Hi guys,
As summer is coming along we will be generating a lot more power than we use.
As our climate is relatively warm I would like to limit the charge output to help preserve my Classic.
I understand it is designed to survive high temp operation however I don't see the point in having the FET's tick away at 60c+ all day for no reason if it can be avoided.
My question is, which is more efficient using onboard programming, to limit input amps or output amps ? Or, will the act of limiting charge output, within the Classic, generate significant heat itself, negating any benifit.
I understand I can disconect strings of panels to limit output but this is not an ideal option as I still want to maintain maximum panel output early/late in the day, or overcast days. Aside from this it would be nice to just have it automatic, not having to push/pull fuses all the time while trying to predict weather.
With the Classic, you can Limit Output Amps to whatever you want.
Local App
Config
Advanced, scroll down just a bit and you can see Input & Output amp + volt limits.
I would limit output amps to what the battery is rated for, about 10-15% of the total amp capacity of the battery is common. Read your battery manual, don't rely on my BS.
Quote from: wrt on September 22, 2015, 08:50:25 PM
Hi guys,
As summer is coming along we will be generating a lot more power than we use.
As our climate is relatively warm I would like to limit the charge output to help preserve my Classic.
I understand it is designed to survive high temp operation however I don't see the point in having the FET's tick away at 60c+ all day for no reason if it can be avoided.
My question is, which is more efficient using onboard programming, to limit input amps or output amps ? Or, will the act of limiting charge output, within the Classic, generate significant heat itself, negating any benifit.
I understand I can disconect strings of panels to limit output but this is not an ideal option as I still want to maintain maximum panel output early/late in the day, or overcast days. Aside from this it would be nice to just have it automatic, not having to push/pull fuses all the time while trying to predict weather.
Hi wrt,
In my opinion, if you are seeing FET temperatures in the high 50s, into the 60 C range, you are hammering on the Classic 150 a bit too hard, perhaps with warm/hot environmental temperatures where the Classic is operation. You may well note that you see greater heat in the Classic 150 under lightly-loaded conditions with good sun, than when the PVs are heavily loaded, and the Vin might be 12 - 15% lower. Large output currents with corresponding lower Vin from the PVs being heavily loaded can yield less heat than light loads with high Vin, from my experience.
Seems that you are running a String Vmp of about 106, or so volts, with a lightly-load Vin to the Classic, into the 120 volt range in customary operating conditions, plus a fairly large array, even if it is a Virtual Tracker. Believe that you have mentioned having something like 4,000 watts of PV power available under good sun.
Am assuming a few things, but seems that operating a Power Electronic device in the corner of High Vin, and High Temperatures is not a secret to long-term success.
Know that the MidNite head Engineering folks, like boB, have not had too many worries about this set of operating conditions ... but, there is this:
http://midnitesolar.com/Forum/index.php?topic=2270.msg21383#msg21383
I, too, had an opportunity to really load-up a Classic 150 with a lot of PV, with string Vmps of about 92 volts at STC Vmps, but decided that it is not a good idea to run any power electronic device in that manner, choosing to run a pair of Classic 150s, each with half of the array, PLUS, running 24/7 Air Conditioning in the power room.
The Classics, batteries, inverters, and I are all much happier ... IMO.
Not to be too judgmental, however. Good Luck! Vic
Quote from: wrt on September 22, 2015, 08:50:25 PM
Hi guys ...
My question is, which is more efficient using onboard programming, to limit input amps or output amps ? Or, will the act of limiting charge output, within the Classic, generate significant heat itself, negating any benifit.
I understand I can disconect strings of panels to limit output but this is not an ideal option as I still want to maintain maximum panel output early/late in the day, or overcast days. Aside from this it would be nice to just have it automatic, not having to push/pull fuses all the time while trying to predict weather.
