Is this controller right for this system?

[kables]

Hiya,
I'm almost ready to spend on my electronics, I thought I was finally grasping things but I'm second guessing everything now.
1. I have (2) 200 watt, 24 volt panels. I am wiring in parallel to stay at 24 volts.
Short circuit current totals only 11.66 amps. Open circuit voltage is 45.4 volts. Based on that, I calculated that I can use the Victron 75 / 15 MPPT controller.
2. I figured my likely energy usage, I'm going to need 200 Ah of batteries. I'm looking at either (2) 12v @ 206 Ah each (in series) or (2) 24v @ 100Ah each (in parallel).
3. Sweet Will Prowse's 24v system videos keep touting a 40 amp MPPT controller, though, and now I'm wondering if my little 75 / 15 MPPT will charge/handle/work best on these batteries. Both types of batteries spec a Recommended Charge Current of 20A and a Maximum Charge Current of 50A. Won't my MPPT only push out 15A? Do the batteries *need* a 40 amp MPPT?
4. I also need a 24v --> 12 v converter and Will again recommends a 40 amp machine. And I'm just unsure why.
I'm hardcore unsure of everything now. I'd appreciate any help.
-kb

 

[Tomthumb62]

Won't my MPPT only push out 15A? Do the batteries *need* a 40 amp MPPT?

Your Victron 75/15 will output a max of 15A.

No, your batteries don’t need 40A of charging, but they would gladly accept it (and charge faster) if you do. 15A might be plenty for you, it depends upon your loads. If you’re drawing more from your battery than your MPPT can put in per day, then over the course of days, your battery SOC will get lower and lower. A well sized system can power all your loads AND charge your battery by midday or early afternoon.

 

[kables]

Thank you for your reply!

 

[dsb]

As Tom said, it’s going to take a long time to fully charge 200Ah at 15A.

200Ah / 15A = 13.3h, that’s 2 good days of sun in summer and maybe 4-5 days in winter, depending where you are. And that’s not using any of your power to power loads while charging.

You might be better off with a Victron 100/30, more panels and a 100Ah battery, which you can charge in 3-4h of sun.

Have you worked out what your loads will draw in a day?

 

[Rednecktek]

A 15a controller gives you no room for expansion or larger panels and since you're spending $$tupid money on Victron, why not give yourself some room to grow. It's also the bare minimum for 200w of panel so you'll be running 100% load under good sun. Not healthy.

The 40a 12v converter is a good idea since you have things that will likely have a bit of startup surge (fridges for example) and since an inverter may be rated for 6000w surge and 3000w running, the step down converters are only rated for 600w surge and 600w running, so you need to make sure you've got plenty of spare overhead.

If you're building a native 24v system go with 24v batteries. It's less points of failure and you can get a larger BMS max in a large 24v than you can in a smaller 12v. A 100a BMS is going to limit you to about 2400w max, and that's the common size for 12v batteries, but 200a and 250a BMS's on a 200a 24v battery is pretty common. Of the two options listed, the 2x 24v 100a setup is the better option.

Make sure you have an inverter generator that can charge those batteries, your solar looks pretty undersized.

 

[fratermus]

I can use the Victron 75 / 15 MPPT controller.
2. I figured my likely energy usage, I'm going to need 200 Ah of batteries. I'm looking at either (2) 12v @ 206 Ah each (in series) or (2) 24v @ 100Ah each (in parallel).

A 15A controller is correctly sized for 400w of panel and 24v bank, IMO. The Victron MPPT calculator will sanity check it for us.

As others have pointed out, 400w of panel is likely undersized for ~5000Wh of bank. It would take >12 hours of full sun equivalent to recharge the bank from 80% DoD, and there is nowhere on the planet that has that kind of insolation AFAIK. Upsizing to 800w (and 30A controller) would do the trick in most places in CONUS in the summer at least. Consult PVwatts or similar models for what to expect where/when, insolation-wise.

TL;DR: I'd seriously consider adding in hefty alternator charging.

 

[mikefitz]

Your application seems to need 12v power, unless there is a need for a 24volt battery system, example inverter over 2000 watts, opting for a total 12v system has advantages .
If you decide on a 24 volt volt lithium battery system use only 24 volt batteries, 12v batteries in series often develop issues .