Design inputs needed

[wine_guy_3]

I'm scrapping my solar panel setup.
6 panels 100 watt each Voc 17.9
PWM controller
4 batteries – 115 Ah each
12 volt inverter – modified sine wave.


new setup      
4 – 375 watt panels – Voc 48.2, Vmp 39.8      
MPPT controller (or 2)            
Add more batteries                
New inverter – pure sine wave      

Because of the higher voltage panels, I will need to go to MPPT,  or a 36 volt system. I think MPPT is the way to go.

I probably need to go to a 24 volt system because I would still have just over 60 amps of charge current (max). Maybe use 2 charge controllers? I have two sets of wires from the panels to the garage.

Putting 2 panels in series would cut down on the current between panels and charge controller, but still will have 60 + amps from the charge controller to the batteries (max).

60 amps of charge current means that I would need at least 600 Ah of batteries for C10, almost 1200 Ah for C20.

Because I have a 24 Volt battery bank, I need to go to a 24 Volt inverter.

Does the logic (math) sound OK, or am I missing something?

[southpaw]

Hi Wine_guy_3    From your post i am going to assume your existing batteries are 12v lead acid. Trojan recommends max. charging rate of C10 for this type of battery. With a 115 ah rating you would need 6 of these batteries in parallel to give you 690 ah at 12v to make C11 , and another bank of 6 parallel batteries hooked in series to get 690 ah at 24v, still C11 with your panel output of 1500 watts. The next consideration is the maximum amp. capacity of the wires from the panels to the charge controller, and from the controller  to the batteries.  If your wires will carry 40 amps with an acceptable voltage drop, (less than 3%) you can connect your panels in parallel and run 40v into your MPPT controller. If the voltage drop is more than 3% you can connect your panels in series/parallel for 80v @20 amps. or series for 160v @ 10 amps. Higher voltages come with higher risk and more restrictions. The charge controller needs to be rated for max voltage in and max amperage out. This might be more than you asked for or less, hard to know. Your logic (math) is definitely in the ballpark and hopefully mine is too. 

[wine_guy_3]

Thanks for the input. I wasn't sure.

so, just to recap my thoughts.
Panel wattage divided by battery bank volts is the charge current.
Using MPPT, you can cut down the current between the panels and the controller by putting panels in series (higher volts).
You STILL have the same charge current to the batteries (MPPT output).
The MPPT output must be able to handle the large input voltage and the large output current.

So with a 24 volt bank and a 60 amp charge controller, the largest array would be 1440 watts.
anything bigger would require ....
   Split the panels - 1400 on 1 set, the rest on another set
   Second set of wires from the second array to the second charge controller
   Parallel the two charge controllers to feed the batteries.

Sorry for the repetition, sometimes it takes a while for stuff to sink through my thick skull.

[SparWeb]

With panels like that (4x375=1500W) in combination with MPPT to run at full current, then you can plan for up to 60A charge current into a 24V bank.  If that's the C10 rate of your battery bank then the bank has to take 60x10=600 Amp-hours.  That's a lot, and it's why southpaw suggested 6 Trojans in parallel.  I would add that you also need 4 in series to get each parallel string up to 24V if you're using 6V batteries.  So that's a total of 6x4=24 Trojans to accept the charge rate that the panels can deliver daily.  This is a pretty big bank.  You would actually be at the point where you should reduce the number of parallel strings to two or three if you can by using batteries with about 200 AH capacity instead.  Beyond this, you will see benefits not only in better battery choices but also in inverter choices if you raise the battery bank voltage to 48V.

In addition, your daily load on a big bank this size can be pretty high.  If you get 5 hours of 1kW every day then 5kW-hr lets you put about 1/4 of your household use on Solar.

[bigrockcandymountain]

If you are buying an inverter anyway, go with 48v as has already been said.  You can use a 30a mppt controller, smaller inverter cables, and most importantly, you won't need a bunch of parallel batteries. 

