Changing from 24 to 48 volt

littleharbor2

I'm a little unsure about how to approach this. I have an array consisting of 8 BP Solar 175 watt 24 volt nominal modules. Presently they are connected in parallel, actually two strings of four in parallel into a Xantrex C-60 charge controller set up at 24 volts and running a 24 volt battery bank and 24 volt inverter.
My plan is to convert the system to 48 volts. Of course I am going to change out the C-60 for a C-40 to run the 48 volts through and swap the 24 volt inverter for a Xantrex 4048 inverter.
Now what I'm unsure about is would it make any difference if I series connect the two 4 panel parallel strings. or should I series connect two panels at a time and end up with four strings of two. Changing to 48 volt will halve the amperage pushing through the wire which is comforting. I just am having a problem wrapping my head around weather either approach is acceptable or if one way is definitely better or way wrong than the other.
Any and all opinions are much appreciated.

Cariboocoot

Re: Changing from 24 to 48 volt

Terminology mix-up: you don't have "strings of four in parallel". Strings are one or more panels in series. Obviously you have none in series because the C60 is PWM controller and it wouldn't work that way.

Going up to a 48 Volt system means you must have two of these panels in series on each string. That way the array Vmp will go from 35 to 70 and you'll have enough charging Voltage for the 48 Volt batteries. Then the array would be four parallel strings of two in series, Vmp 70 Imp 20 (those panels are 35 Vmp 5 Imp I believe).

I have to say that with eight 175 Watt panels you should be considering an MPPT controller here. That is a 1400 Watt array. On the C40 it will put out 20 Amps. On a small MPPT controller like the Rogue 3048 or MidNite Kid it will put out about 22 Amps. It may be worth it for that extra 10% (which could be higher under certain conditions).

Keep in mind you may need to change fuses/breakers/disconnects to accommodate the higher system Voltage.

zoneblue

Re: Changing from 24 to 48 volt

As you dont have a good grasp of the basics yet, before you go any further youve got some significant reading ahead of you.

PWM controllers are rarely used these days except in very tiny systems. If you use them you have to be careful to match the panels Vmp to the system battery voltage. The mppt gain over pwm is a lot more if your voltages are mismatched, which is easy to do. But any system upgrade would upgrade to mppt for lots of reasons.

Why upgrade to 48V at all? If you plan to expand the pv, or have heavy loads maybe, otherwise probbaly better staying where you are at 1400W.

If you add a sig with all your gear we will be in a better position to assist you. Also some info about teh extent of your loads is the good old place to start.

littleharbor2

Re: Changing from 24 to 48 volt

My apologies for using the wrong terminology.
Back to my initial question, I now have two groups of 4 panels connected in parallel. They are hooked up to multi branch MMMM/F and FFFF/M harnesses. I could simply series the output of the branch connectors keeping two sets of 4 in parallel. Is there a problem in doing this that you can see? It would be much simpler than re-configuring the whole array.
I already have the C-40 so I'm not really interested in buying an mppt controller.

littleharbor2

Re: Changing from 24 to 48 volt

I basically listed the gear involved. The reason I'm changing to 48 volt is the fact I have the Xantrex 4048 and as far as I'm concerned there isn't anything wrong with a tried and true Xantrex C-40. I believe I posted a pretty basic question seeking a basic answer.

zoneblue

Re: Changing from 24 to 48 volt

littleharbor2 wrote: »

My apologies for using the wrong terminology.
Back to my initial question, I now have two groups of 4 panels connected in parallel.

With PV we say N strings of X panels, to mean X panels in series, in N number of groups in parallel. So are you saying you have 2 strings of 4? If so that doesnt make much sense with your PWM controller. You need to get the Vmp specs of those panels.

They are hooked up to multi branch MMMM/F and FFFF/M harnesses. I could simply series the output of the branch connectors keeping two sets of 4 in parallel. Is there a problem in doing this that you can see? It would be much simpler than re-configuring the whole array.

With PWM you tpyically have lots of parellel strings of one, maybe 2 panels. That means you must have a fused combiner to protect against panel fires.

I already have the C-40 so I'm not really interested in buying an mppt controller.

Sorry thats not very sensible. Your talking about a major upgrade of inverter etc, and your gona let a 20 year old 100 dollar controller compromise the entire design?

Cariboocoot

Re: Changing from 24 to 48 volt

littleharbor2 wrote: »

My apologies for using the wrong terminology.
Back to my initial question, I now have two groups of 4 panels connected in parallel. They are hooked up to multi branch MMMM/F and FFFF/M harnesses. I could simply series the output of the branch connectors keeping two sets of 4 in parallel. Is there a problem in doing this that you can see? It would be much simpler than re-configuring the whole array.
I already have the C-40 so I'm not really interested in buying an mppt controller.

Oh I see a problem: no over-current protection on the parallel panel connections. That should have been in place on the initial system with its eight parallel connections. Really you should have been using a combiner box there, not just stacking the connection with those connectors. When you put four of those panels in parallel you are then feeding 20 Amps into that one connection. You then add in the other four and now you've got 40 Amps. One panel shorts and 35 Amps can be dumped through it, which is more than twice what it was designed to handle. That's how fires start. As for the wiring ... I hope it's all 6 AWG from those connections onward.

What you're wanting to do is eliminate one set of those branch connectors so that there are only four parallel strings coming together, but now they will be strings of two. Good idea except for that little issue of no over-current protection.

But yours is the house at risk so it's up to you.

mike95490

Re: Changing from 24 to 48 volt

1400W of array (apx 70V)

PWM Controller [ C-series ] @ 53V = 20 amps
MPPT controller @ 53V = 26 amps.

" free " amps for the price of a new controller. The amp gain does slowly diminish as the batteries come up to full. But when they are low, you get the full power of the panels (depending on sun conditions)

littleharbor2

Re: Changing from 24 to 48 volt

Understood, a fused combiner box will be incorporated into the mix and I will eliminate the branch connectors.

jcheil

Re: Changing from 24 to 48 volt

littleharbor2 wrote: »

Understood, a fused combiner box will be incorporated into the mix and I will eliminate the branch connectors.

Just in case you did not know, be sure it is a REAL DC rated combiner/fues/breaker box. You can't use cheaper AC rated breakers with DC. And don't try to use automotive fuses either. And seriously, get an MMPT controller (like the Kidd). It is WELL worth the extra few $.

littleharbor2

Re: Changing from 24 to 48 volt

I already have a disconnect in front of the cc with midnite solar 30 amp 150 vdc breakers and the combiner box I have and will use is a PV multi string fused combiner.
I know the merits of MPPT charge controllers. I have a Midnite solar classic 200 controller in my system at home. I won't go into the reasons why the system I'm asking about is going to stay with the PWM, suffice to say down the road it may get upgraded.