Help required to design off grid system

syedbukhari · September 2012

Re: Help required to design off grid system

Photowhit wrote: »

Is the 12.6 Volts across the bank?

Yes this is across the bank

westbranch · September 2012

Re: Help required to design off grid system

here is a good battery layout site:http://www.smartgauge.co.uk/batt_con.html

Note the max number of batteries in a bank!!!

syedbukhari · September 2012

Re: Help required to design off grid system

Thanks for the link
Kindly tell me can I use 36 volt battery bank for my system to get right charging
And also guide can I use 48 volt battery bank after getting MPPT charger ?

westbranch · September 2012

Re: Help required to design off grid system

you can not use a 36v battery bank until and unless you have an MPPT charge controller.

Please use a standard charger from mains current to quickly charge those batteries for the interim, otherwise they will die from being undercharged.
Please post a list of all components, make and model, when you get the MPPT charger and we will assist you in building the best system with what you have...

Photowhit · September 2012

Re: Help required to design off grid system

syedbukhari wrote: »

Yes this is across the bank

Your batteries are dieing (or dead) figure out some way of charging them now. I guess I haven't said it, but remove all loads!!!!

If your current charge controller is set up correctly for 48 volt batteries, you can run strings of 3 panels and likely not have a problem with the over voltage. If you will tell us the brand and model of the charge controller you are using we/I will look it up and check to make sure you won't damage it, doing this.

If you have to remove the batteries one at a time and put them on a truck or car's system and get some current running into them!!!, Likely they won't start the car, so you can use cable made for giving a car a boost. If you have a garage associated with your fuel station, likely they have a charger for cars. It's not ideal, but it will help. Leave one string attached to the solar array which you have changed to strings of 3, I know you'll only have 12 attached, but they will do something...

If someone has a warehouse that uses electric forklift, go ask if they would charge these batteries, don't save face/worry about embarassment, these batteries may be dead weight if you don't do something NOW!

niel · September 2012

Re: Help required to design off grid system

i should add that the voltages i mentioned would be the battery voltage for a 48v battery bank. charging voltages from pvs are much higher than the battery voltages.

you may have something radically wrong in your system that either is a result of your misunderstanding of what is right or you are misscommunicating to us the circumstances and i suspect both.

batteries are not a finite voltage as i said, but are slightly variable and this is differences in battery makes and types, but are nominal at the voltage of 12v or 6v or whatever they were made for. the state of charge has quite a bit to do with it too for a battery low on charge will show a lower voltage than one that is fully charged and i am talking of batteries without anything connected to them for several hours of rest. in a 48v battery bank you have 4 12v batteries in series or maybe 8 6v batteries in series and some opt for 24 2v batteries in series. in a series arrangement you can check individual batteries, but you must consider it as 1 whole battery too operating at 48v. checking either the battery bank as a whole or its individual batteries can indicate things to those more familiar with batteries. one such thing would be that for the whole 48v battery bank to show 12v or so means the whole thing is dead, but if that is the voltage of the individual batteries that make up the battery bank then this is indicative of more normal voltages, but that they would be quite low on charge. also note that showing different voltages on batteries in a series string indicates a problem too as they should be nearly identical in voltage.

for general speak, most individual 12v batteries can be anywhere from about 12.7v fully charged to about 10.5v totally dead with no charging or discharging sources to it for many hours. to charge a 12v battery to full it must often go to about 14.4v give or take a few tenths of a volt and then it'll stay there for awhile in an absorb charge stage before reverting to a float stage or no stage as in no charging. for some batteries this can vary as some batteries are know to require nearly 15v rather than the 14.4v i mentioned, but to charge the battery to that voltage much higher voltages from pvs are used to insure that under its full power loading during most conditions that it will continue to deliver its power. this is almost always over 17v from a pv and is now typically 18v+ for the 12v nominal pvs out there today and is known more commonly as the vmp. keep in mind i gave rough figures for a 12v battery and you said you have a 48v battery bank so these figures get multiplied by 4. same applies to the pvs' voltage requirements being multiplied by 4 in comparrison to a 12v nominal pv.

in addition, one should check the specific gravity of the cells to be sure of the state of charge present as voltages can be misleading at times. if you have vrla type batteries then you can only rely on the voltages and possibly a battery monitor.

i sincerely hope there's nothing fatally wrong going on in your setup.

syedbukhari · September 2012

Re: Help required to design off grid system

I think i am not able to elaborate the things rightly.