I have wrecked a pile of batteries by paralleling them with poor/corroded connections.  A single series string is way more forgiving. 

So, what i would do. 

4x375 panels 2 strings 2 per string 100voc 80vmp

30a mppt controller midnite kid but they are expensive for the amps.  Outback fm60 are good value for a 60a and then you could expand.  Tristar mppt have the best reputation by far.

48v pure sine inverter.

Batteries...I charge a 428ah 48v bank at C6.8 max.  That really only happens a couple times a year though.  Panels rarely put out rated watts.

For you i would do your existing batteries all in series plus another identical parallel string.  If your batteries are half shot, sell them and do single series 8 6v golf cart batteries approx 200ah at 6v.  Thats 200ah 48v 9600wh.  C10 of 960w.  Probably close enough.  They are WAY better than the 12v batteries i have used. 

That's what i would do.  I usually warn people to not do what i do so there is that...

[Scruff]

In most cases series is better than pparrallelle. When ya get up to 400V then that might not apply any more.

Don't mind that C10 charge. That's just advisory. Plentya folks use C3.3.
I push C2 on 6Vs. It just costs more water.
Besides compensating for running loads and solar not running 100% 100% of the time. Don't leave yourself short.

Generally speaking it's self-regulating, high current chargers are 'spensive.

+1 for 48V, think of the future. Everything is cheaper up there and 12v/24v gets expansion prohibitive right fast.
Any time I'm routing 95mm² cables I can't help but think I'm doing it wrong.

[wine_guy_3]

Wow.
You guys have been REALLY helpful.
This is a system for a cabin up in Northern Wisconsin.
I was just trying to keep it as a 12 volt system just for lights, but the wife wants "more power".
I had never considered a 48 volt system, but the items you guys have brought up have kind of convinced me.
THANK YOU.

You know any good 48 volt battery chargers? I have a 5700 W generator.
Any other things I might need to keep the batteries happy? Protected?

Once again THANK YOU.

[bigrockcandymountain]

https://www.ebay.com/itm/184600785866

These are very good inverters.  Not sure how much power you need.  I would buy a used outback before a new chinese any day.   

[Mary B]

If you need to run 12 volt load sin the cabin Samlex makes good 24to 12 and 48 to 12 volt DC to DC converters. I run 24 volts and use one to provide 35 amps at 12 volts for the ham radio gear.

[Scruff]

I'd steer clear of 12v, everything is small and 'spensive and lossy.
You can use domestic 30 cell solar panels with 24v + PWM.

At 24V: 300W €100 panel + €50 PWM controller...for instance
compared to "12volt"
150W €200 panel + €50 PWM controller with half the overhead or
300W domestic €100 panel + €200 MPPT controller.

I only buy inverters that give me a hernia picking them up. (Low frequency; Stronger, more reliable, more robust, better surge)
48V chargers are hard to find. Solar controller or inverter-charger combi.

[wine_guy_3]

I'm going to single step the project
First get a MPPT controller to see difference

Second re-mount my panels to get more from them
    Possibly rewire them into series sets

Third, Get a new inverter
   Modified sine wave does not have a true neutral.
      Pops the inverter when neutral touches ground (ask me how I know)
   Rewire the batteries into 24 or 48 volt config

fourth, get more batteries

fifth, get new solar panels.
   Add charge controller, and/or replace the existing panels.

I figure all of the above steps are isolated enough that I won't wreck the existing system while working on it.
All a matter of time and money ..... and the 300 miles the cabin is away from home.

Thanks again for all of the wonderful thoughts.
I still need to think about other "things" I don't know I need.
  48 volt charger?
  battery balancer ?
  more fuses between batteries?

I'm still learning

[Scruff]

I'm going to single step the project
First get a MPPT controller to see difference

Depends on your climate and panel to battery voltage.
If overcast skies and low light are a regularity MPPT kicks bottom.
If it's usually sunny then PWM with a panel Voc ~3V above absorption can hold it's own.

