Charging current and other newbie stupid questions

[wa1gon]

Greetings,

I currently have a 245 watt panel (have 4 total but 3 aren't deployed yet) connect via C40
charge controller to 4 new 6 volt Duracell 225 AH batteries (two strings in parallel in a 12 volt configuration)

I haven't been able to find any specs on the batteries that I purchased at Batteries Plus.

I am guess when we talk about c/10, c/12, etc we are talking about the C of the entire bank.
So with my 4 225 6v (ie 2 12v) 450 AH recommend charge c/12 would be 37.5 amps. 450/12 = 37.5.
Is that correct?

What is coming out of the CC is about 6-8 amps @ 12-13v. I am assuming that is because it is winter and not getting
enough sun, but I was also thinking that the current might be limited because of the state of charge of the batteries.
Right now I have a fairly low power budget, but I am thinking if the batteries get very low, then it may take more than
one day of sun to charge them with a single panel. Right now they completely charge about about 1 PM. How every
even when I put a load of say 80 watts, it doesn't seem to increase the current much coming from the CC so again I am
assuming that the panels are at full throttle for the amount sun.

Does the batteries limit the current as they get charged?

Right now everything is just an experiment, so that I can fully understand all the moving parts.

Am I on the right track?

I am also thinking about using one of my C40 as a load control to do a low voltage disconnect. What would you recommend as
a good cut off voltage for the FLA GC2 batteries?

thanks and 73
-darryl WA1GON

[SunEagle]

Greetings,

I currently have a 245 watt panel (have 4 total but 3 aren't deployed yet) connect via C40
charge controller to 4 new 6 volt Duracell 225 AH batteries (two strings in parallel in a 12 volt configuration)

I haven't been able to find any specs on the batteries that I purchased at Batteries Plus.

I am guess when we talk about c/10, c/12, etc we are talking about the C of the entire bank.
So with my 4 225 6v (ie 2 12v) 450 AH recommend charge c/12 would be 37.5 amps. 450/12 = 37.5.
Is that correct?

What is coming out of the CC is about 6-8 amps @ 12-13v. I am assuming that is because it is winter and not getting
enough sun, but I was also thinking that the current might be limited because of the state of charge of the batteries.
Right now I have a fairly low power budget, but I am thinking if the batteries get very low, then it may take more than
one day of sun to charge them with a single panel. Right now they completely charge about about 1 PM. How every
even when I put a load of say 80 watts, it doesn't seem to increase the current much coming from the CC so again I am
assuming that the panels are at full throttle for the amount sun.

Does the batteries limit the current as they get charged?

Right now everything is just an experiment, so that I can fully understand all the moving parts.

Am I on the right track?

I am also thinking about using one of my C40 as a load control to do a low voltage disconnect. What would you recommend as
a good cut off voltage for the FLA GC2 batteries?

thanks and 73
-darryl WA1GON

A single 245 watt panel will not be enough to charge your 450 AH battery system. Simple rule of thumb would be to have at least the same amount of panel wattage with respect to battery AH rating. So you should at least hook up a second 245 watt panel.

Post both the panel information (Vmp, Imp) as well as the Charge controller data (max DC voltage input, amp output, etc.) That way someone will be able to give you a better idea on how to hook up your system to properly charge your battery system.

[wa1gon]

re: Charging current and other newbie stupid questions

A single 245 watt panel will not be enough to charge your 450 AH battery system. Simple rule of thumb would be to have at least the same amount of panel wattage with respect to battery AH rating. So you should at least hook up a second 245 watt panel.

Post both the panel information (Vmp, Imp) as well as the Charge controller data (max DC voltage input, amp output, etc.) That way someone will be able to give you a better idea on how to hook up your system to properly charge your battery system.

CC: Xantex C40 Max DC volts: 125 V DC, load current 40A
PV: 246.2 Watts
37.31 Voc
8.21 Im

However my questions are more educational at this point. My current power requirements on most days would be less than 500 w/hrs which
is why I only have one panel deployed.

Just by the math, my batteries provide 5400 watt/hrs (450 Ah*12 volts).

So wanting to keep my power budget to 1/2 discharge then my power budget would be 2700 watt/hrs.

As of 10 AM my panel was putting out 84 watts which I expect to continue as least though 2 pm or least 336 watt/hrs per sunny day, which is kind of disappointing because I hoping to get close to 1000 watt/hrs per day, per panel.

Here is my questions:

1. What voltage is a good rule of thumb for 50% discharge?
2. If I put in 10 AH into a battery how much is lost?, I am guessing about 20%
3. If I have a 2 A load connected to the batteries, and the PV is putting out 8 A, does that mean the battery is being charged at 6A?


