Best system voltage to use

dogman · September 2010

Hello, I am new to solar and have build my own system. I started with 12 volt RV type equipment and have since moved up to a full house type system.

I have a morningstar TS-MPPT-60 controler hooked up with 8 220 watt 30 volt each panels, they are connected in 4 strings of 2 each for 60 volts. This charges a 12 volt battery bank of 2760 amps. My inverter is an RV type sunforce 2500 watt, that is only 12 volt.

The TS-MPPT-60 is limiting my charging to 60 amps during the peak of the day and I am wasting the extra generation of my panels. I see that the extra panels cause the watts genertated to come up faster in the mornning when there is not as much sun and stay higher in the evening.

I don't think that the over current limit regulation is hurting anything, but I would like some one to comfirm that my thougts are correct or not ?

I am at the point of wanting to replace my inverter with a much better one.
At this point I am thinking of changing the system to 48 volts and getting a 48 volt inverter, however I have 12 volt radios and equipment that I want to be able to run direct from DC.

So here is the main question, Is there any advantage to going 48 volts over say 12 volts other than the controler will be able to make use of all of my panels and smaller wire size will be smaller ?

I could also just add another control to make use of the panels but I some how think going 48 volts might be a better idea. If I just add another control then I would be at the sampe point laster if I added more pannels.

The next questions is what inverter would be recomended ?
I would like following:
1) Network monitoring so that I can keep track of my power usage and status.
2) 2000 or more watts full sine wave.
3) not so important but maybe future grid tie.

I have been looing at the OutBack FX and Xantrex XW.
I might even think about replacing/adding another control to match the inverter.
I have not had very good support responses from morningstar and they do not have a good high end inverter to match the controler.

Thank you for any help.

Cariboocoot · September 2010

Re: Best system voltage to use

If you really need that 2760 Amp hours of battery you should definitely go up in system Voltage. Right now your batteries being charged at 60 Amps maximum are doomed to an early death. That size bank should be charged at a rate of 276 Amps, and that is impractical.

I have to ask why you need 33,120 Watt hours of battery? That's HUGE!

But if it were a 48 Volt system the capacity would be 690 Amp hours and could be charged at 60 Amps without much fuss. Unfortunately your existing array is 1760 Watts and is not going to produce 60 Amps @ 57-58 Volts for charging a 48 Volt system. It would need to at least double in size (depending on how much in efficiency losses you are suffering).

Maybe you need to re-evaluate from Step One: calculate your loads. Both in terms of maximum Watts used at any given time and total Watt hours used over a day. Since you're planning on a re-do, now is a good time to go back and re-check the whole design.

Also, if you have a lot of 12 Volt equipment you want to run from a higher system Voltage you either use a DC-to-DC converter or a separate 12 VDC system (which could be recharged via an MPPT controller from the main system or have its own panels). Depends on how much 12 Volt power you need.

niel · September 2010

Re: Best system voltage to use

you are losing at least 40a by having a 12v battery arrangement with 1 controller and you want to get as much as possible from your investment. 48v is good to have for a battery bank, but you could split your systems altogether and run the necessary 12v stuff from one with it's own batteries, controller, and pvs and the other with its 48v battery bank, controller, inverter, and pvs giving you the best of both. as coot said you are too low by trojan's charge recommendations of 10% of capacity. you might get away with a lower charge percentage for less pvs than they recommend, but what you have in pv is definitely too small for those batteries and i'd recommend at least doubling the pvs if you can with any increase being a good thing at this point.

dogman · September 2010

Re: Best system voltage to use

The batteries are 27DC-6 at 115 amp each times 24 batteries. I have a Trimetric meter and I have been getting 100% charge, also the TS-MPPT-60 has been going into absorption so I am think I am getting a full charge. Voltage shows 14.4 volts.

The meter shows that I only use about 20% each night and it was getting 100% charge back each day until this last week when I added 4 more batteries. It still gets to about 98% and if I don't run the refrigerator on it while charging I still get 100%.

Is this only because I am not running them down so low that it's getting a good charge or am I looking at things wrong ?

If I change to 48 volts with the batteries and current panels will the amount of use and charge getting to the batteries % wise still be the same ?.

