Posts: 349
Threads: 7
Joined: Aug 2014
40 amp DC circuit breakers are few and far between; I ordered a $60 monster from Amazon, it never shipped so I canceled the order.
Then I did some research:
https://www.youtube.com/watch?v=kRB7Z66brO0
AC breakers will trip with twice the current DC.
Anyhoo the real problem inbound to the controller from the array is lightning, so at worst what you have by using a non-functioning breaker there is a disconnect.
A surge from lightning along the negative wire (I.E. THHN 8 AWG Black) previously upset much equipment to the point of marginal functionality, so we need a double-throw for both positive and negative...
But YEA!!!! Added another rack and Saturday, the amperage exceeded 40 amps ... and she blew!
Bought a 25amp GE breaker at HD for $9, had it installed in 5 minutes, in the standard house box. The controller is 40 amp, but according to the documentation it can take 50 ok. Can't have the damn thing tripping all the time.
Just thought you would all like to know. Off grid engineering... it's about innovation!
***Still can't figure out how to spell 'car' correctly***
***Still can't figure out how to spell 'car' correctly***
Posts: 5,083
Threads: 84
Joined: Feb 2009
A youtube video is not really a good source.
Because of the difference in the way they work, a very large surge would weld the contacts on an AC breaker and burn your house down.
Posts: 349
Threads: 7
Joined: Aug 2014
Been struck by lightning, dude. 30 Volts isn't that much.
***Still can't figure out how to spell 'car' correctly***
***Still can't figure out how to spell 'car' correctly***
Posts: 349
Threads: 7
Joined: Aug 2014
Main problem I see is in fact the lower voltage, the AC breaker contacts are cheap and don't conduct as well as a DC magnetic contact. (& @ lightning voltage, Neither are guaranteed to do diddly squat)
***Still can't figure out how to spell 'car' correctly***
***Still can't figure out how to spell 'car' correctly***
Posts: 14,138
Threads: 424
Joined: Aug 2012
40 amp DC circuit breakers are few and far between
Gexpro.
AC breakers will trip with twice the current DC.
Square D.
Posts: 2,485
Threads: 10
Joined: Feb 2008
I spent a lot of time fooling around with fuses until I found these. Never looked back.
https://www.solar-electric.com/mnepv.html
Posts: 349
Threads: 7
Joined: Aug 2014
Another 'thang... Praises and Hallelujah for the Xantrex C-40 controller. Mine is set at 24V; with bulk charge set to 28.5V (only 2v less than panels at max). This beast is the Jeep of controllers; it's rock solid reliable and survived a lightning strike. I tried an off-brand MPPT controller; the C-40 performed much much better in my humble opinion. The big seller of the MPPT controller is it's ability to convert excess voltage into amperage. I found that this actually is not that useful, the main problem not being use of energy when conditions are optimal but performance when conditions are cloudy. Because the C-40 does nothing period with the power under those conditions it is more efficient, delivering 100% of what power is comming in under low light conditions.
It's also cheaper; you can pick up a C-35 for less than a hundred dollars.
On the downside, you can't monitor your power remotely from the internet or get a composite image of power from all arrays in the area.
Oh well, life is suffering!
***Still can't figure out how to spell 'car' correctly***
***Still can't figure out how to spell 'car' correctly***
Posts: 2,485
Threads: 10
Joined: Feb 2008
My understanding of solar PV theory is admittedly crude and it goes something like this. Picture photons of light coming in like hailstones and hitting the PV cells like hailstones hitting water. Each photon "splashes" an electron up through an electric field like the hailstone splashes drops of water up into the air against gravity. If a collection device (cup, bucket, gutter) is placed where the droplets of water can be collected before they fall back down to the water's surface from which they started out, then you could run a water wheel and get useful work. Similarly if the electrons are captured somehow they can be used as "electricity" as they attempt to return to the voltage they were originally dislodged from. The million dollar question in each case is how high to you hang the bucket? The higher you collect the droplets/electrons, the more energy to be had per droplet/electron as it rides the waterwheel/motor back down but the fewer make it that high. It's a "how sharp is the bell curve" thing.
In the case of the hailstones hitting the water I can see less intense hail splashing the rain less high meaning a wider bell curve. If there is a similar relationship between less intense light and less energetic electrons then as the light weakens due to clouds the panels will put out both fewer electrons and less energetic electrons so you should hang the bucket lower.
TL/DR: An MPPT charge controller will allow you to add an extra panel to each string since it automatically adjusts the height of the bucket. The benefit on cloudy days when the panels might otherwise not produce at all more than makes up for the modest penalty for doing so extracted when the light is high. Truest if VOC varies a lot with light intensity. Less true if all electrons knocked loose by each photon are equally energetic (always splash equally high) and vary only in number of electrons produced. The salient point is that there is a rather precipitous cut-off in production as the light dims and the native voltage produced by the panel string is no longer enough to push electrons into the battery bank. MPPT controller lets you design for that worst case.
Posts: 14,138
Threads: 424
Joined: Aug 2012
The big seller of the MPPT controller is it's ability to convert excess voltage into amperage.
Oversimplification of best use case.
MPPT allows your panel array to run at a different voltage than your batteries.
- smaller combiner boxes -- or none at all
- surplus panels with "weird Voc" work great
- reduced power loss and/or wiring expense for distant panels
- freedom to change battery voltage
Several years ago, I picked up some surplus panels that were custom-designed for a specific grid-tie application: they had a Voc of 25V -- useless with a PWM controller. With 3 in series (no combiner box!), an MPPT controller could charge batteries at 12V, 24V, or 48V; the high array voltage made for negligible loss over a long run of #10 wire. In this design, the "expensive" MPPT controller saved way more money than it cost -- and the system still charges (a little) when the panels are in the shade.
Posts: 7,795
Threads: 694
Joined: Jun 2011
The simplistic way I interpreted it was that PWM charge controllers function by limiting the amount of energy that goes to the batteries, and MPPT controllers function by maximizing the amount of energy that goes to the batteries. There are several reasons why but once I got to that level of knowledge it gave me a raging brainer.
