Adding 200 watts more than maximum?

bbbuddy

I have seen several internet discussions about true maximum array size versus "name-plate" maximum array size for the FM 60 and 80.

We have an FM60 and 800 watts of 75 volt panels paralleled to it...which on a 12 volt system is what Outback says is max- 800 watts.

However these discussions point out that with higher voltage arrays the amps in is much less than planned for by the manufacturer...in the FM60 it is 48 amps in maximum.

Now, our panels' IMP is 1.34, and ISC is 1.66, so with 8 panels we never get anywhere near the max.

When we bought our 8 panels a neighbor also bought two of the exact same ones. He changed his mind about them later (I think he didn't want to pay for a more expensive controller, which I had warned him he would need) so he asked us if we would buy them...

we did, but they are just stored right now since we are supposedly "maxed out" according to Outback. Waste of good wattage...

With our array at 75 volts in, we have never seen the voltage go above 99 in winter.

Any way, I was thinking maybe we should just go ahead and hook up these two extra panels, for a total of 1000 watts, but only 13.4 amps into the controller.

Oh, also our 8 panels are over a year old so running at less then 800 watts, the 2 extra ones would not take us that far over 800, maybe to just under 900 watts max to start, and after several months we probably would "settle" at just over 800 watts.

Opinions?? Bad idea or go for it?

raydias

Re: Adding 200 watts more than maximum?

From what I have read you would benefit from extra wattage at the early and late part of the day. The charge controller might clip the power down at some point and that would increase the heat coming from the controller. At least that is how I have read the documentation and various posts

Cariboocoot

Re: Adding 200 watts more than maximum?

Charge controller Amperage ratings are for the maximum Amps out, not in.
Basically a 12 Volt system @ 60 Amps is 720 Watts on the output side. You can run more panel than that on the input because panels do not average their STC rating. Outback recommends 800 Watts maximum on an FM60. You can run a bit more than that, but any power available over 60 Amps out will just be "clipped" (lost). Going to 1000 Watts will result in a lot of clipping on sunny days; about 4 Amps lost.

Also, since it is an MPPT type controller the input Voltage and current is not strictly relevant to the output. All that matters on the input side is that the array Voltage is above the battery Voltage so that charging can occur and that the Voc isn't above the controller's input max so it won't shut down.

Couple of other things. Your array at 75 Volts isn't going to be the most efficient on a 12 Volt system. Your battery bank @ 790 Amp hours will want that FM60 maxed out - and about 19 Amps more.

Vic

Re: Adding 200 watts more than maximum?

Charge Controllers, like the OB FMs are rated on OUTPUT current.

It is not the best idea to rely on the CC to always limit the current to a safe value, altho, some folks do do it.

Under certain conditions, PV modules can output more than rated current and so on. So you cannot alays rely on PV outut being well below ratings.

Have you tried asking this question on the OB Forum?:
http://www.outbackpower.com/forum/index.php

Some there recommend against relying on the CC to always limit output current to a self-protective value. BTW, what is the rating of your output circuit breaker (fuse)?

Vic

BB.

Re: Adding 200 watts more than maximum?

In general, a true/good quality MPPT charge controller is designed to limit its output current to the rated maximum. You should be able to put a lot more power on the input and not have any problems (other than you should have a fuse that is ~Rated-Current*1.25 if the source has a lot of current potential on the input).

As I understand, many times mfg. of MPPT controllers test them with battery inputs... And some are even rated/spec'ed (MorningStar MPPT controllers--at least as I understand) for use as down converters for battery to battery charging.

So, 60 amps FM controller and 800 watts with 95% efficiency:

• 800 watts * 1/60 amps battery = 13.3 volts nominal battery voltage

So, 800 watts of "true power" on the output will give you give you ~maximum output current to a 1/2 charged battery bank.


Now, I like to use 0.77 as a derating for solar array and controller losses... That means, on a "nominal" day/temperature, the "optimum" battery bank would be:

• 800 watts * 1/0.77 nominal losses = 1,039 watt solar array

So, the 1,039 watt solar array is the nominal maximum for charging based on 13.3 volt battery bank.

Or, you can also use:

• 60 amp output * 14.5 volt battery bank * 1/0.77 = 1,130 watt solar array

In the end, you can go higher, and everything will be OK... But you start losing a bit of power mid-day as the controller limits to 60 amp rated output current.