The Output current Limiting on the Classics works very well, but, of course, it does set the Maximum current it will output for any/all reasons. AND, this Limiting, will often cause the PV modules to be relatively lightly-loaded. Light loading on the PVs will usually cause the Vin to the Classic to rise. This higher voltage will cause almost any MPPT CC to operate less efficiently, causing the CC to run hotter, in many cases.
One additional thing, is that using fuses as switches is a BAD idea, generally. The fuse contacts do not have any arc-quenching in their design. So, they MUST NOT be connected or disconnected, with any real DC current flowing in that circuit (you probably know this). And, there is probably a fairly low number of connect/disconnect cycles that the contacts are designed to endure.
Circuit breakers, like the MNEPV, and MNEDCs are designed for DC Switch service, AND will endure 5,000 - 10,000 switching/trip cycles.
Vic
Thanks for the reply Vic,
Regarding the fusing issue, I am aware of the need to not disconect under load, hence the comment above about having to "predict the weather" early mornings (no current) is the time for this.
As an aside, I used to have rated DC breakers and was advised that they where not ideal in this application, something to do with only tripping in one direction and the possibility of reverse current under certain fault conditions, so swapped them out for fuses.
As per your link, you can understand why I am a little nervous about temps even if they are designed to handle it.
We havnt had any hot days yet, but current temps are (taken yesterday) ambient 18c batt 11.5c PCB 46c FET's max 62.7c. At that time Vin ~107v output to battery ~4150w.
Reducing output to 3500w takes FET's to high 50's further reduction to 2000w means high 40's
One recent 26c day at ~4000w output Vin ~96v did see FET temp no higher than the cooler day, but I don't think panel volts drop much more than this as temps increase above the 30c mark.
The idea of a second Classic is not a bad one, financially as it would not cost a lot more than reconfiguring the array, but a LOT less work and not on the roof :)
Just curious as per my OP question, is there any efficiency difference in this instance between limiting the amps input, or, the amps output when using the Classic's menu.
Quote from: mike90045 on September 23, 2015, 02:39:28 PM
I would limit output amps to what the battery is rated for, about 10-15% of the total amp capacity of the battery is common. Read your battery manual, don't rely on my BS.
Hi mike,
I am running 400ah of lithium, so rated charge is far greater than the Classic can supply :-)
I have found the options you are referring to, hence my question about the difference between input/output limiting.
Quote from: wrt on September 23, 2015, 08:59:54 PM
...
As an aside, I used to have rated DC breakers and was advised that they where not ideal in this application, something to do with only tripping in one direction and the possibility of reverse current under certain fault conditions, so swapped them out for fuses.
that sound like questionable advice. Properly rated and installed DC breakers would very rarely have issues.
Quote
We havnt had any hot days yet, but current temps are (taken yesterday) ambient 18c batt 11.5c PCB 46c FET's max 62.7c. At that time Vin ~107v output to battery ~4150w.
All looks normal to me and well within acceptable temp range of FETs. Seems like a lot of unnecessary messing about switching in/out parts of the PV array.
You could try some passive cooling such as mounting the Classic on an aluminum plate. Or if the Classic's fans are always running then an external fan could help.
An A/C in room with controller would be ultimate solution but somewhat energy expersive.
dgd
Hi dgd,
Switching parts of the array is a lot of messing around, this is why I am avoiding it.
As for the cooling solutions suggested all but the aluminium plate (was tried but not enough contact area from unmodified case to make a noticable difference) are good ideas, however I think reducing the heat produced seems far more efficient than trying to get rid of it once it is created.
Our energy requirements are less and sun hours longer when it gets hotter, so we can do without the higher charge currents in summer. Using the Classics programming to reduce peak charge appears to be the least hasle, most efficient way to do this.
The anticipated result should be, no switching and a flat, moderate charge current for most of the day.
I do understand the unit is designed to withstand the temps above, however why shorten life needlessly running at high output and temps. Keep in mind the stated temps above are at 18c it will get to double that in a few months making the turbo fans job a little harder.