Thanks
Darryl

[br2an]

Greetings,

I currently have a 245 watt panel (have 4 total but 3 aren't deployed yet) connect via C40
charge controller to 4 new 6 volt Duracell 225 AH batteries (two strings in parallel in a 12 volt configuration)

I haven't been able to find any specs on the batteries that I purchased at Batteries Plus.

I am guess when we talk about c/10, c/12, etc we are talking about the C of the entire bank.
So with my 4 225 6v (ie 2 12v) 450 AH recommend charge c/12 would be 37.5 amps. 450/12 = 37.5.
Is that correct?

What is coming out of the CC is about 6-8 amps @ 12-13v. I am assuming that is because it is winter and not getting
enough sun, but I was also thinking that the current might be limited because of the state of charge of the batteries.
Right now I have a fairly low power budget, but I am thinking if the batteries get very low, then it may take more than
one day of sun to charge them with a single panel. Right now they completely charge about about 1 PM. How every
even when I put a load of say 80 watts, it doesn't seem to increase the current much coming from the CC so again I am
assuming that the panels are at full throttle for the amount sun.

Does the batteries limit the current as they get charged?

Right now everything is just an experiment, so that I can fully understand all the moving parts.

Am I on the right track?

I am also thinking about using one of my C40 as a load control to do a low voltage disconnect. What would you recommend as
a good cut off voltage for the FLA GC2 batteries?

thanks and 73
-darryl WA1GON

Hi Darryl,

I think I can answer some of your questions. I am no expert and if I am wrong I am positive someone will flame me; actually a good thing about this site.

The CC will limit the current to the batteries depending on the charge state of the batteries. Low batteries (70% or less) will take all the current available in your system. As the battery charge level increases they will take less. Fully charged will take almost nothing.

As I understand it, four 225AH 6v batteries connected 2 series-2 parallel as you have is a total of 450AH at 12V. For quite some time I was simply adding the AH values and over estimating my AH capacity. You seem to have it right.

I believe you may have more panels bought (not connected) than you need for your batteries. 4 x 245W/panel = 980W of power. 980W/12V = 81.7A and 450AH/81.7A => C/5.5 This would be too high a charge rate and would damage your batteries.

So I am guessing you are planning on buying more batteries and another CC. Correct?

One question for you, how are you measuring the current output of you panels? Is that a readout on your CC or do you have a separate meter?

I find this a very good site, there is a ton of good reading here.

Good luck.

Brian

[SunEagle]

Hi Darryl,

I think I can answer some of your questions. I am no expert and if I am wrong I am positive someone will flame me; actually a good thing about this site.

The CC will limit the current to the batteries depending on the charge state of the batteries. Low batteries (70% or less) will take all the current available in your system. As the battery charge level increases they will take less. Fully charged will take almost nothing.

As I understand it, four 225AH 6v batteries connected 2 series-2 parallel as you have is a total of 450AH at 12V. For quite some time I was simply adding the AH values and over estimating my AH capacity. You seem to have it right.

I believe you may have more panels bought (not connected) than you need for your batteries. 4 x 245W/panel = 980W of power. 980W/12V = 81.7A and 450AH/81.7A => C/5.5 This would be too high a charge rate and would damage your batteries.

So I am guessing you are planning on buying more batteries and another CC. Correct?

One question for you, how are you measuring the current output of you panels? Is that a readout on your CC or do you have a separate meter?

I find this a very good site, there is a ton of good reading here.

Good luck.

Brian

If Daryle had a MPPT type charge controller you would be correct that panel wattage into the charger would be equal to the wattage into the batteries. But he has a PWM type controller.

Each panel is rated at 8.21 max amps. And the charge controller only puts out battery voltage or about 13 to 13.5 volts at full charge. If you multiply the panel amps x the charge voltage you get 8.21 x 13.5 = 110 watts max from one panel, not 245 watts. So the max amount of amps for all 4 panels being used to charge the batteries would be 440 watts / 13.5 volts or less than 33. That comes to approximately 450AH / 33A = C/13.6 which is not enough to really charge his battery bank.

[wa1gon]

If Daryle had a MPPT type charge controller you would be correct that panel wattage into the charger would be equal to the wattage into the batteries. But he has a PWM type controller.

Each panel is rated at 8.21 max amps. And the charge controller only puts out battery voltage or about 13 to 13.5 volts at full charge. If you multiply the panel amps x the charge voltage you get 8.21 x 13.5 = 110 watts max from one panel, not 245 watts. So the max amount of amps for all 4 panels being used to charge the batteries would be 440 watts / 13.5 volts or less than 33. That comes to approximately 450AH / 33A = C/13.6 which is not enough to really charge his battery bank.

