Outback FM-80 Maximum Parallel unit?

Re: Outback FM-80 Maximum Parallel unit?

Welcome to the forum Joe.

That is a huge (and expensive) system. And probably beyond the size most of us here have every worked on. And there are huge safety concerns with large/high voltage battery banks and DC current.

We certainly can help you with some of the basic issues/questions, but you will have to work with a professional engineer/engineering firm with experience on such large systems. Circuit breakers/fuses, wiring choices, etc. are all going to be critical for a safe system.

Some questions... 150,000 Watts (will have to be careful here, in the use we use commas "," and decimal points "." backwards from how you do) is a "rate"--We also need to know if that is your peak value (starting motors, etc.) or if this is an average value for over 14 hours of use (watts*hours of use = Watt*Hours of energy). Knowing Watt*Hours (or kWH) so we can help size the battery bank and solar array.

Next, you say this is for backup power? So will you be generating power for 14 hours (or 24 hours) maximum? Then use a generator or shut down the system until main power is restored? What will the solar array be used for? Slow charging after power is restored? Or to supply 100% of the load during the 24 hour outage?

Are you assuming that the battery bank will supply 100% of the power during the outage if there is no sun (such as during stormy weather)?

Don't get into picking hardware just yet--Sizing the system (battery bank, inverters, solar array, charge controllers) for your loads is the first step.

Can you tell us what this system is for? Battery backup / green energy for a computer farm? Backup power for a remote town, etc.?

-Bill

Re: Outback FM-80 Maximum Parallel unit?

Some basic numbers (assuming you discharge battery to 50% level for occasional backup power--If done daily, I would suggest discharging by 25%:

150,000 Watts * 14 hours * 1/0.85 inverter eff * 1/2 battery capacity * 1/384 VDC battery bank = 3,217 AH @ 384 battery bank

I am not a fan of large numbers of parallel battery strings. It can be very difficult to ensure that the batteries always share current properly (dirty connections, open/shorted cells, temperature variations, etc.).

I would be suggesting something larger than 200 AH @ 12 volt batteries... I would get industrial batteries ("traction" or fork lift type) in much larger AH per cell ratings:

Surrette Battery - 2 Volts, 2430 Amp-hours (Weight: 285 lb / 129.27 kg per cell)
Crown Industrial Battery - 24 Volts, 1875 Amp-hours (Weight: 2844 pounds / 1,290 kG per battery)

Two or three parallel strings of the above batteries would probably a whole lot more reliable and easier to maintain. 1/12th the number of cells and cables to monitor. (of course, it depends on what is available in your corner of the world). Also depends on transportation (i.e., 130 kG per cell vs 1,300 kG per battery).

Charging wise--Deep cycle traction batteries probably need 10% (some want a whole lot more) minimum charging current. A typical solar array would be:

3,217 AH * 480 volts charging * 1/0.77 panel+charge controller derating = 200,540 Watt array minimum

If you doubled the size of the battery bank (25% maximum discharge per day), then you would need ~400kWatt array minimum.

The size of the array to support your loads--Depends on if this is a daily discharge all recharged by solar or something else.

Using the Solar Electricity Handbook link, for Palembang, Indonesia, fixed array tilted basically flat (you might want a 5% tilt or so to make the array more or less self cleaning when it rains):



Jan
Feb
Mar
Apr
May
Jun


4.16

4.62

4.71

4.80

4.63

4.52



Jul
Aug
Sep
Oct
Nov
Dec



4.52

4.87

4.95

4.80

4.49

4.23


Assuming charging during day and discharging at night:

150,000 W * 14 hours * 1/0.52 system efficiency * 1/4.16 hours of sun (January break even month) = 970,784 Watt array.

You will have to look very carefully at how you recharge the battery bank... You may need a larger bank to allow it to be recharged fully in one day (assuming you need to recharge every day). Industrial battery chargers have "unlimited" power to recharge the battery bank with optimum current for ~8 hours. Solar has to make do with available energy. A larger bank that has shallower discharge can be recharged quicker.

