one battery bank two inverters

aspiringpisces

can I  connect two 48 volt 4000 watt inverters to one battery bank of 8 6volt  350 amp batteries to supply two RVs each running off 110 30 amp outlets running off 14 275 watt 31v panels all wired for 48 v including batteries

Estragon

Short answer, I think so if I'm understanding the setup.
Assumptions:
- Each inverter is supplying each RV separately and independently.
- There are no alternate AC sources (generator/shore power)
- AC neutral/ground & DC ground considerations per inverter manual are understood.

On the DC side, having multiple loads (inverters) on a bank is fine in principle (make sure wiring & OCP can handle the current though).

Obviously running 8000w of inverters off 350ah@48v bank is another question.  Ok if it's just for each rv to make toast in the morning, not so much if you expect to run anything like 8kw loads for long.

BB.

Can you, yes. Should you... Less than an optimum solution.

More or less, for a flooded cell lead acid battery bank running at 48 volts... The maximum (reliable over time, able to supply rated power over time, and rough maximum solar array) is around 1,000 Watts per 100 AH of battery bank capacity. Or ~3,600 Watts maximum.

Your 3.3 kWatt solar array is, nominally, OK.

2x 4 kWatt inverters is a lot of energy. And the battery bank would be hard pressed to support that sort of current, especially on a 50% or less depth of discharge battery bank.

Another issue is that two large AC inverters can draw upwards of 40-80 Watts (or a lot more) just "turned on"... 80 Watts * 24 hours per day = 1,920 WH per day or enough power to keep one or two full size refrigerators running.

A better fit would be a single ~1,800 to 3,500 Watt AC inverter supplying the two RVs. Of course, that does limit your energy usage. Which for solar power is a better idea (unless you plan on running a genset a fair amount of time to support A/C, deep well water pump, or other heavy loads).

Ideally, it is better to start from your loads, design the battery bank, then design the solar array that keeps the loads running and your battery bank "happy".

So, lets just throw some numbers around. Using our rules of thumbs to design a reliable/optimum power system (best bang for your buck, will run for many years without "surprises"--Other than the usual that happen when you are your own power company)...

Generally, we start with a flooded cell lead acid battery bank (relatively rugged, reliable, and not too expensive). And suggest 2 days of stored energy (no-sun stormy weather), and 50% maximum discharge nominally (longer battery bank). That gives us:

• 350 AH * 48 volts * 0.85 AC inverter eff * 1/2 days storage * 0.50 max discharge = 3,570 WH per day ~ 3.6 kWH per day

That is enough energy to run one cabin/small very energy efficient home with Energy Star Refrigerator, clothes washer, well pump, laptop computer, LED TV, etc.... A "near normal" (with lots of conservation) home (I actually use ~3.3 kWH per day as my "starting point").

Next, sizing the solar array for your battery bank. 5% to 13%+ rate of charge is recommended. For full time off grid, 10%+ rate of charge (vs summer cabin, weekends non-winter usage can work OK with 5% rate of charge).

Lets say fixed array, near Tucson Az:
http://www.solarelectricityhandbook.com/solar-irradiance.html

Tucson
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 58° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
4.95	5.63	6.61	7.07	6.91	6.56
Jul	Aug	Sep	Oct	Nov	Dec
5.77	5.66	6.09	5.84	5.30	4.76

For 3.6 kWH per day, assume you use a genset when needed in December/winter (you have lots of sun):

• 3,570 WH per day * 1/0.52 AC off grid system eff * 1/4.76 hours per day = 1,442 Watt array minimum

And there is sizing the array to support your battery bank AH and Voltage capacity:

• 350 AH * 59 volts charging * 1/0.77 panel+controller * 0.05 rate of charge = 1,341 Watt array minimum
• 350 AH * 59 volts charging * 1/0.77 panel+controller * 0.10 rate of charge = 2,682 Watt array nominal
• 350 AH * 59 volts charging * 1/0.77 panel+controller * 0.13 rate of charge = 3,486 Watt array "cost effective" maximum
  

So, if this was a full time off grid system, I would be suggesting a minimum array of 2,682 Watts as being a "balanced" system that would support the recommended usage/capacity of your battery bank.

You have 14 * 275 Watt panels (Vmp~31 volts). Output wise, during winter, the average maximum daily output would be (you can never harvest 100% of your daily harvest):

• 3,850 Watt array * 0.52 off grid AC system eff * 4.76 hours of sun (December) = 9,540 WH per day

So, during sunny weather, you can harvest and use (during the day) upwards of 9.5 kWH of power during the day, and use something like (a recommended) 3.6 kWH overnight.

For an off grid system, your base loads (the suggested amount of "base load" energy you cannot live without--Refrigerator, well pump, LED lighting at night) would be ~50% to 65% of your daily harvest. 50% of 9.5 kWH is 4.75 kWH per day base load.

Other questions... you have 14 solar panels. Nominally, for a 48 volt battery bank, I would be suggesting 3 panels in series... That gives you 14/3=4 parallel string of 3 (12 panels total) with 2 left over.

2 panels in series (Vmp-array~62 volts) is not high enough "working voltage" to charge a 48 volt battery bank. You really need >~72 volts for PWM type controller and would be better off with Vmp-array>~78 volts for a MPPT type charge controller. Typically somewhere 78 to 100 VDC Vmp-array is "optimum" for a typical mid-range voltage MPPT charge controller.

Details matter here... Brand/model of charge controller. Actual installation location (nearest major city--Don't know if Tuscon, Phoenix, Atlanta or elsewhere). As well as what your daily loads expectations are...

Anyway, lots of guesses and suggestions. Your corrections to my guesses, and such?

-Bill

BB.

New posts/comments from original poster:

-Bill "moderator" B.

aspiringpisces said:

I want to supply ac for 2 RVs using 1 air conditioner each plus normal rv refrig tv microwave lights. I have 2 xantrex 4048 inverters ,3 xantrex mppt 60-150 charge controllers 8 250ah 6v batteries do I have what I Need or do I need more batteries and how should I wire this setup with additional batteries do I have enough inverter power ,panels

aspiringpisces said:

I forgot to add I have 5 sunmodule mono black panels 39.4 open circuit v 31 vmpp v 9.58 open A  ,8.94 impp and 9 REC Twin Peak 2 panels 290w 38.8 open circuit 32.1 vmpp v  , 9.71 open A   9.05 impp A

Estragon

It really starts with loads, as per Bill's post #3, and your ability/willingness to run a generator.