proper sizing of controller to array

kylew · December 2011

hello, I live on a small sailboat in SoCal and have been living off the grid (for about a year now) with a small 110W array of panels. I run them into a 10 Amp morningstar controller to a bank of 140 AMPHR AGM batteries. My battery bank is sized so that I rarely if never ever go below 50% and they almost always recharge fully within a day. I try to keep the panels facing the sun, unshaded by anything, but being on a moving boat I know I am never getting full juice out of them very often. I came into another 68W panel recently and am wondering if I were to occaisionaly wire it into the array when my needs may require a bit more joy, or when the sun was weak and a bit more surface area may help, what would this do to my 10 amp controller? I suspect I would rarely ever exceed 10 amps but if all the stars (and panels) were to allign and I happened to get that theoretic perfection, I could exceed 10 AMpS. Would it blow out my controller, or simply limit my input to 10 amps?

thanks for any help
kyle

icarus · December 2011

Re: proper sizing of controller to array

My guess is that you would probably be fine,, but I would consider getting a 15 amp controller instead. A PWM MS controller is pretty reasonable.

You are discovering the major pit fall of off grid,,, the loads always grow with time, and controllers are always the limiting factor when your try to grow a system. ( I have been through 4 each bigger than the last.). I would consider getting either a MS MPPT or a Rouge now, and assume th at your system will eventually grow into it.

Tony

RCinFLA · December 2011

Re: proper sizing of controller to array

For a PWM controller you should not exceed the rated input current of the controller. A good PWM controller will sense too much current, or temp, and shut down to keep from overheating the series MOSFET. In this case you just lose power because of the duty cycling to keep from overheating.

Pretty sure the Morningstar has these limit checks.

A cheap PWM controller may not have the overcurrent sense and just overheat, damaging the unit.

MPPT controllers just go to their limit and no further. They will keep producing at their rated output current limit.

westbranch · December 2011

Re: proper sizing of controller to array

Adding that 68w panel with the 110w panel will give you some but no much additional power. For multiple pv systems the panels should be 'matched' in output to maximize your production.
You could get a cheap PWM controller (5 A or so) for the small panel, that would give you the most bang for the buck with unmatched panels. But you could not expand again. Same problem you have now, 1 panel 1 CC.
Also if possible try to not use as much power daily to get more life out of those batteries. Since you are in a daily warm SUNNY location, I would seriously consider a Rogue MPPT CC and a matched 110 panel to max out your harvest and get to float daily . Under charging is the battery killer (sulphation).

HTH

Cariboocoot · December 2011

Re: proper sizing of controller to array

For those who like to see theoretical calculations

110 Watts + 68 Watts = 178 Watts.
@ 77% efficiency = 137 Watts.
137 Watts / 12 Volts nominal (minimal) = 11 Amps potential current.

That's under good conditions. Low sun, poor panel angle, high temperatures will all reduce the maximum potential. It is unlikely such an arrangement would exceed the controller's 10 Amp max long enough to do any damage. If you're really worried about it, run it through a 10 Amp fuse and let that blow before the controller does.

Be sure to check the Vmp on all the panels to be certain there aren't any that are far out of spec (more than 5% say) and thus not really adding anything to the mix.

niel · December 2011

Re: proper sizing of controller to array

coot,
that might be too borderline to chance damaging the controller. either buy a bigger controller for them all or another small controller for just that 68w pv when you want to add it.

Cariboocoot · December 2011

Re: proper sizing of controller to array

Well if it's the Sunsaver 10 Amp it actually has a 5 minute 25% overload tolerance. In conjunction with the OP's original desire to "occasionally" wire it in and the other mitigating factors it doesn't look like too big of a risk to me.

But as they say in France chacun à son goût.

kylew · December 2011

Re: proper sizing of controller to array

thanks everyone for the good suggestions. I do have another 10 amp sunsaver i keep as a spare and it would be easy to just wire it in with the extra panel when needed, just to be on the safe side. (funny, i am rarely on the safe side....)
I did consider getting an MPPT CC at the git-go but for a system this small I figured it wouldnt compensate for the 4X price tag. As prices drop and my needs increase i may need to re-examine that. Are they really that much better than PWM?
cheers
kyle

Cariboocoot · December 2011

Re: proper sizing of controller to array

kylew wrote: »

Are they really that much better than PWM?
cheers
kyle

No, not on a small system. You are quite right that the extra price is hard to justify.
They would give you an advantage if you sail in cold climates or need to run higher Voltage array to overcome long wiring distances, et cetera. But for the most part the MPPT doesn't add much until you get into larger arrays.

Someone is bound to disagree with this.

niel · December 2011

Re: proper sizing of controller to array

don't forget that there are smaller mppt controllers out there, but he would need to watch mixing and matching different pvs to use on them. for series you want the currents to match up and for parallel you want the voltages to match up. some say within 10% and i like within 5% tolerance. the closer the match the better, but just be aware that the farther out the worse it is.

a few example mppt controllers currently would be the non certified rogue and the morningstar sunsaver 15.

proper sizing of controller to array

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