Another option is to run the battery bank from ~50% to 80% rate of charge daily, and recharge back to 100% once a week (may work well if weekend load is less than workday load).

Of course with large systems like this, you need to run your back of the envelope calculations past a battery engineer to confirm expected battery life/cycle life under your planned conditions.

Charge controllers are going to be a bit tough--The highest voltage battery charger we use around here is the Midnite classic family (90-150 VDC battery depending on model--higher voltage, lower current rating). And these controllers do not have electrical isolation between the PV Array and the battery bank--That means that there is no "common ground" between controllers (each controller recharges just a part of the series string). Communications and signal voltages will have issues with offset "reference grounds".

There are other ways to get energy from the solar array to the battery bank (one way is to Connect the solar array to large Grid Tied AC inverters which then "back drive" the Off Grid AC inverter (many TSW AC inverters actually have bi-directional power capability). And you can recharge the battery bank through the AC Off Grid Inverter (you need to check with your Inverter supplier and have the system rated/tested to support reverse power flow).

Otherwise, you should find Solar Charge Controllers that are compatible with your high voltage battery bank.

You might try and see if this link can give you any useful information regarding suppliers (380 VDC data center alliance):

http://www.datacenterknowledge.com/archives/2010/10/20/alliance-boosts-380-volt-dc-power-standard/

Otherwise, you will probably have to check with your various major suppliers and see if they have any solar charger suppliers for 380 VDC.

Good Luck,
-Bill

Re: Outback FM-80 Maximum Parallel unit?

You are very welcome "J",

I forgot to put a link in for the solar hand book in my previous post (so you can do the calculations for the installation and see where I got my numbers)... Here it is:

http://solarelectricityhandbook.com/solar-irradiance.html

I used to design DC powered equipment (for telephone/computer networking systems)--And I am still very scared with using 48 VDC rated charge controllers on a 380 Volt battery bank. The chances of even a trained electrician of screwing something up with offset "grounds" is very high over the life of the system. Try very hard to find 380 VDC rated charge controllers or even making your own 380 VDC PWM type charge controller (less efficient--But possibly doable).

Operating at upwards of 480 VDC (when charging) a battery bank creates a whole new set of dangers. Arc Flash (high voltage/high current sparks) are extremely dangerous for folks working on/around systems (special suits and face masks are required in many countries when working on "life equipment" with Arc Flash dangers). Add DC current (very nice arc welder) and the high current available from a shorted battery bank (you can get 10,000's of amps if your bank is shorted), and hydrogen gas from battery charging--This no "toy".

http://www.geindustrial.com/publibrary/checkout/ArcFlash4?TNR=White%20Papers|ArcFlash4|generic
Arc flash - Wikipedia

Make sure whomever you choose for equipment and performing the "engineering" for your system has other systems successfully installed and operating (for at least, 1 year--With data showing performance). And that whoever will be working/maintaining your system is well trained and you have training books/videos in the language of your electricians to keep them trained in the decades to come (well documented system).

Best of luck,
-Bill

Re: Outback FM-80 Maximum Parallel unit?

Hi Joel,

Several things... First--Wire the battery banks "optimally" to your AC inverters. The FM 80 charge controllers will each connect to the battery bank via separate cables to each battery bank.

Be sure that you treat each FM 80 +/- connections as high voltage. You cannot connect any common communication cables (RS 422, RS 232, etc.) between sets of charges (ground offsets).

Second, your AC inverter's 285 to 415 VDC input... Talk with the vendor and figure out exactly what is the input voltage range to the inverter.

Next, figure out what your battery bank operating voltage will be. A "12 volt" lead acid battery bank will run from 10.5 (dead) to ~16.0 volts (charging/equalizing). That is a very wide operating range. Assume that the minimum operating voltage (under load 50% - 80% discharged or so) is 11.5 and the typical AC inverter operates down to 10.5 volts (inverter cutoff).