Cable costs and the ability to oversize and hybridise my controllers has me phasing out all my PWMs

Second re-mount my panels to get more from them
    Possibly rewire them into series sets

Earlier start up and later shut down with series strings. If you go too far the controllers start singing something woeful.

Third, Get a new inverter
   Modified sine wave does not have a true neutral.
      Pops the inverter when neutral touches ground (ask me how I know)

That's the high-frequency topography not the sine resolution. If it's transformerless and or centre tapped earth then smoke will escape.

   Rewire the batteries into 24 or 48 volt config

24v is very handy because truck appliances and switchgear control convenience.
If your needs will never, ever, ever exceed 4kVA then it's ok.
If they conceivably will, make the move early because the later you do it or pursue a too low voltage the more expensive the legacy gets.

   Add charge controller, and/or replace the existing panels.

More is more. unless you use them to leverage more hardware/need the real estate yer probably better off keeping them and throwing them into the mix...on a separate controller.

I figure all of the above steps are isolated enough that I won't wreck the existing system while working on it.

The amount of gear I've blown working on live circuits...

  48 volt charger?

Feed an MPPT solar controller a PSU and current limit the charge controller to 80% of the PSU output.
You can use a transfer switch/relay between a solar array.

  battery balancer ?

No need unless the battery is made of orphans of mixed providence.
 

more fuses between batteries?

One per string and cable size reduction.

I'm still learning

Me six.

[wine_guy_3]

PSU ?
never heard of it.

[Scruff]

Power Supply Unit.

Adjustable bench power supply.
Constant Voltage PSU (ubiquitous..from wall warts/laptop power bricks & beyond)
Constant Current PSU (Led Drivers)
Rectifiers (industrial grade)

...think of it as an artificial solar array.
I have more solar controllers powered by mains than solar panels. I usually allow them to be changeover because it's practically free functionality.

MPPTs do it well. The capacitance of some PSUs can kill the FETs in PWM controllers. Also PWM controllers don't often have current limiting so less practical applications.

[bigrockcandymountain]

I like your plan wine guy.  You'll see exactly which component does what.  24v is ok too depending on loads.  There are lots of DC loads that are easier to buy in 24v.  Lights and water pumps come to mind.  I just prefer 48v for the single string batteries. That can be done with big 6v or 2v batteries at 24v also.

It is best to decide early on voltage.  Lots of charge controllers don't do 48v. 

I helped a guy over text messaging convert from 12v to 48v.  He has 21 panels 100w each like yours.  I think we ended up with 7 panel strings x3 on his and it works well.  He went 48v to run a 240v well pump off the inverter. 

I, too, would keep the panels you have and add to them.  Unless you are limited on roof space or something. Just have them on their own charge controller and all will be well. 

[Scruff]

You can run pparrallelle series 24v chargers on a centre tapped 48v battery as long as they are identical and have identical isolated inputs. I've thought about it..the balancing might be better. Waaaayyy too messy though. KISS won.

Edited...was thinking parallel operating. ...scientifically series operating.

[wine_guy_3]

You guys are WAY cool.

The reason I thought of scrapping the 100 watt panels is the racking layout.
I want to hang them between 2 poles, and use a something similar to a steering mechanism on a boat to allow me to tilt them.
Racks of the same length would be easier, but I will see how 3 100 watters measure up to 2 375 watters.
100 watt are about 4 feet each, 375 watt are over 6 feet. Sounds close.

Thanks again for all of the inputs.

[wine_guy_3]

One more question.
VOC of the existing panels is 17
Putting 3 in series is 51.
Will this work well using MPPT to charge a 48 volt bank?

[bigrockcandymountain]

Nope that won't work very well at all for 48v.  You want the vmp (volts max power) to be over 60v but preferrably much higher.  100v works well for mppt charging 48v.  Depends on how many volts the charger is rated for too.