48 V battery bank and two 24 V inverters ??

mrleemus1 · July 2011

Let me describe my situatiuon - i need to upgrade my current PV system that is suffering from a poor design and not adequately chargeing my 24V battery bank; due to a lower than needed Vmp. See my other post for more info and discussion and responses if needed. The batteries are now configured to one large 24V bank that supply TWO 4,000 watt inverters.
I want to rewire and series up the voltage on my panels and then use MPPT charge controllers to charge my battery bank which consist of sixteen L-16 batteries. From the specs on MPPT controllers i see the 48 volt units can supply twice the wattage.
OKAY --heres the question -- can i reconfigure the batteries in such a manner that they can supply the 24 volts to the inverters -- and YET be in series to make a 48 volt bank for the charge controllers? Is this feasable or fantasy?---advisable???

Cariboocoot · July 2011

Re: 48 V battery bank and two 24 V inverters ??

You may have got a bit confused by the MPPT function. It can take a higher-than-system Voltage input and downconvert to system Voltage, turning "extra" array Voltage into additional charging Amps.

It is not a good idea to try to split a 48 Volt battery bank to run two 24 Volt inverters: the load on the two "halves" of the battery bank will inevitably be uneven. So whereas you'd be charging a 48 Volt bank evenly, it's individual batteries would be discharged at different rates. This will rapidly destroy the batteries. Nor would there be any power advantage to doing so.

There's nothing wrong with upping the nominal array Voltage to "48 Volts" and feeding that through an MPPT controller to a 24 Volt battery bank and running one or two inverters off that.

mrleemus1 · July 2011

Re: 48 V battery bank and two 24 V inverters ??

Thanks for the prompt reply ---yes i am confused !! ---easily so the older i get.. Heres what i find confusing from the tech specs on a Tristar MPPT i read the following:
• Maximum Battery Current : 45 amps
• Nominal Maximum Solar Input :
At 12 Volt - 600 Watts
At 24 Volt - 1200 Watts
At 48 Volt - 2400 Watts
• Peak Efficiency : 99%
• Nominal System Voltage : 12, 24, 36 or 48 volts DC
• Max. Solar Open Circuit Voltage : 150 volts DC
• Battery Operating Voltage Range : 8-72 volts DC
• Maximum Self-consumption : 2.7 Watts
• Transient Surge Protection : 4500 Watts/port
Doesnt that indicate if its a 48 volt SYSTEM it will handle twice the wattage????????
Thanks for pointing out the pitfalls of the 48 volt battery bank and (2) 24 volt inverters: i can see thats not acceptable at all.
Now getting back to selecting the correct size and number of MPPT controllers. The way the panels are now arranged there are 10 strings. When i series them i will wind up with 5 strings. I will have to send 3 strings to one controller and 2 strings to the other. The OPEN CIRCUIT VOLTAGE at the combiner box is 36 Volts. Total amps available should be around 116.
Should i use one 80 amp controller for the 3 strings and one 60 amp controller for the 2 strings?

Cariboocoot · July 2011

Re: 48 V battery bank and two 24 V inverters ??

Charge controllers' Amperage rating is based on their output. An MPPT controller that can handle 45 Amps can handle 45 Amps @ 12 Volts, @ 24 Volts, or @ 48 Volts. The corresponds to more Watts on the input side:
45 * 12 = 540, 45 * 24 = 1080, 45 * 48 = 2160
And yes the array Wattage given by the manufacturer is higher than that because they actually consider the panel efficiency. You can to some extent always "over panel" a controller; "excess" power simply doesn't get used.

Open circuit Voltage (Voc) should not exceed the controller's maximum input Voltage, including allowances for cold temperature compensation (when panels get cold their Voltage goes up). When you put panels in series the Voltages go up. If the Voc on your panels is 36 you could put four in series without exceeding the 150 Volt input limit. When reconfiguring an existing array it becomes a matter of trying to divide up the existing number of panels into a new arrangement of equal numbers. It doesn't always work. Your ten panels limits you to five strings of two each like you said, or three strings of three and leave one panel out.

I can't remember your panel specs 'cause I'm old and confused too and answer about 50 questions a day. They all start to run in together after awhile.

But that's the basics. Repost the panel specs and we can juggle the numbers about.

BB. · July 2011

Re: 48 V battery bank and two 24 V inverters ??

Solar charge controllers are output current limited... So, the controller will output 45 amps at 12 volts, 24, or 48 volts... Because power is:

Power = Voltage * Current

If you change from a 12 volt to 24 volt bank, the power through/managed by the controller will double. And if you go from 24 volts to 48 volts, it will double again.

So, with a high voltage battery bank, you will need fewer/smaller charge controllers for the same wattage array.

For your system, with a 150 VDC maximum input charge controller, you are able to put 5 of your panels in series--and almost able to do 6 (depending on how cold it gets in your area--perhaps 6 is good down to near 14F or so--depending on the exact panels you have).

6 panels would give you 7 parallel strings which you can divide among several controllers (depending on battery bank voltage and specific controller output current rating).

The maximum array / minimum output rating would be:

4,200 watts * 1/59 volts charging * 0.77 panel+charger derating = 55 amps day to day expected maximum current

So--depending on your choice for derating (0.77 is about the maximum derating I would suggest before it becomes an expensive over-paneling to charge controller ratio)... You could, if you wanted do all this on one 60 amp MPPT charge controller or a pair of 45 amp MPPT controllers.

I am getting a little confused--In your earlier thread you talked about 42x 100 Photowatt panels--Is this a different installation?

-Bill

mrleemus1 · July 2011

Re: 48 V battery bank and two 24 V inverters ??

Can MPPT charge controllers and PWM controllers be used together on the same battery bank --- since i have 10 strings at my combiner box now , could i series up 8 strings --That would give @ 72Voc on those 4 strings. i measured the Open Circuit Voltage again today (midday 90 f.) and it 35.8 volts. Put those 4 strings into one combiner box and send to a new 60amp MPPT. Leave the other 2 strings where they are (in a different combiner box) going down to the existing C60?

waynefromnscanada · July 2011

Re: 48 V battery bank and two 24 V inverters ??

mrleemus1 wrote: »

Can MPPT charge controllers and PWM controllers be used together on the same battery bank --- since i have 10 strings at my combiner box now , could i series up 8 strings --That would give @ 72Voc on those 4 strings. i measured the Open Circuit Voltage again today (midday 90 f.) and it 35.8 volts. Put those 4 strings into one combiner box and send to a new 60amp MPPT. Leave the other 2 strings where they are (in a different combiner box) going down to the existing C60?

Shouldn't be a problem using different controllers as long as they are not in any way connected together until their output hits the batteries, or the big cable feeding the batteries. Just that the MPPT controllers will be able to take advantage of extra voltage, tuning it into extra amps, while the PWM controllers will not. Other thing to note - - - it's when the panels are at their coldest that they produce the highest voltage. The warmer they get, the lower their voltage. Just mentioned that as you stated you checked their open circuit voltage at mid day and 90 deg F, when the panels would be quite hot.

48 V battery bank and two 24 V inverters ??

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