Charge controller for large 1766AH, 48V battery bank with 6 hour peak sunshine

[SillySnakes]

Wondering if someone can weigh in on the following...

I am planning on getting a 1766AH, 48V battery bank (24x 2V, 1766AH batteries) that will be discharged daily to about 70% (Dod=0.7)...so I will be using about 1160AH every 24 hours.

My peak sunshine window is about 6 hours....that means that I will need to recuperate my 1160AH of energy in a 6 hour window. I will have 300W panels (300W is the STC rating...at NOCT it's more like 240W).

The MidNite Classic 150 can give me 3 x 5 panel config (3 in series...then 5 such strings in parallel) using the online MidNite string sizing tool. One such controller can provide me with 60A, if the 15 panels can put out 240W. It can put out 80A if the panels approach STC rating...but that will probably not happen Math for one controller:

15 x 240W = 3600W. Then divide by 60V charging voltage:

3600W/60V = 60A

Obviously 60A is not enough to put back 1160AH into my 1766AH bank... so how many such MidNite controllers do I need to be able to recharge my battery bank in a 6 hour sunshine window ?

If I do my math correctly, would it be:

Amount of charge = AH capacity removed / 6 hour window = 1160AH / 6 hrs = 193.3A. So I need to have 193.3A pumped into the bank to recharge it in 6 hours. Taking into account charge efficiency (93%) and battery bank efficiency (90%):

193.3A / (0.93*0.90) = 231A

So would I need to pump realistically 231A into my battery bank to be able to recharge it in 6 hours ??

Does this sound right ? I would need about 4 MidNite 150 Classics, right connected parallel into the battery bank ? Or can I use a better/larger charger ? Wouldn't 231 A be too much for the battery bus bars/cables ??

Much appreciated!

[SunEagle]

I see two issues with that plan.

The first is your batteries will barely last a year or so being discharge 70% daily. Check the specs and you will see the number of life cycle drops like a rock once you discharge more than 50%.

The second issue is except for maybe a couple of months a year you will not be getting 6 hours of sunlight strong enough to produce full nameplate panel wattage. You need to design around your shortest time of sunlight which usually happens in the Winter to determine how big to size your panel wattage.

Oh another item. You will need an 80A CC for 3600 watts of panels for a 48v battery system.

[SillySnakes]

I see two issues with that plan.

The first is your batteries will barely last a year or so being discharge 70% daily. Check the specs and you will see the number of life cycle drops like a rock once you discharge more than 50%.

The second issue is except for maybe a couple of months a year you will not be getting 6 hours of sunlight strong enough to produce full nameplate panel wattage. You need to design around your shortest time of sunlight which usually happens in the Winter to determine how big to size your panel wattage.

Oh another item. You will need an 80A CC for 3600 watts of panels for a 48v battery system.

These are Rolls Surrette deep discharge batteries and according to the battery specs, at 70% daily discharge, they will last ~2400 cycles, which is about 6.5 years..with maintenance.

There is no Winter at 20N of the Equator...the ~6 hour sunshine window should be fairly steady throughout the year.

[SunEagle]

Ok. So you want to use 1160Ah of a 48v battery system. That calculates to over 55kWh a day yet you only have 3600 watt (peak) for 6 hours (maybe) that comes to about 22kWh.

Due to efficiency losses you always need to put back more that what you take out of a battery system so I would say you do not have enough panels wattage.

[NEOH]

SillySnakes,

Trojan states, "The 2,400 Deep Cycles are for estimation purposes ONLY".
Those cycles were performed in a temperature controlled Lab, as quickly as possible, not over 6.5 years - you mileage may ( will ) vary.
The Trojan Premium L16RE-2V has a 7 year warranty = 2 years 100% replacement and then 5 years pro-rated - but only if you keep precise records of each cell.
It will be interesting to see, if you can actually get both: 2,400 cycles at 70% DOD and 6.5 years of aging.


Here is my quick & dirty Watt-Hour math ...

59,337 Watt-Hours = 24 Batteries x 2 Volts x 1,766AH x 70% DOD = Energy Consumed from the Battery Bank every Night ( you said,"every 24 hours" HUH? )
21,600 Watt-Hours = 6 Hours (sun equiv) x 15 panels x 240 watts = PV Energy Generated every Day

You need, at least, TRIPLE the PV Panels ( ~ 45 ), to just "break-even" every day.
Or maybe even 50 Panels = ( 45 Panels / 90% battery efficiency )
I used the NOCT rating, not the STC rating, because your panels will be HOT at 20 Degrees North latitude.

But what about Daytime loads?
Will the PV Panels be required to supply any power to the Daytime Loads and simultaneously re-charge the 70% DOD battery bank?
You seem to have left out some significant details about this project.

The answer to your question about, "... buss bars and cables ..."
No, 231 Amps is not "too much for buss bars and cables", if they are sized correctly.

You will be charging the battery bank over a 10.5 - 13.5 hour period of varying sunlight and varying energy,
which is ~6 Hours of Solar Equivalent time, not 6 hours of clock time.

[Sunking]

You will kill your batteries in less than a month. You maybe at 20 degrees with lots of sun, but you also have a lot cloudy and rainy days.

Rolls requires at least a minim C/10 charge current. That means with 1776 AH of batteries you will need 180 amps of charge current. Do the math 180 amps x 48 volts = 8640 watts of solar panels. 3600 Watts is just enough to keep those batteries alive for a couple of weeks before you destroy them.

Your math really sucks To generate 58 Kwh from solar panels where you live will requires (58,000 x 1.5) / 6 Sun Hours = 14,500 watts. That will require 3 very expensive 100 Amp MPPT Charge controllers, and minimum size battery is 48 volts @ 3000 AH

You are not even close to reality.

