How do i know when battery is @ 50% discharge

dishxpert

Hello Guys,

I am a newbie to this forum and to PV in general. Just had my system setup and trying to fully understand how to manage it.

I am located in the Caribbean where i get about 5 sun peak hrs/day (930am - 2:30pm).
Additionally, i asked a company to design a system that would give me about 150Kwh/mth (40% of total consumption).

This is what they sold me:

5x 230W Canadian Solar 24V panels (7.78A/29V ea) in parallel
8 x 6V/225Ah US-225 Ah connected as 24V system -- total 450 Ah
1 x TS-60 Tri Star Morning Star Charge Controller
1 x Samlex 3000W 24V inverter
1 x Gentran 30A Automatic transfer switch with 6 circuits pre-wired.

Here are my concerns/questions:
1) My charge controller never reads above 29A though 5 panels in parallel should be giving me 7.78A/panel x 5 panels = 38A. Why is my total current so low?
Panels are mounted facing south with an inclination of about 22o.

2) In a given day i collect 140 -180Ah depending on weather conditions. Isnt this low based on the specs of the panel?

3) My battery voltage went as high as 29.5V one day. Problem is, i am not sure when i am at 50% discharge hence when to switch over to o my utility company. Do i look for a particular voltage? For now, i am using 24.2V but not sure if this is too high or low.

4) Does my battery bank of 450Ah match the output from the panels? If not, what should i change?

5) For now, i am running my refrigerator and Office from the system, a total of 380VA/260W (reading from my kill-a-watt). For how many hours should i run these before 50% discharge?

6) I am thinking of upgrading my system before Christmas to produce 220Kwh/mth, up from 150Kwh. How many more panels and batteries would i need?

I will stop there for now but would appreciate detailed answers/comments to my questions. I am continuously reading the articles online to find some answers in the interim.

Thanks much.

Dishxpert

Cariboocoot

Re: How do i know when battery is @ 50% discharge

Welcome to the forum.

Might as well get this part out of the way first: you don't have enough panel. Now let's answer the questions and deal with the 'why'.

dishxpert wrote: »

Here are my concerns/questions:
1) My charge controller never reads above 29A though 5 panels in parallel should be giving me 7.78A/panel x 5 panels = 38A. Why is my total current so low?
Panels are mounted facing south with an inclination of about 22o.

Panels do not put out their rated power level except on rare occasions. What we would typically expect from five 230 Watt panels is:
1150 Watt total array size * 77% efficiency (typical - would be worse if the weather is hot) = 885 Watts effective / 24 Volts (minimum Voltage level you should allow) = 36 Amps potential peak charge current. If your panels are hot or the batteries do not need more current the current will not be there.

2) In a given day i collect 140 -180Ah depending on weather conditions. Isnt this low based on the specs of the panel?

What you can expect from 1150 Watts of panel in terms of daily AC Watt hours:
1150 * 5 hours equivalent good sun * 0.50 (over-all system efficiency) = 2.875 Watt hours. Multiply by 30 and you get 86.25 kW hours per month; not what you were looking for.
With a battery-based system you will only see the Amp hours used/replaced, not the total capacity of the panels. If there's no place for the power to go (loads or charging batteries), they just don't 'harvest' it.

3) My battery voltage went as high as 29.5V one day. Problem is, i am not sure when i am at 50% discharge hence when to switch over to o my utility company. Do i look for a particular voltage? For now, i am using 24.2V but not sure if this is too high or low.

Batteries ought to hit around 28.8 Volts every day for at least as long as it takes to reach that level. This is the Absorb stage. You can get a good idea of what the batteries should be doing from the deep cycle battery FAQ's: http://www.windsun.com/Batteries/Battery_FAQ.htm
The best way to know the SOC of your batteries is with a battery monitor.

4) Does my battery bank of 450Ah match the output from the panels? If not, what should i change?