Also, Lead acid battery charge voltage (and operating voltage) change with battery temperature. For example, charging goes -.005 volts per cell per degree C. (raise from 25C to 35C, charging voltage needs to fall by 0.050 volts per cell (24 cells for a 48 volt battery bank = 2.4 volt reduction in charging voltage for a "hot" battery bank). And if the bank is below 25C, the charging temperature increases... This further can push battery "nominal" voltages higher or lower. For the moment, I will assume that your battery bank will be held to a nice 25C.

So, we need to figure out how this maps to your AC inverter's input. Low voltage is 285 Volts and we have 10.5 minimum inverter voltage (cable voltage drop):

• 285 volts / 10.5 volt min per 12 volt battery bank = 27.14 "12 volt battery modules" for low limits
• 27.14 "12 volt modules" / 4 = 6.79 "48 volt modules minimum

And the high voltage battery bank sizing... 415 VAC and the top charging voltage for industrial batteries may be ~16.0 volts maximum:

• 415 VDC / 16.0 volts per 12 volt module = 25.94 "12 volt battery modules" for high limits
• 25.94 "12 volt modules" / 4 = 6.48 "48 volt modules maximum" for low limits

So--We already have an issue here--the minimum number of 48 volt modules is 6.79 (low voltage+wiring drop) for low inverter voltage vs low battery bus voltage.

And for high voltage charging of battery bank, 6.48 x 48 volt modules to ensure that charging/equalizing batteries will not "over voltage" the AC inverter's DC input voltage.

So--You are going to have to make some limits (perhaps limit to 15.0 volts charging when the AC inverter is running--using 12 volt battery reference), or you will have to make different assumptions on minimum battery voltage and wiring voltage drop (heavier cables for example).

But in either case, a 6x 48 volt battery bank may not "work" over the entire operating range for the battery bank.... What is the minimum voltage for 6x 48 modules?

• 285 VDC inverter minimum voltage / 6 "48 volt modules" = 47.5 volts @ 48 volts minimum per bank

So, my assumption was you would run the battery bank at (4x11.5 volts=) 46 volts without accounting for any wiring loss at all. Or (6x46v=) 276 volts which is less than the 285 VDC inverter minimum (paper spec?).

If you are using 2 volt cells, you could use 25x 2 volt cells for a 50 volt battery bank:

• 11.5 volts per 6 cells "12 volt battery" / 6 cells = 1.916667 volts per cell (minimum operating voltage)
• 1.916667 * 25 cells * 6 battery banks = 287.5 volts minimum (at battery posts, no voltage drop)

Anyway, you can see where I am heading. Batteries have a very non-linear set of operating curves. Here is one set generated for a "typical" off grid deep cycle battery:

New poster "leaf" has a really nice set of charts that compare battery voltage against different rates of discharging and charging (as well as resting voltage readings).

leaf wrote: »

Am trying to upload the charts I am using...

Attachment not found.Attachment not found.

[note charts are from: I believe those charts are from Home Power #36, August- September 1993. Lead-Acid Battery State of Charge vs. Voltage ©1993 Richard Perez.
Here is a link: http://www.scubaengineer.com/documen...ing_graphs.pdf

vtMaps]

I don't quite a agree with the resting voltage line (I think the voltage is a bit low)--But it shows how to estimate a battery's state of charge while operating.

Note, where the charts "flatten out"--the room for error estimating state of charge is pretty high.

You will need to work with both your battery vendor and the inverter manufacture to see what is the "exact" battery bank voltage you will need to operate the system reliably (and why engineers are no fun at parties )

-Bill

Re: Outback FM-80 Maximum Parallel unit?

You are very welcome Joel.

And you owe us pictures/write up on your system if it goes forward and is installed!

Have fun.
-Bill