[SillySnakes]

Thank you all for the reply, but I think some important info stated in my first post went un-noticed:


I clearly stated and proved that ONE MidNite Classic 150 with 15x 240W panels (@NOCT) will give me about 3600W, which is 60A of charging current.


I then asked the questions 'Does this sound right ? I would need about 4 MidNite 150 Classics, right connected parallel into the battery bank ? " Implying that YES, I am planning on using more than 15 panels (more than 3600W).


4 x 15 = 60 panels @ 240W = 14400W


14400W/60V = 240A -> 90% Eff -> 216A of charging current total



Will 216A of charging be enough to put ack 1160AH of consumed energy back into the battery bank during the 6 hour sun window ?


Thank you very much. Appreciate the patience...trying to learn as much as possible

[NEOH]

Will 216A of charging be enough to put back 1160AH of consumed energy back into the battery bank during the 6 hour sun window ?

Yes, it is possible.

[Sunking]

4 x 15 = 60 panels @ 240W = 14400W


14400W/60V = 240A -> 90% Eff -> 216A of charging current total

Where are you coming up with 60 volts? Your thread title is 48 volt battery.

14,400 watts / 48 volts = 300 Amps.

[SillySnakes]

Where are you coming up with 60 volts? Your thread title is 48 volt battery.

14,400 watts / 48 volts = 300 Amps.

A 48V nominal battery needs to be charged at 57-60V to force the current into it.
A 26V nominal battery needs to be charged at 28-30V to force the current into it
A 12V nominal battery needs to be charged at 13-14V to force the current into it.

The MidNite Classics actually charge a 48V at 57.5V

Any battery needs to be charged slightly higher than its nominal voltage, otherwise current will go the opposite direction. For example, it would not make sense to connect a 12V solar panel to a 26V battery.

The max current that the solar charge controller will pump into your battery bank at any instant in time is calculated by taking the total cumulative wattage of all the panels connected to that controller in that instant of time and dividing by the charging voltage for the battery bank. Might as well take into account the 90% MPPT current pumping efficiency.

So if I will have a 3600W total power being produced at 9:36:15AM and that is connected into my MidNite Classic 150, the current flowing at that instant of time from the controller and into the battery bank will be:

3600W/57.5V=62.6A --> 90% 0.9*62.6A = 56.34A

[Sunking]

Only one minor little problem You do not know what you are talking about. The Voltage of a Charging battery = OCV + (Charge Current x Ri)

Where OCV = Open Circuit Voltage
Ri = Battery Internal Resistance

A 48 volt battery OCV at 50% SOC is 48.4 volts. The Internal Resistance (Ri) of a 1700 AH 48 volt battery is roughly .0006 Ohms. So with 300 amps of charge current the voltage is:

48.4 volts x (300 Amps x .0005) = 48.55 volts.

MPPT Output Current = Panel Wattage / Nominal Battery Voltage.

The nominal voltage of a 48 volt battery is really hard to figure out. It takes 16 years of school to learn it is 48 volts. As the battery voltage rises while charging, charge current tapers down. When Battery Voltage = Charge Voltage, Charge Current = 0 Amps.

So if your battery is 48 volts with a 14,400 watt panel, and the battery OCV is 48 volts or 45% SOC charge current will be 14,400 watts / 48 volts = 300 Amps. As the battery charrges and voltage goes up to:

50 volts; 14,400 / 50 = 288 amps
51 volts, 282 amps
52 volts, 276 amps
60 volts, 240 amps.

If your Controller is set for 60 volts, and the battery voltage is fully charged at 60 volts = 0 Amps. It is clear you do not understand how a battery charges. Try reading this.

[LETitROLL]

The max current that the solar charge controller will pump into your battery bank at any instant in time is calculated by taking the total cumulative wattage of all the panels connected to that controller in that instant of time and dividing by the charging voltage for the battery bank. Might as well take into account the 90% MPPT current pumping efficiency.

Sources of electricity do not push or pump electricity into anything, Loads (light bulb, charging battery, stereo, etc.) "DRAW", or pull electricity from the source based on their needs, which is controlled by their resistance or impedance.

[Sunking]

Sources of electricity do not push or pump electricity into anything, Loads (light bulb, charging battery, stereo, etc.) "DRAW", or pull electricity from the source based on their needs, which is controlled by their resistance or impedance.

No Sir that is not true. Voltage is pushing Current into Resistance. With a battery Ohm's Law still applies, but you have to account for the battery Open Circuit Voltage aka OCV. An example I like to use because the math is simple is a 12 volt 100 AH battery with an Internal Resistance (Ri) of .01 Ohm.

Voltage of a Charging Battery = OCV + (Charge Current x Ri)
Voltage of a Discharging Battery = OCV - (Discharge Current x Ri)

Open Circuit Voltage of 12 volt battery at 50% SOC = 12.1 volts. If we charge at 10 amps the voltage is 12.2 volts. If we discharge at 10 amps is 12.0 volts.

Voltage is PRESSURE, the driving force called EMF and it certainly pushes. Current is just the result of the resistance the Voltage is pushing against.

[bcroe]

As far as I'm concerned, Ohms law has 3 terms which all function together. Bruce Roe

[SillySnakes]

According to Trojan's website, they clearly state that the battery charging process has to be done at a higher than nominal voltage. For a 48V battery, the charging has to be set to 59.3V during Bulk phase and 64.8V during EQ. So that means that the MPPT Controller will pump out current at the 59.3V... no ? What am I missing here? Look: 1.png
At what point in time does the MPPT controller actually sets the 59.3V charging voltage ? Early on in the charging process or near completion ?