In my opinion, no. Two things to look at: peak potential charge current and daily Amp hours.
As mentioned before the best you can expect for current is 36 Amps. You would be better off with 10% of the total capacity: 45 Amps. You're at least one panel short of that figure. Your 29 Amps observed is about 6% of the battery capacity, and that's before any loads take their toll. That is barely above the 5% recommended minimum.
In terms of Amp hours, if your DOD is 25% that's 112 Amp hours. Over five hours of sun you would need to average 23 Amps to make this up. If the DOD is 50% you couldn't do it with a 29 Amp peak. That 112 Amp hours is rough 2.2 kW hours AC; far below what you hoped to achieve.

5) For now, i am running my refrigerator and Office from the system, a total of 380VA/260W (reading from my kill-a-watt). For how many hours should i run these before 50% discharge?

You would be doing good to get ten hours at that rate. But that is a rough calculation and quite a number of things will affect it. I based it on 50% DOD and 85% inverter efficiency.

6) I am thinking of upgrading my system before Christmas to produce 220Kwh/mth, up from 150Kwh. How many more panels and batteries would i need?

If you really want the 220 kW hours per month potential:
220 / 30 = 7.3 kW hours per day. That's about a 3kW array and around 1350 Amp hours of battery.
This is large for an off-grid system and it would be expensive. You should first spend the money doing whatever you can to reduce the consumption figure; it will pay back better.

Frankly I think whoever design your system neglected all the loss factors that eat up power. This would be panel temperature, wiring length and size, battery and inverter efficiency, et cetera.
For instance, with that size system there should be six panels wired three in a string and two strings in parallel feeding an MPPT charge controller. That would be more efficient right there.

Vic

Re: How do i know when battery is @ 50% discharge

One additional thought,

The Canadian Solar panels that I find on line have a 29.6 Vmp. IMHO, this is a bit too low for use "in parallel" on a 24 volt FLA battery.

Perhaps, the battery is always HOT (and the panels COOL), and perhaps there is another charge source that can perform an EQ ... but ... seems a bit too close to the edge in my book.

Havind the Panel and Battery P/Ns would help.

Perhaps I am being too literal, reading just what was posted by the OP. Whatta I know ?!? Vic

dishxpert

Re: How do i know when battery is @ 50% discharge

Thank you for the reply so far.

Am here trying to make enough sense out of them. What i hear is that my designed system (1150W) wont give me what i want due to loss of energy from various sources.

The current (of max 29A) is lower than anticipated value of 36A plus too close to the battery voltage of 24V.

So i need two strings of 4 panels in series, then both strings connected in parallel? This will give me what? About 81V and how many amps? 15?

How many 6V/225Ah batteries will i now need?

This is draining me!!

SO, can i stick with a 24.2V to achieve 50% DOD?

Thanks guys and please send me additional pointers.

Dishxpert

petertearai

Re: How do i know when battery is @ 50% discharge

SOC is best checked with a hydrometer and a battery monitor. Buy a battery monitor, you will never regret that purchase. Also more PV is required .
You have found a great site for help with solar power.

BB.

Re: How do i know when battery is @ 50% discharge

There are two major classes of battery charge controllers... Simple PWM (Pulse Width Modulation)--Where there is basically a "switch" (older ones could be relay, newer ones are transistor) that simply turns on and off... Solar panel with Vmp=35 volts and battery that needs ~29 volts to fully charge (and around 30 to 31 volts for equalization).

Switch closes, current flows from solar panel to battery bank (basically at the charging voltage of the battery). Switch opens, charging stops. Turn the switch on and off several times to hundreds of times a second, you get a reduced average charging current. Maximum current from a PWM controller is simply the maximum current from the solar panels/array.

MPPT charge controllers are much more expensive and are based on (typically) some sort of Buck Mode switching regulator.... For the sake of argument, the charge controller can behave like a DC version of a variable step down transformer. If the Vmp-Array Vmp~60 volts, the firmware in the charge controller Down Convert (at ~95% efficiency) from high voltage/low current (solar array) to low voltage/high current (charging battery bank).