I am using 12 batteries making 48 volt system. I was using second hand APC inverter and a chinease solar controller. In the morning time the controller showed 30-35 charging amp and 52 charging volt. At this stage the cut off voltage were set on 54 volts. At this time charger showed SOC 50 % after 3-4 hours it showed SOC 100 % and charging amp droped to 20 amps. Today i changed the setting of controller and increased the cut off voltage up to 60 volt and also i bought today an inverter of a chinease EAST company with buit in MPPT charger. After completing this i divide the seven strings and put the load of 3 strings on chinease inverter and the load of remaing 4 strings on old chage controller. In the morning chinease inverter showed 20 amp and apc inverter controller showed 25 amp and the batteries starts charging and after 3-4 hour charging amp reduced to 50 %. At this time inverter showed the SOC 100 % and the voltages were 13 volt but the gravity was still 1.150. Then i disconnect the 4 four batteries from the system and check the voltage of individual battery it was 12.6 volt. I started these 4 batteries to charge by AC power after 5 hour the voltage of the battery were 13.7 and the gravity was 1.175.
Please guide i am not getting the exact reason of this problem.
Please also guide about MPPT charger
if i use the array of 82 pv input volt to charge the battery bank of 48 volt
Will the charger convert the extra voltages into amp to increase the efficiency of the battery or not?
if i use current array of 54 PV volts to charge the battery bank of 48 volt then
Will the charger convert the amp into voltages to full fill the charging volt requirement or not?

westbranch · September 2012

Re: Help required to design off grid system

syedbukhari wrote: »

Please also guide about MPPT charger
if i use the array of 82 pv input volt to charge the battery bank of 48 volt
Will the charger convert the extra voltages into amp to increase the efficiency of the battery or not?

YES

syedbukhari wrote: »

if i use current array of 54 PV volts to charge the battery bank of 48 volt then
Will the charger convert the amp into voltages to full fill the charging volt requirement or not?

NO

MPPT will only change excess voltage into Amps.

the voltage you have on those AC charged batteries is much better. you should do the same treatment on the rest of the battery bank. Then you will have hopefully rescued them.

I will leave the determination of charge settings to the others.

syedbukhari · September 2012

Re: Help required to design off grid system

Then i have to increase panels from 14 to 15 to ensure the connections or to decrease them from 14 to 12. or is there any way to manage the connections because Vmp of one panels is 27.9

westbranch · September 2012

Re: Help required to design off grid system

before this question can be answered properly we need the specs for the MPPT charger to ensure that the charger can even handle 15 panels in at least 3 in series per string (array).

There may be other combinations that would work.

stephendv · September 2012

Re: Help required to design off grid system

syedbukhari wrote: »

I am using 12 batteries making 48 volt system. I was using second hand APC inverter and a chinease solar controller. In the morning time the controller showed 30-35 charging amp and 52 charging volt. At this stage the cut off voltage were set on 54 volts.

What exactly is this "cut off voltage"? Is it where the inverter cuts off AC power to the loads - or is it where the charge controller keeps this voltage constant on the batteries?
Usually a charger lets you set the Absorb voltage and Absorb time -which values are you using?

syedbukhari wrote: »

In the morning chinease inverter showed 20 amp and apc inverter controller showed 25 amp and the batteries starts charging and after 3-4 hour charging amp reduced to 50 %. At this time inverter showed the SOC 100 % and the voltages were 13 volt but the gravity was still 1.150.

The SOC of the inverter does not actually measure soc, it just measures the voltage of the battery so don't rely on it to tell you whether the batteries are charged or not. The specific gravity is the one to use.

Cariboocoot · September 2012

Re: Help required to design off grid system

syedbukhari wrote: »

Then i have to increase panels from 14 to 15 to ensure the connections or to decrease them from 14 to 12. or is there any way to manage the connections because Vmp of one panels is 27.9

You are right; you either need to add one more panel (preferable) or subtract two in order to get strings of 3 panels in series = 83.7 Vmp.
There is no way to get the right Voltage for properly recharging a 48 Volt bank (Vmp 70) from any fewer, regardless of the charge controller type.

The best thing to do is get one more panel and have five strings of three in series. Each of these five strings should have its own fuse or breaker on it before connecting to the charge controller input.

This will work with a PWM type charge controller, although you will be losing some of the charge potential due to the inability to down-convert the higher Voltage (83.7 vs. the 'ideal' 70). Using an MPPT type controller with this array would be best.

syedbukhari · September 2012

Re: Help required to design off grid system

westbranch wrote: »

before this question can be answered properly we need the specs for the MPPT charger to ensure that the charger can even handle 15 panels in at least 3 in series per string (array).

There may be other combinations that would work.