It isn't a max of 8.1A x 13.5V, it is 8.1A * 30V per panel, however the most I have seen has been about 8A @ 14.4V coming out of the controller. which I think has been due to winter and high clouds. I am hoping to get closer to 15A in the summer, but I am not banking on it.

I understand that one panel or even two will unlikely provide a total charge in one day, but this begs another question. If max current to safely charge a battery is C/10 then it will take 10+ hours to charge the battery which is more than the usable sunlight in a day. Of course this would be full discharge which is a bad plan. However a 50% discharge cycle would be 5+ hours.

What I am trying to plan for is a 50% discharge cycle, right now I am closer to 25% discharge cycles, so one panel charges the batteries by about noon.

Now the question that I have burning is:

At what battery voltage should I set my Low Voltage cut off at for a 50% duty cycle?

thanks
-darryl

[SunEagle]

It isn't a max of 8.1A x 13.5V, it is 8.1A * 30V per panel, however the most I have seen has been about 8A @ 14.4V coming out of the controller. which I think has been due to winter and high clouds. I am hoping to get closer to 15A in the summer, but I am not banking on it.

I understand that one panel or even two will unlikely provide a total charge in one day, but this begs another question. If max current to safely charge a battery is C/10 then it will take 10+ hours to charge the battery which is more than the usable sunlight in a day. Of course this would be full discharge which is a bad plan. However a 50% discharge cycle would be 5+ hours.

What I am trying to plan for is a 50% discharge cycle, right now I am closer to 25% discharge cycles, so one panel charges the batteries by about noon.

Now the question that I have burning is:

At what battery voltage should I set my Low Voltage cut off at for a 50% duty cycle?

thanks
-darryl

No that low voltage is not because of clouds or Winter it is because of the type of Charge Controller you are using. Your Charge Controller only puts out a voltage high enough to charge the battery which is about 13 to 14 volts not the panel voltage. Because the CC limits the voltage down the calculation for wattage charging your battery is the Imp of the panel x the charge voltage or 8.21 x 13.5 = 111 max wattage you will get out of one panel. You are losing a lot of panel wattage because of the PWM type charger. To make up for that you need to use more panel wattage.

The charge value of C/10 relates to the amount of amperage not hours. Your battery C = 225AH. C/10 = 22.5 amps. If you had C/6 = 37.5 amps which would over charge your batteries. Or C/25 = 9 amps which is more than one panel can put out and will never be enough to properly charge your battery system.

The low voltage cut off is specific to the battery. You should have paperwork concerning your Duracell 6v battery. That will give you the voltage values for specific discharge %.

[wa1gon]

No that low voltage is not because of clouds or Winter it is because of the type of Charge Controller you are using. Your Charge Controller only puts out a voltage high enough to charge the battery which is about 13 to 14 volts not the panel voltage. Because the CC limits the voltage down the calculation for wattage charging your battery is the Imp of the panel x the charge voltage or 8.21 x 13.5 = 111 max wattage you will get out of one panel. You are losing a lot of panel wattage because of the PWM type charger. To make up for that you need to use more panel wattage.

I wasn't talking low voltage, the voltage coming from the panel is ~30 volts, it was the current coming from the CC which is around 1-7A depending
on the amount of sun.

I am sorry, but that doesn't make any sense. That would mean that all panels would have to be connected in parallel and there would be no point in
having a panel over 120 watts or so. There would also be no point in having a CC with max V over 50 volts.

The charge value of C/10 relates to the amount of amperage not hours. Your battery C = 225AH. C/10 = 22.5 amps. If you had C/6 = 37.5 amps which would over charge your batteries. Or C/25 = 9 amps which is more than one panel can put out and will never be enough to properly charge your battery system.

It does relate, because current at C/10 will charge in fewer hours than current at C/20.

The low voltage cut off is specific to the battery. You should have paperwork concerning your Duracell 6v battery. That will give you the voltage values for specific discharge %.

In my first post I said I didn't have the datasheet on the battery. Does it really vary enough that I couldn't take a good guess to set the load controller?

thanks
Darryl WA1GON

[Mike90250]

Panel design voltage is dependent on the number of series connected wafers inside, each wafer (cell) producing about 0.55V. The open circuit voltage is rapidly reached with very little illumination.

Panel amps is dependent on the physical size of the cells (2x2 or 2x3, 4x6....) larger cells producing more amps. Production is dependent on the brightness of light, brighter light produces more amps.