-Bill

bbbuddy

Re: Adding 200 watts more than maximum?

Cariboocoot wrote: »

Charge controller Amperage ratings are for the maximum Amps out, not in.
Basically a 12 Volt system @ 60 Amps is 720 Watts on the output side. You can run more panel than that on the input because panels do not average their STC rating. Outback recommends 800 Watts maximum on an FM60. You can run a bit more than that, but any power available over 60 Amps out will just be "clipped" (lost). Going to 1000 Watts will result in a lot of clipping on sunny days; about 4 Amps lost.

Also, since it is an MPPT type controller the input Voltage and current is not strictly relevant to the output. All that matters on the input side is that the array Voltage is above the battery Voltage so that charging can occur and that the Voc isn't above the controller's input max so it won't shut down.

Couple of other things. Your array at 75 Volts isn't going to be the most efficient on a 12 Volt system. Your battery bank @ 790 Amp hours will want that FM60 maxed out - and about 19 Amps more.

Well Cariboocoot, I know the FM60 rating is for output, but these discussions were about Outback's INPUT amperage maximums, which they must have gotten from Outback since I can't find it in the manual...

Outback efficiency charts show we would gain 3% in efficiency by going to 24 volts instead of 12 volt.

Also, because of panel derating, there is no way I ever get 60 amps out, thus the idea of adding these two extra panels...getting the output amps closer to 60...right now the highest I see is about 49 amps out.

bbbuddy

Re: Adding 200 watts more than maximum?

Vic wrote: »

Charge Controllers, like the OB FMs are rated on OUTPUT current.

It is not the best idea to rely on the CC to always limit the current to a safe value, altho, some folks do do it.

Under certain conditions, PV modules can output more than rated current and so on. So you cannot alays rely on PV outut being well below ratings.

Have you tried asking this question on the OB Forum?:
http://www.outbackpower.com/forum/index.php

Some there recommend against relying on the CC to always limit output current to a self-protective value. BTW, what is the rating of your output circuit breaker (fuse)?

Vic

umm, I kinda figured they'd stick with what their manual says....

right now the most amps out I see is around 49, so my charge controller is most definitely not current limiting my output...

my panels are all in parallel, with an 8 amp breaker for each panel.

Cariboocoot

Re: Adding 200 watts more than maximum?

bbbuddy wrote: »

Also, because of panel derating, there is no way I ever get 60 amps out, thus the idea of adding these two extra panels...getting the output amps closer to 60...right now the highest I see is about 49 amps out.

That is something to be concerned about.
Now it is possible that the batteries never get low enough to demand full current. In which case you are fine.
It is also possible that the panel angle isn't the best or that there is too much power loss in the wiring. That is not fine.
By my calculations seeing about 51 Amps peak could be expected, so 49 is not that far off.

I don't know what they've done to the FM60 as there was never an input Amp limit on the MX60: you want to put 60 Amps in and 60 Amps out you can.

To go for the short answers:
Can you add 200 Watts more? Yes.
Will it work? Yes.
Do you want to do this? No; because it is never a good idea to run any piece of equipment at its maximum consistently.

bbbuddy

Re: Adding 200 watts more than maximum?

BB. wrote: »

In general, a true/good quality MPPT charge controller is designed to limit its output current to the rated maximum. You should be able to put a lot more power on the input and not have any problems (other than you should have a fuse that is ~Rated-Current*1.25 if the source has a lot of current potential on the input).

As I understand, many times mfg. of MPPT controllers test them with battery inputs... And some are even rated/spec'ed (MorningStar MPPT controllers--at least as I understand) for use as down converters for battery to battery charging.

So, 60 amps FM controller and 800 watts with 95% efficiency:

• 800 watts * 1/60 amps battery = 13.3 volts nominal battery voltage

So, 800 watts of "true power" on the output will give you give you ~maximum output current to a 1/2 charged battery bank.


Now, I like to use 0.77 as a derating for solar array and controller losses... That means, on a "nominal" day/temperature, the "optimum" battery bank would be:

• 800 watts * 1/0.77 nominal losses = 1,039 watt solar array

So, the 1,039 watt solar array is the nominal maximum for charging based on 13.3 volt battery bank.

Or, you can also use:

• 60 amp output * 14.5 volt battery bank * 1/0.77 = 1,130 watt solar array

In the end, you can go higher, and everything will be OK... But you start losing a bit of power mid-day as the controller limits to 60 amp rated output current.