MPPT voltage range 48-90 Vdc
Max. charge current 50 amp
Conversion efficiency 98 %

syedbukhari · September 2012

Re: Help required to design off grid system

Cariboocoot wrote: »

You are right; you either need to add one more panel (preferable) or subtract two in order to get strings of 3 panels in series = 83.7 Vmp.
There is no way to get the right Voltage for properly recharging a 48 Volt bank (Vmp 70) from any fewer, regardless of the charge controller type.

The best thing to do is get one more panel and have five strings of three in series. Each of these five strings should have its own fuse or breaker on it before connecting to the charge controller input.

This will work with a PWM type charge controller, although you will be losing some of the charge potential due to the inability to down-convert the higher Voltage (83.7 vs. the 'ideal' 70). Using an MPPT type controller with this array would be best.

Thanks and also guide in the light of charger specifications

syedbukhari · September 2012

Re: Help required to design off grid system

stephendv wrote: »

What exactly is this "cut off voltage"? Is it where the inverter cuts off AC power to the loads - or is it where the charge controller keeps this voltage constant on the batteries?
Usually a charger lets you set the Absorb voltage and Absorb time -which values are you using

Sorry i am not sure about this

westbranch · September 2012

Re: Help required to design off grid system

Yes you are limited to 3 panels in series. so you need 15 panels minimum.

Now what brand of battery so we can confirm the correct charging Voltage.

PS> I will be off line for the next few hours

syedbukhari · September 2012

Re: Help required to design off grid system

westbranch wrote: »

Now what brand of battery so we can confirm the correct charging Voltage.

I am using local Pakistani lead acid flooded battery. There is no specification material along with this battery. I get only one information written on it is
Cut off charging voltages is 14.4. The battery capacity is 205 amp and its weight is approximately 35 KG.

Cariboocoot · September 2012

Re: Help required to design off grid system

syedbukhari wrote: »

I am using local Pakistani lead acid flooded battery. There is no specification material along with this battery. I get only one information written on it is
Cut off charging voltages is 14.4. The battery capacity is 205 amp and its weight is approximately 35 KG.

In that case you will have to use 'generic' settings for your 48 Volt system. It sounds like "cut off Voltage" is the Absorb set point; the highest Voltage the battery is meant to be at during charging.

So you get specs like these:
Absorb Voltage (4 * 14.4) 57.6
Float Voltage (4 * 13.6) 54.4

You will have to watch them to see that they do not use too much water while charging. You will also have to watch the charge current and see that the Absorb stage does not go for too long. Not knowing the specifications of the charge controller it is difficult to say what if anything can be done about these factors.

syedbukhari · September 2012

Re: Help required to design off grid system

Cariboocoot wrote: »

Not knowing the specifications of the charge controller it is difficult to say what if anything can be done about these factors.

The spec are
MPPT voltage range 48-90 Vdc
Maximum charge current 50 amp
Maximum efficiency. 98%
Sorry I have above information only
I will try my best to inform you more about the charger

Cariboocoot · September 2012

Re: Help required to design off grid system

syedbukhari wrote: »

The spec are
MPPT voltage range 48-90 Vdc
Maximum charge current 50 amp
Maximum efficiency. 98%
Sorry I have above information only
I will try my best to inform you more about the charger

Yes, I understand that. The issue is one of how the charge controller works; what can be adjusted on it. There may be only a few increments for Absorb Voltage.
Some have a fixed Absorb time length, some have a few selections for time, some have a widely adjustable amount. Others have an End Amps function which can be used separately or together with the time limit. And so forth.

There is one possible problem with that controller's 90 Volt input maximum: your array Voltage open circuit will exceed that with three of the panels in series. How the controller will respond to that is another question: some will just ignore the higher Voltage and wait for it to drop before turning on. Others will fry.

On the whole I don't think you have a very good charge controller and you should seriously consider buying a better one.

BB. · September 2012

Re: Help required to design off grid system

One other voltage would be handy--Maximum input voltage to the charge controller and how cold it gets in your area (at sun rise).

As solar arrays get cold (below ~25C), their output voltages rise. We are looking for the panels' Voc-cold voltage vs the maximum input voltage (never exceed) for the charge controller. Vmp (panels under load) will always be lower than Voc or Voc-cold.