The PWM controller, for best use of your solar panel power, needs a Vmp~17.5-18.x volts (12 volt battery) to be able to charge the battery in most conditions (remember, Vmp falls as the panel gets hot in sunlight--The panel may be upwards of 180F in full sun on a very hot/windless day). Vmp can fall below Vbatt-charging if Vmp is not around 17.5 volts or higher (12 volt battery).

If you put your panels in series (or series parallel), the charge controller during maximum charging current (battery less than ~90-80% state of charge) is a constant power device--or in math:

• Vmp-array * Imp-array = Vbatt-charging * Ibatt-charging (ignoring losses)

So, if you have Vmp array of 60 volts and 2 amps, you will have at the battery 15 volts and 8 amps charging current:

• 60 Volts * 2 amps (array) = 120 watts = 15 volts * 8 amps (battery)

So, if you have non standard solar panels (over ~200 watts), then it usually makes sense to get a MPPT charge controller to match the solar array Vmp/Imp to the battery bank Vcharge.

Also, a MPPT controller is nice because you can run Vmp~100 volts and charge a 12/24/48 volt battery bank. This allows you to use much smaller wire from the solar array to the charge controller. And/or you to put the solar array farther away from the battery shed (better sunlight, etc.).

All About Charge Controllers
Read this page about power tracking controllers

Anyway, you are sort of in the middle and trying to grab all of the "Facts" of designing a off-grid solar system... And it is confusing because, depending on specific choices/needs, the design can be done with different components/sizing of the parts/wires/batteries/array/etc...

I would suggest that there are two ways to design your system... The "best" is to measure your loads and define your needs (on grid, off grid, emergency backup, etc.). Then size the battery bank. And lastly the charge controller. If you have the need for a backup generator/AC mains as backup--then we finish off with that.

The second is to pick your core "hardware" you already have on hand (or is the only components you can obtain locally). That may be your battery bank, or your solar panels, or other expensive part(s) of the system that you do not want to part with.

With the core parts picked, we can then suggest the balance of system and estimate how much power you will get from it for your area. At that point (before you have spent any more money), you can decide if the system will meet your requirements or not. And proceed or not...

We can help either way--But it will be less confusing for you if we walk through one paper design first.

Also, battery wise, here are a couple good FAQs:

Deep Cycle Battery FAQ
www.batteryfaq.org

The battery is really the "heart" of your off grid system... If treated well (operated within its capabilities), the batteries will last you many years. If under or over charged, left setting for months without charging, run dead by somebody forgetting and leaving the lights one, etc.--The battery bank will only be so much scrap lead.

That is why we spend so much time discussing the batteries and how best to charge and discharge them. And the maintenance required.

-Bill

stephendv

Re: How do i know when battery is @ 50% discharge

Vic wrote: »

The Canadian Solar panels that I find on line have a 29.6 Vmp. IMHO, this is a bit too low for use "in parallel" on a 24 volt FLA battery.

Nice catch Vic. So I'll state it a little less humbly Those panels are not suitable for charging a 24V system. Since you have a PWM controller, the panels should have a Vmp rating of about 36V. Now if you had an MPPT controller, then you could rewire those panels by putting 4 strings of 2 panels in series. You'd have to check the Voc rating of those panels, but at a glance I don't think you can put 4 in series as this would take you over the Voc limit of the MPPT controller (140V).

But first things first, whoever sold you that system should correct their mistake and either change the panels for some that will actually charge a 24V battery - or they need to swap out your PWM controller for an MPPT version (which is more expensive) and then you can rewire your panels.

dishxpert

Re: How do i know when battery is @ 50% discharge

So, to charge my 24V, 5 x 230W panel system i need a MPPT charge controller? What voltage rating?

Say i get a 6th panel, how should i wire them? 3 in series + 3 in series, then make parallel the two strings? How many Amp and Volt will i end up with and why is that a better system?