PV panels are CURRENT Sources, and so their output voltage is easily limited by the amount of load the panel is presented. A battery will pull the voltage instantly to the battery voltage, with no damage (panels can be shorted indefinitely without damage) and then you are just pumping the panels rated amps into the battery, till the battery voltage rises close enough to the panels rated voltage that charging stops.

With a 30V panel, you have traded cell surface area (amps) for higher voltage. That means you can only obtain 8.2 amps from the panels (unless you illuminate them with large mirrors). If you obtain a true MPPT charge controller, now you are going to be using the controllers MPPT function (a very special DC-DC converter usually spec's at about 95% efficient) as a DC transformer, and you can now get wattage in = wattage out (minus 5% losses) and you will be able to fully utilize your panels.

Peukert's law http://en.wikipedia.org/wiki/Peukert%27s_law covers the losses involved with
discharging batteries and means it's very hard to discover the "right" voltage disconnect set point for varied loads.

So, this is not an "answer" but it's a start at an explanation of why the answer is complex.

[SunEagle]

I wasn't talking low voltage, the voltage coming from the panel is ~30 volts, it was the current coming from the CC which is around 1-7A depending
on the amount of sun.

I am sorry, but that doesn't make any sense. That would mean that all panels would have to be connected in parallel and there would be no point in
having a panel over 120 watts or so. There would also be no point in having a CC with max V over 50 volts.

Your limitation is the type of charge controller you are using. It doesn't matter what voltage the panel puts out the charge controller will only put out battery charging voltage. If you go with a MPPT type charger you will get more out of your panels

It does relate, because current at C/10 will charge in fewer hours than current at C/20.

That is because there are more amp at C/10 then at C/20. More amps equals less charging time

In my first post I said I didn't have the datasheet on the battery. Does it really vary enough that I couldn't take a good guess to set the load controller?

I could give you a value but it may be wrong and I don't want you to ruin your batteries. I have seen a value of 12.06 volts at 50% discharge but it relates to a specific FLA type battery

thanks
Darryl WA1GON

See my comments in bold

[Martshmoo]

My two cents, change to 24 V System

Hi,

If I were you I would Hook all four Batts in Series, and get a 24V Inverter.

That way you will get 24V at 8 Amps Charging from each Panel. 32 A at 24 V is double the watts as 12V. So you will charge twice as fast. It IS that way for me, since I doubled Voltage a long time back. also, with the OB Controller, I can hook up panels in 60V or even 90V Conficuration, forcing amps faster in the Batteries than ever before with Xantrex.

Your Charge Controllers (If I understood correctly, they are Xantrex C40's), can switch to 24V, you will need a 24V Inverter, but you should have 2.4 Kwh of Power before you hit 50%. 80 Watts X 30 Hours or so)

One question you asked was about the controller putting the excess amps to the Battery while the Load is on, your Math was correct, But for some reason Xantrex are not as efficient as outback in this process. It falls to about half the amps you would expect (with the Math) in my experience with a Load. Might be my Turbines getting overloaded tho.

On the other hand, Outback's C60/ or C80's are awesome at it.

2 Cents...

[thastinger]

OP, you bought the wrong stuff but it would help to know your loads so we could help you configure your current equipment as best it can be. Also need distance from panels to CC. The PWM CC is reducing your panel to half but if you can reconfigure your batteries to 24V you can get closer to the rated panel output.

[wa1gon]

Thank you all for your patients!

It took a bit of everyone hammering at me and doing a lot of research to understand what everyone was
saying about the PWM controllers.

The fact is that 5 amps in to a PWM control will be 5 amps out regardless of the voltage on either side.

Part of the problem was the specs for the controller doesn't make sense. The control will take 125 V
in, but the max out would be 48 V. I can't think of a good reason to have 60 or more volts input.

What also confused me was hearing that MPPT was about 15% more efficient than PWM controllers.
That didn't seem hardly worth the price difference, but with my size of panels it is about 60% more efficient.

I guess my next questions is there any good low cost MPPT controllers that will ~20 or ~40 amps output?

Can more than one CC be connected to the same battery bank (sounds like a really bad idea)?

thanks

[paulcheung]

I have 2 charge controller connected to my battery bank.

[SunEagle]

I have 2 charge controller connected to my battery bank.

Not a good idea to have more than one charger unless they are exactly alike and talk to each other otherwise they will fight each other to charge the batteries.

Better to use one charge controller sized appropriately for your batteries.

A quality MPPT type charger is the Morningstar but our Sponsor sells all types. Check out this link.

http://www.solartown.com/store/catalog/charge-controllers/