-Bill

niel · September 2012

Re: Help required to design off grid system

i think this is the problem,
"and 52 charging volt. At this stage the cut off voltage were set on 54 volts."
that is too low to fully charge your batteries and they have been consistently undercharged. it should be 14.4v x 4 = 57.6v and that is at 25 degrees c. for hotter temps you might want to lower it and in colder temps raise it, but how much depends on the battery and the temps of course. if the cc has temperature compensation that attaches to the batteries then the temperature will automatically be compensated for. depending on how long this has been going on may determine how much sulfation has occurred and i think i can guarantee they have sulfated some.

bill,
do we also know the max input voltage to the apc as some may not like the high charging voltage?

westbranch · September 2012

Re: Help required to design off grid system

Is this the MPPT CC you have?

http://lahore.olx.com.pk/solar-charge-controller-mppt-50a-48v-iid-365932958

Seems to match up to your specs posted.

BB. · September 2012

Re: Help required to design off grid system

Most small UPS type systems (APC?) tend towards ~14.2 to 14.4 volts charging as they normally use some sort of sealed battery (AGM, GEL, VRLA, etc.) battery technology--Which usually do not like higher charging voltages.

So--It would not be a bad assumption to guess that a UPS was not designed to run at 15-16 volts (based on 12 volt multiples).

I did a quick look, and could not find a spec. for min/max DC input voltage. They do seem to have an active forum.

-Bill

syedbukhari · September 2012

Re: Help required to design off grid system

BB. wrote: »

One other voltage would be handy--Maximum input voltage to the charge controller and how cold it gets in your area (at sun rise).

As solar arrays get cold (below ~25C), their output voltages rise. We are looking for the panels' Voc-cold voltage vs the maximum input voltage (never exceed) for the charge controller. Vmp (panels under load) will always be lower than Voc or Voc-cold.

-Bill

Voc. 34
Vmp 27.6

Our area is very sunny and temp. remains above 25 c but from Nov to Feb it remain below 25 c.

stephendv · September 2012

Re: Help required to design off grid system

syedbukhari wrote: »

Voc. 34
Vmp 27.6
Our area is very sunny and temp. remains above 25 c but from Nov to Feb it remain below 25 c.

That's the Voc for the panel, but what is the voltage input limit for the MPPT charge controller you have? Earlier on you said:

syedbukhari wrote: »

MPPT voltage range 48-90 Vdc

So 3 x 34V = 102V which is too high for that charge controller. If you exceed the voltage limit on a charge controller, the controller will burn out- there is no protection against this.
What's important about the temperature is the record low temperature, not the average temperature. For example, if it ever gets to 0 degrees, then Voc will increase by 9%, so instead of 34Voc you'll have 37Voc per panel and 111V for every string. The controller's input voltage MUST be above this value, else it will burn out.

BB. · September 2012

Re: Help required to design off grid system

I assume that 15C is about the minimum temperature that you get for your area (Jiddah Saudi Arabia)? If so, that is not very cold, so Voc does not increase very much (looking for temperatures well below freezing to dramatically increase array voltage).

-Bill

westbranch · September 2012

Re: Help required to design off grid system

Bill, way back he said he was in Pakistan. no location though from what I remember.

niel · September 2012

Re: Help required to design off grid system

yes, he's in pakistan, but temps can vary widely even there as there are mountains too. my guess is for him to put it about 57.5v and see if things improve.

i was not asking of the cc input v max, but that is important too. i was asking of the v max to the inverter as ups type inverters may not extend as far out in voltage capability as some of our other inverters do.

syedbukhari · October 2012

Re: Help required to design off grid system

Sorry for late responding. I belong to Multan (Punjab). Here in Multan temp. remains above 40 centigrade and there is very few chances for the temp to go down to zero centigrade.
According the advice of the forum i connected three panel in series and got aprox. 83 volts and i used East company inverter with built-in MPPT charger. Mean while i disconnected 4 batteries from the system and charged them on utility grid. From the morning i started to supply PV power to the remaining 8 batteries and withdraw all the load from these batteries. Before charging the Gravity was 1.120. After 1 hr. the inverter showed SOC 100 % at this stage charger started to increase the PV input voltages and the voltages increased up to 98 volts. At this stage the PV charging current goes down to zero. After some time i again checked gravity of these batteries it was 1.150 and the batteries those were charging on utility grid were enjoying with the gravity of 1.210. Next day i connected these 4 batteries with PV supply and charged them with this. At the same time started to charge other 8 batteries with utility grid. This day the inerter worked efficiently and charged the batteries @ 70 volt and convert the remaining voltages in to amp. Today evening i checked gravity of all 12 batteries it was 1.200 approx. Now my system is working smoothly but i wants to increase the gravity of the batteries up to 1.250. Please advice how can i do this and why my charger increases the PV input voltages.

Help required to design off grid system

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