Pardon the ignorance.

stephendv

Re: How do i know when battery is @ 50% discharge

With a 24V flooded lead acid battery, you'd typically have to charge it at 28.8V - and occasionally, you'd need to equalise it at about 32V. Assuming you have the "CS6P-230" model of panel, they have a Vmp rating (their voltage at normal standard test conditions under load) of 29.9V, that's at 25 degrees C panel temperature. This voltage will usually be lower by the time it reaches the battery because of:
1. voltage drop in the cabling
2. voltage drop in the charge controller
3. voltage drop due to temperature (the panels also have a Normal Operating Cell Temperature rating of 27V for those panels)

So with all in parallel, you'll never be able to charge at 28.8V, meaning the batteries will never fully charge and will sulfate in short order.

If you want to keep those panels, then yes you'll need an MPPT charge controller. Most of those available in the 60A range have a voltage limit of 140V. If you get 1 more panel then you can put them in 3 parallel strings of 2 in series or 2 parallel strings of 3 in series. The "CS6P-230" panels have a Voc of 37.1V so a 2 x 3 configuration gives you a Voc of 111V and a 3 x 2 would give you 74V - both of which are below the 140V limit of the controller so both will work.

niel

Re: How do i know when battery is @ 50% discharge

3 in series x2 gives the benefit of overcoming v drop losses better (or smaller wire) and no need for fuses/circuit breakers being you only have 2 strings here.

2 in series x3 gives a bit more efficiency on the controller and any future expansions would be in multiples of 2 rather than 3 making it cheaper in the cost of the pvs.

dishxpert

Re: How do i know when battery is @ 50% discharge

Isnt the current important as well in the charging the panels or more so the voltage?

If i get a 6th x 230W panel and have 3 x 2 in series, with MPPT, what V & A should i expect vs 2 x 3 in series?

Can my present TS 60A charge controller handle either config until i get the MPPT?

On the battery side, i have 2 x 4 in series 6v/225Ah batteries giving me 450Ah. With a sixth panel and 2 x 3 vs 3 x 2, how many more 6V batteries should i order?

Thank you guys for helping me to redesign my system. Pity i didnt meet my "brothers" before i bought my system.

Dishxpert

Cariboocoot

Re: How do i know when battery is @ 50% discharge

dishxpert wrote: »

Isnt the current important as well in the charging the panels or more so the voltage?

Both are important, but in different ways. The current is a maximum potential current which will ensure the batteries charge quickly enough to complete a full charge before you run out of sun. The Voltage is necessary to make the set point of the Absorb stage. With panels that have a Vmp of 29 it is too easy to lose Voltage by the time you get to the battery so it becomes impossible to achieve 28.8 Volts for the Absorb cycle.

If i get a 6th x 230W panel and have 3 x 2 in series, with MPPT, what V & A should i expect vs 2 x 3 in series?

The simple version of the math: six 230 Watt panels = 1380 Watts. Multiply by typical 77% efficiency derating and divide by nominal system Voltage and you get about 44 Amps. Likewise three 29 Vmp panels in series will supply 87 Volts more or less to the charge controller, well above the needed level for Absorb and Equalization. This will only work with an MPPT type charge controller.

Can my present TS 60A charge controller handle either config until i get the MPPT?

No. Absolutely not. If you put panels connected in series on your PWM controller you will be throwing away a lot of Watts as the panels will effectively not operate above the maximum battery Voltage. In other words two panels in series will be 58 Vmp but the charge Voltage will still be only 28.8. In essence you will lose the power of the Voltage difference * current: about half the available power in this case.

On the battery side, i have 2 x 4 in series 6v/225Ah batteries giving me 450Ah. With a sixth panel and 2 x 3 vs 3 x 2, how many more 6V batteries should i order?

None. Adding the sixth panel will just bring you up to the amount you need for the 450 Amp hours of batteries you have now.

Thank you guys for helping me to redesign my system. Pity i didnt meet my "brothers" before i bought my system.

Dishxpert

I think whoever designed your system originally ignored all the losses along the way, from the panel temp to the wiring losses, et cetera.

Photowhit

Re: How do i know when battery is @ 50% discharge

I just wanted to state the "located in the Caribbean" with 24v nominal panels (34-35volt) a MPPT charge controller might not be cost effective, since you are only likely to see a modest 10% gain in warm/hot temps. It might be more cost effective to buy more panels....