I was figuring the following:
220 * 8 = 1760 / 12 = 146.6 amps charge from panels but the charge controler is regulating down to 60 amps meaning 86.6 amps wasted ?

If I am correct wouldn't that mean that with current equipment I would be gaining the 86.6 amps % wise to what I was storing ?

Are you saying I could feed a 48 volt battery bank into the MPPT charge controler on the panel side and charge a 12 volt battery bank on the output side ?

Cariboocoot · September 2010

Re: Best system voltage to use

Ah, yes but no.

You're "putting back the used Amp hours" which looks like the batteries are fully charged.
But you're not getting sufficient charge rate to re-mix the electrolyte and slow the damage from sulphation. This will show up as shortened battery life.

Again I have to wonder why such a big battery bank. Normally a 12 Volt system would have a 2000 Watt or smaller inverter, and probably would not be asked to provide more than 4kW hours per day - a less than 1000 Amp hour battery bank.

If you have big Wattage needs, going up in system Voltage reduces the current needed and thus reduces heat, improves efficiency, and simplifies large battery bank wiring. Your bank must have a significant number of parallel batteries, whereas the same Watt hours could be supplied @ 48 Volts with an "all batteries in serial" arrangement, which improves current handling between batteries.

And yes you could feed a 48 Volt battery bank (charged by solar) into a small MPPT controller to charge a 12 Volt "sub system" if you like. But there must be a 12V battery.

dogman · September 2010

Re: Best system voltage to use

I kept making the battery bank bigger and bigger because I don't use much during the day while there is sun, Most of my use is during the night. I watched the Trimetric meter and the charge controler and could see that I was generating much more then I used during the day so I wanted to store it for night use.
I also read that you should have about 5 days of storage so you can keep going during bad weather with no sun.

Before I learned how to cut my power usage I used about 17 - 20 kwh from the grid and now by cutting my use/wast I am at about 9kwh.

My grid usage has been about 1 kw for the whole of last week since using the solar. I would like to be able to shut the grid off if need be, but being realistic I realize how cheep grid power really is if you stay within the baseline limits.

My goal has been to limit my grid to baseline or less usage. I was planning on just using the larger loads for short times, microwave, washer etc.

I was also planning on using excess solar to run a cooling/heating system in the afternoon after the batteries got fully charged. Up until the last week when the season started changing and the sun has dropped lower I had no problem getting the batteries to read 100% full by about 1 PM and the charger would go into absorption mode for 2 or three hours not doing much. On weekend when this was happening I have been running most of my loads while the battery continued to get an absorption charge so this made me think I was getting enough from the solar. I guess I was wrong !.

I am still thinking I need a new inverter and should get another MPPT charge controler made by the same company so that they can work together,
what are your thought on that ?.

Cariboocoot · September 2010

Re: Best system voltage to use

5 or even 3 days of "no sun" storage may be what some people recommend, but it's about as practical as a bicycle for fish. You want back-up power? Buy a generator. It's way cheaper per Watt for occasional use than solar. My own formula is to plan on drawing the bank down 25% for a day, know that you can take it down to 50% maximum - there's your second day - and if you need that third day, start the generator.

In off-grid terms your power consumption is still large. About 3X mine, in fact. Mine could stand to go up a bit, though.

Since you have grid, the obvious question is: Why not install a grid-tie inverter and sell back any solar surplus to the utility? It is possible they don't allow it where you are (I don't know - it's impractical here in BC due to really cheap rates). You can even have battery back-up grid tie with a hybrid inverter like the Xantrex XW series.

A larger bank for storage is fine, but it is better to go up in Voltage to lessen the current. The equivalent power is the same. And you must have enough charge rate to remix, as mentioned before, in addition to sufficient 'replacement' Watts.

Time to re-evaluate the over-all plan!

BTW; you can mix different charge controllers without trouble. They won't "fight" one another. The Outbacks of course can actually talk to each other and any OB inverters via the Hub & MATE connector, but it's money.

mike95490 · September 2010

Re: Best system voltage to use

The more batteries you add, the more losses. 1% or more, for each battery. If you have grid, go grid tie, or a hybrid system with backup batteries.

Best system voltage to use

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