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Rick F avatar image
Rick F asked

Picking a proper charge controller

Hi all.. I'm new here and this is the first time I'll be setting up a small off-grid system of my own.. I'm fairly experienced with computers and electronics..

So.. my question.. I'm thinking about buying some 350 or 400W bifacial solar panels I'd like to put to use, brand new (Vmax: 40.8v, Voc: 48.9, Imp: 9.81A, Isc: 10.33A). If I'm going to get 4 of these to start with then the Nom PV Power is 400W/panel. I'm assuming that I'd have 1600W nominal for all 4 of them..

In looking at the different MPPT charge controllers, I was looking at the 250/70 configured to use 24V.. That would put my max nominal PV power at 2000W for that config. I know that this is a kinda spendy controller.. Am I looking at this the wrong way or am I about on target? I know that I could run a smaller controller at a higher voltage rating (e.g. 48V on the 150/35) to get the same 2000W max power rating.. I suppose that perhaps these controllers might have the ability to handle multiple strings but I don't think that's the case here.

I just wanted to check with you all to make sure I wasn't missing something and perhaps ordering something I'd regret if wrong. Right now I'm just doing my homework and trying to put the pieces together on paper.. Thanks!

MPPT Controllers
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2 Answers
Kevin Windrem avatar image
Kevin Windrem answered ·

How big of an off-grid system are you planning? That is, what is your maximum consumption? This may dictate your choice of battery voltage. But yes, you get more output power as the battery voltage goes up.

The solar calculator here will help predict solar output:

https://www.victronenergy.com/mppt-calculator

You'll need to put in specs for your panels and your location. Use -0.5%/degree C for voltage and current temperature coefficient.

Max PV input voltage is critical and can't be exceeded. Panel open circuit voltage goes UP as temperature goes down. So on the 150/35 you could wire at most 2 of those panels in series, then put 2 of those strings in parallel.

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Rick F avatar image
Rick F answered ·

Well.. we've got a mixed household with a handful of items I'd love to run off-grid to keep our summer energy bills a bit lower as mostly a science project... I'd like to be able to power 2 freezers and a fridge -- all of which are roughly 6A each.

So, initially I'm thinking about getting 4 panels but if I can fit a few more in my yard I'll do that.. I've found a local place selling some 200Ah AGM batteries that are brand-new -- so I'd like to have a nice bank of them to hold whatever I can.. I'm planning on peace-mealing this system together as time & money permits but want to design it for the worst case so I don't buy things twice if I outgrow.

I ran the calculator you linked to and need to run either a 36v or 48v system. The 48v gives more options -- basically the same device (150/35) with either MC4 or terminals -- with or without bluetooth. My voltage and C coefficients are -0.29 and .05 respectively per the spec sheet.

I put a string in the calculator containing 2 in series and 4 in parallel.. which I assume is talking about 8 panels in total (2x4). With that config, it seems like the 150/35 ought to be ok -- but I'm curious about one thing..

The spec sheet for some of these indicates that they're fanless and can run up to 104F.. I live in the high desert of Los Angeles and it can easily get 110F here in the summer.. Does that mean that this thing is going to have heat issues unless the charge controller is kept cool by means of an external fan or something? If it's going to start limiting its output due to heat then I need to look at other options.


One other thing.. since these panels are bifacial, the max power rating is a bit odd -- the table looks like :

(implied 0% bifacial gain) - 350w

5% bifacial gain - 368w

10% bifacial gain - 385w

20% bifacial gain - 420w

30% bifacial gain - 455w

With that data I'm assuming my power (Wp) in the calculator should be 455w and NOT 350.. correct?

Thanks in advance!!

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Kevin Windrem avatar image Kevin Windrem commented ·
The advantage of bifacial panels will vary depending on your installation. To get the 30% increase, you need a high reflectivity surface below the panels. I don't know for sure what you can expect.

I'm not sure what happens at high temperatures for the MPPT. If you are in that hot an environment, I recommend fan cooling, or try to locate the controllers in a cooler environment. The inverter's output will be reduced at high ambient temps also. Maybe the batteries also.

Get yourself a watt-hour meter. They are inexpensive and you can plug your appliances in one at a time and get a daily energy consumption figure for each. This is more accurate than taking the max current off the name plate since they probably won't run all the time. E.g., your two freezers and refrigerator may run 60% of the time, so your daily energy consumpiton would be 120 * 6 * 3 *.6 * 24 or 31,000 watt-hours. With 4 400 watt panels catching 6 hours of sun a day, you'd be getting 4 * 400 * 6 = 9,600 watt-hours or 22% of your consumption. But make your energy measurements to see what you actually need.

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