...of course the panels you have, require you to have an MPPT type charge controller to effectively charge your batteries.

[let us not go there... -Bill B.]

niel

Re: How do i know when battery is @ 50% discharge

i do concur that to stay with the pvs you have and may need to expand upon that an mppt controller should be utilized. as it is the pvs are quite marginal in charging with cooler temps. hotter temps will lower the vmp and the result may be that you can't achieve full charge on your batteries.

i agree that somebody messed up in the design, but all is not lost as it is correctable, even though it may cost a few more $.

dishxpert

Re: How do i know when battery is @ 50% discharge

SO, in going forward i need a MPPT Charge controller and one or few more panels.

I am seeing MPPT for $500+shipping. In the future which >200w panel would you recommend thats cost effective?


Thanks again for your help.

dishxpert

Re: How do i know when battery is @ 50% discharge

Just a thought, can i more that get by on a MPPT 45A with a change in configuration to series/parallel or the 60A is necessary? If so, up to how many strings of 2 or 3 in series array?
Currently, I have a regular PWM 60A controller?

stephendv

Re: How do i know when battery is @ 50% discharge

6 x 230W = 1380W charging at roughly 24V = 57.5A, so you'd need the 60A controller at a minimum. If you're thinking of expanding the array in the future then it might be worthwhile springing for an 80A controller like the outback FM or midnite solar classic so that you have space to add 2 more panels.

Best to use exactly the same panel as you have now when you expand. Or find one that's within 5% tolerance for both A and V as that which you have. If not, they'll have to be connected via a different controller.

niel

Re: How do i know when battery is @ 50% discharge

my guess is 1 more pv for a total of 6. your choice of 2x3 or 3x2 as it won't matter much to us and would need to be determined by you. now 6 pvs at 230w is 1380w. derating at 77% is 1063w.

figuring backwards on your 150kwh/m requirement it is 5kwh per day based on 30 days in a month. if you get 5hrs of sun per day on average then you need 1kw of power to be supplied by pv and 1063w is enough on paper. now i can't account for your weather and other little idiosyncrasies, but this is a good start. you can add more pv if you like or wait and see, but the controller and wiring would need to be able to be accommodated with such an expansion.

the pv derating is fairly typical, but isn't in stone as you may get better (or worse too) efficiency or even times of more intense sun that exceeds 1000w/m^2. assume for rating the controller that there isn't any derating. the 1380w stc rating divided by the nominal output voltage of 24v is 57.5a. obviously a 45a controller won't due here and a 60a controller would fail upon inspection if held to the nec rules to be no more than 48a. you are close with a 60a mppt cc and it would work, but just so you know there's no room for expansion without getting a higher rated cc or have a 2nd one to handle the excess power from any additional pvs.

edit to add: i had a smart a**ed thought to the question of How do i know when battery is @ 50% discharge? keep in mind i am not being serious here, but you know it when it starts to see the light on the other side.:p it's half dead so it'll start seeing the light of the spirit world. i know bad joke.

telljf

Re: How do i know when battery is @ 50% discharge

I read it somewhere in this forum that 24V is 50% SOC charge for 24V battery. But is this 24V under discharge or no discharge state?

Cariboocoot

Re: How do i know when battery is @ 50% discharge

telljf wrote: »

I read it somewhere in this forum that 24V is 50% SOC charge for 24V battery. But is this 24V under discharge or no discharge state?

That would be a resting Voltage SOC: no current in or out of the batteries for at least a couple of hours.
If you use it as an "under load" guideline SOC you can be reasonably certain of not actually being below 50% SOC - depending on how accurate your meter is. Not the best, but if it's all you've got it's what you work with.

dishxpert

Re: How do i know when battery is @ 50% discharge

Hello Guys,

I am back. I've just bought some more equipment but trying to decide with my semi-experienced installer how best to configure the new equipment with the existing. I therefore need your input.

