering the overall charging output too much?
112.5 Voc vs 118 Voc and the small amperage difference don't seem like a big deal to me.
All help is appreciated. If this isn't doable I will have to decide between a second CC and just not using all of the panels.

• Banned Posts: 17,615 ✭✭
Re: Charge controller for uneven array of panels

Let's look at it like this: a 60 Amp charge controller on 48 Volt system can handle about 2880 Watts of array. In addition, the maximum Voc for the controller is 150. That means strings of no more than three of the 250 Watt panels (37.5 * 3 = 112.5, add one more and you are right on 150 with no margin for error). Basically with the panels you've got you could have three parallel strings of three on that controller. That would be 2250 Watts, leaving roughly 630 Watts to the limit.

Basically, this is not a set of panels that will play well together. The Vmp of the 150 Watt panels is too low, even when put together in series (17.5 * 2 = 35, significantly higher than the 30.4 of a single 250 Watt). Since you don't have workable multiples of either panel type, any combination of the two will come with a power loss over the maximum potential.

Best way that I can think of is to use the nine 250 Watt panel array on the C60, then put together another array with the remaining 250 Watt panels and 150 Watt panels (the Imp's are close enough to series connect without significant loss). That would be 800 Watts and could be handled by another MPPT charge controller, but it wouldn't put out much more than 16 Amps @ 48 Volts.

Ideally you'd sell/trade the 150's for one more 250 and put together four strings of three (3 kW array). That would be just slightly above the 2880 "max" and wouldn't be a problem.

But your battery bank doesn't really need more than the 2250 Watts.
• Solar Expert Posts: 10,300 ✭✭✭✭
Re: Charge controller for uneven array of panels

maybe i'm missing something here, but i think it would work even though it may be more power than the cc can deliver at times. you have 2 extra pvs at 250w and essentially would need a 3rd to make things identical to the other strings. put the 2 150w pvs in series and then place them in series with the 2 250w pvs. the imp of the 2 150w pvs is a tad larger than that of the 250w pvs, but is within tolerance even for my standards at 4.13%. the voltage will also go a tad higher with this string because of the extra the 2 150 watters will have making this oddball string 30.4v x 2 + 17.5v x 2 = 95.8v vmp and the other strings will be 3 x 30.4v = 91.2v vmp. the oddball string is just a bit over 5% higher in voltage than the rest, but doable. if the cc can tolerate the top end charging for long terms then go for it.
• Registered Users Posts: 27
Re: Charge controller for uneven array of panels

Well I just read through the xantrex moot manual and it says it can harvest a maximum of 3500 watts but doesn't say where that number comes from. I'm guessing it is 60 amps x 58v charging. And cariboocoots estimate of 2880 is 60a x 48v. Both are probably right depending on the state of the batteries.
I would be fine with less than max output anyway since 60a into my batteries is just over 13% the capacity an hour which is about the max I see suggested.
But now I'm worried about it outputting it's max constantly. Has anyone had problems with this? Could it stress it somehow to failure?
• Banned Posts: 17,615 ✭✭
Re: Charge controller for uneven array of panels

I keep forgetting not everyone lives in a frozen wasteland. My objection to the two 250 Watts + two 150 Watts string is that the Voc would exceed the C60's max in cold conditions. If this is not an issue where you are it will work as Niel says.

As a rule, keeping any type of electronics running near its maximum current limit for extended times is bad. But really this isn't going to happen with a charge controller because as the batteries charge the current will go down regardless of the potential from the panels. Most of the time the controllers will not see maximum current anyway (in this case an 'ideal' of 45 Amps at 48 Volts), as the batteries won't need it.

Many of us who use the old MX60 have found they are most efficient at about 75% peak capacity (45 Amps), but that is another issue and doesn't necessarily apply to the Xantrex C60.
• Solar Expert Posts: 10,300 ✭✭✭✭
Re: Charge controller for uneven array of panels

are you having a senior moment coot?:p he never mentioned a c60, but did say the mppt 60a cc.

generic,
i guess you could fire off an inquiry to xantrex and ask them if it is rated to be continuous at max output. it would be they who would warrant it and know its abilities best. if you should hear back from them then let us know what it is that was said.
• Banned Posts: 17,615 ✭✭
Re: Charge controller for uneven array of panels

All my moments are senior, Niel.
But it still beats the alternative.
• Solar Expert Posts: 2,348 ✭✭✭✭
Re: Charge controller for uneven array of panels
All my moments are senior, Niel.
But it still beats the alternative.

Amen to that Coot!
• Solar Expert Posts: 10,300 ✭✭✭✭
Re: Charge controller for uneven array of panels
All my moments are senior, Niel.
But it still beats the alternative.

very true. i guess my comment would be my senior moment as all of mine are senior as well.:p or was that senile? hmmm.