-Bill

It looks like you are suggesting that 1000 watts could be ok in my case...the derating is what I was figuring. 1000 watts would derate at or just to under their 60 amp max out.

It just seems silly to have them here and not use them IF it isn't going to hurt my FM60...

Cariboocoot

Re: Adding 200 watts more than maximum?

Keep in mind you could add 200 Watts of panel and still see only 49 Amps of current if that is all the batteries need.

BTW, the typical "12 Volt" panel has a Vmp of 17.5. Multiply that by the "48 Amp input limit" and see what you get: 840 Watts.

bbbuddy

Re: Adding 200 watts more than maximum?

Cariboocoot wrote: »

That is something to be concerned about.
Now it is possible that the batteries never get low enough to demand full current. In which case you are fine.
It is also possible that the panel angle isn't the best or that there is too much power loss in the wiring. That is not fine.
By my calculations seeing about 51 Amps peak could be expected, so 49 is not that far off.

I don't know what they've done to the FM60 as there was never an input Amp limit on the MX60: you want to put 60 Amps in and 60 Amps out you can.

To go for the short answers:
Can you add 200 Watts more? Yes.
Will it work? Yes.
Do you want to do this? No; because it is never a good idea to run any piece of equipment at its maximum consistently.

Panel angle: we change it 4 times per year, right now it's at 10% angled due south for summer.
The wiring is fine, in fact it's mostly overkill...we measure virtually the same at the charge controller input as we do from the combiner box. Remember, we are running high voltage panels. We don't have the "normal" wire loss. In fact we could go more than 3 times further before we would hit a 1.5% loss according to Outback charts.

We "may" go higher than 49 amps sometimes, but I don't watch it continuously. That is the highest I've ever seen lately since the panels got past the break in period. We were seeing higher in the beginning.

We generally don't go below 80% on the batteries. Sometimes a bit lower, but not often or much...

Cariboocoot

Re: Adding 200 watts more than maximum?

Sounds like you're good to go (especially with not discharging below 80%).
The extra panels may add just a bit more kick on those cloudy days making the difference between "uh oh" and "okay".

niel

Re: Adding 200 watts more than maximum?

"Panel angle: we change it 4 times per year, right now it's at 10% angled due south for summer."

now if you meant 10 degrees then you are overshooting on the angle. assuming you are at about the 36 degree latitude mark then subtracting 23.5 degrees for the solstice then it's 12.5 degrees. anything below this angle will not be going in a good direction as the sun will never be that directly overhead in your location. i would advise at least the 12.5 degrees, but seeing as that is the highest point in the sky the sun will be then i'd recommend at least a 15 to 20 degree angle for your summer angle with 15 degrees good primarily around the solstice only.

Cariboocoot

Re: Adding 200 watts more than maximum?

Check your panel angles here: http://www.macslab.com/optsolar.html

It gives a good explanation of the why as well; that the average daily production is often had at a different angle than what is "ideal" for precisely solar noon.

bbbuddy

Re: Adding 200 watts more than maximum?

niel wrote: »

"Panel angle: we change it 4 times per year, right now it's at 10% angled due south for summer."

now if you meant 10 degrees then you are overshooting on the angle. assuming you are at about the 36 degree latitude mark then subtracting 23.5 degrees for the solstice then it's 12.5 degrees. anything below this angle will not be going in a good direction as the sun will never be that directly overhead in your location. i would advise at least the 12.5 degrees, but seeing as that is the highest point in the sky the sun will be then i'd recommend at least a 15 to 20 degree angle for your summer angle with 15 degrees good primarily around the solstice only.

We did use http://www.macslab.com/optsolar.html for the angles. With our latitude and their formula, we should be at 7.34 at this time of year, which we are actually pretty close to...

bbbuddy

Re: Adding 200 watts more than maximum?

Cariboocoot wrote: »

Check your panel angles here: http://www.macslab.com/optsolar.html

It gives a good explanation of the why as well; that the average daily production is often had at a different angle than what is "ideal" for precisely solar noon.

Yes we used their formulas for changing the tilt angle 4 times per year. That's a VERY good resource.

niel

Re: Adding 200 watts more than maximum?

i am in disagreement with that as it makes no sense to aim it at a high point in the sky that the sun will never be at.

i am not going to argue over it either as we've had discussions on it before, but i will just disagree with it.