Remember, i have 5 x 230W CS 29v/7.8A panels, 60A PWM charge controller, 3000w/24v pure sine wave inverter, 8 x 225Ah/6V batteries connected in two string of 24v.

I wasn't getting the efficiency i desired and was told that i needed a MPPT controller and some more panels.

So, i went out and bought an Outback FM80 MPPT, 5 x 280w 35v/7.7A panels, and 4 more of same batteries.

My installer says that the FM80 wont be able to handle all that load at 24V as the cap is 2500W @24v. He wants me to use only 3 of the 5 x 280W panels or buy a higher rating (48v) inverter. He is also concerned about the mismatch of panel voltage, i.e, 35V and 29V as they are too wide apart.

I therefore turn to you for your analytical advise on how to best use what i have so i can reason with him. Hopefully, i wont need anything else for now except some more batteries.

Thanks

Cariboocoot

Re: How do i know when battery is @ 50% discharge

Your installer is correct: 80 Amps * 24 Volts = 1920 Watts, factor in the panel derating and you get 2493 Watts - close enough to 2500. He is also correct that 29 Volts and 35 Volts are too far apart for efficient operation (more than 10% difference).

I'd suggest one charge controller for one panel array and a separate controller for the other.

mike95490

Re: How do i know when battery is @ 50% discharge

Cariboocoot wrote: »

....I'd suggest one charge controller for one panel array and a separate controller for the other.

All that, and aim one array a few degrees east (170d), and the other a few degrees west (190d), and you could have a bit more power to use throughout the day, instead of it all peaking at noon (180 deg)

icarus

Re: How do i know when battery is @ 50% discharge

Adding batteries to an existing string is not generally advisable?

Do your load calcs suggest you need more batteries?

Tony

jeffkruse

Re: How do i know when battery is @ 50% discharge

Cariboocoot wrote: »

Your installer is correct: 80 Amps * 24 Volts = 1920 Watts, factor in the panel derating and you get 2493 Watts - close enough to 2500. He is also correct that 29 Volts and 35 Volts are too far apart for efficient operation (more than 10% difference).

I'd suggest one charge controller for one panel array and a separate controller for the other.

He has 2550W of panels. Subtract the ineffiencies because the voltages don't match and wont the controler work just fine?

dishxpert

Re: How do i know when battery is @ 50% discharge

In spite of the mismatch of the panel voltages, the fact that the current are almost identical doesn't matter?

If i am to make two strings of panels with two charge controllers (80A MPPT & 60A PWM), how best to configure the 10 panels that i have?

RandomJoe

Re: How do i know when battery is @ 50% discharge

So how about wiring them as five strings of one old / one new panel (the two styles are very close in current rating) giving you five 64V strings, parallel those together to the FM80 (64V x (5 x 7.7A) = 2464W, after derating should be right to the top end of the FM80's capabilities)?

Just my amateur enthusiast off-the-cuff take...

BB.

Re: How do i know when battery is @ 50% discharge

Should be fine (if I got all the Voc/Vmp and Vbatt numbers correct).

-Bill

niel

Re: How do i know when battery is @ 50% discharge

dishxpert,
i had a long reply for you, but i lost it somehow. in a nutshell, i am not following you as to why you bought all of that stuff. that's a far cry from 1 more pv that's the same as the others and an mppt controller that can handle that power and a bit more for possible expansion. i don't think anybody suggested to you the stuff you bought and i don't have a clue what you are doing and why. as such i don't know what to say to you in advice. as it was you should've run it past us first as to what we thought before you bought something. maybe you are misinterpreting us or not understanding us for i really don't know. if this were a car forum and somebody recommended maybe brake pads and new tires i certainly wouldn't go buying a whole new car as you practically bought another whole system. it wouldn't have taken all of that to get you a working viable system.

dishxpert

Re: How do i know when battery is @ 50% discharge

Hi Niel,

I am continuing to upgrade my system until i reach self sufficiency. My monthly usage is about 420KWh and i started out with 1150 W panels/C60. With these 5 additional panels i should be at 60-70% sufficiency.

Dishxpert