Calculating proper charging wattage and amp hour storage

[aerodyn]

Hi, I am a new poster on this forum. I got the urge to build some panels and power outside led lights on my property, so I started building the panels with cells and materials from ebay. I know it is totally not cost-justified, but it is a little hobby that I am doing in my retirement. That said, here is my current situation and what I am trying to figure out:

I have 10 60w panels which produce about 31 amps at peak sunlight. In the morning here it is always overcast and the panels put out about 8 amps for a couple of hours, then it goes up depending on how sunny it gets. I have 480 amp hours (theoretical max) in battery storage. I have 4 different light circuits, which are all independently controlled with light switches, so I can run 1 to 4 of these circuits each night. The total amp draw of all 4 circuits is 7.5 amps. I run the lights from 9 pm to 6 am, 9 hours. My math tells me 9 X 7.5 = 67.5 amp hours of draw. The problem is that the batteries can not charge that much in one day. I have watched and monitored my system carefully and once the batteries are charged up to a certain point, the charger begins cycling and limiting the voltage delivered to prevent over charging. The result is that the most amp hours that my amperage monitor shows delivered by the charger in one day is 50 - 55 amp hours. The batteries never get back up to 100%. So, I run fewer circuits-1 or 2, depending on where the specific gravity in the batteries is before I turn on the lights.

What do I need? More panels, or more batteries, or both?

[Mike90250]

Welcome

You need more panels

More batteries will not solve the issue . There is a stickie in the OffGrid section about battery sizing

[aerodyn]

Welcome

You need more panels

More batteries will not solve the issue . There is a stickie in the OffGrid section about battery sizing

Thanks Mike. I am not too savvy, so bear with me a little more. I looked at the battery charging rate info and find that batteries between 10% and 85% of full charge can be charged at C/5. In my case, "C" would be 480, so the bulk charge rate for my system could be as high as 96 amps. I guess if I had 96 amps coming in from the panels that the charge controller would limit the amount going to the batteries once they reached 85%. Is that correct? Is my reading of this charge rate info correct?

That still leaves me a little confused. These figures are about the max charge rate. Even though my system is delivering more than 20 amps for at least 3 peak hours and less before and after that, the charge controller slows it way down. How would more amperage input from the panels change that? Wouldn't the charge controller just limit it and I would wind up in the same place as I am?

[inetdog]

Wouldn't the charge controller just limit it and I would wind up in the same place as I am?

If you have a 20 amp charge controller, yes it would. You might get more total charge out of more panels just from the low sun hours.
But to do the job right you would need at least a 30 Amp capacity charge controller, and you will probably benefit from an MPPT controller as long as you have to buy a new one.

What is your charge controller? Make and model or specifications.

You never want to pull your batteries down below 50% (less if you want long life and do not have an emergency need). So you would not be spending much time at the C/5 rate anyway. But the C/10 rate is still almost 50 Amps, and the sooner you get to the point where the charge controller is limiting the current/voltage, the longer you keep the final part of the process going before the sun goes down.

[aerodyn]

If you have a 20 amp charge controller, yes it would. You might get more total charge out of more panels just from the low sun hours.
But to do the job right you would need at least a 30 Amp capacity charge controller, and you will probably benefit from an MPPT controller as long as you have to buy a new one.

What is your charge controller? Make and model or specifications.

Thanks again. I am using Flexcharge 35 amp and within the next few days I am upgrading it to 60 amps. I believe it is MPPT. This still leaves me confused about increasing the input to the charger.

[Sunking]

I guess if I had 96 amps coming in from the panels that the charge controller would limit the amount going to the batteries once they reached 85%. Is that correct? Is my reading of this charge rate info correct?

No it is not correct, at least in the terms you are thinking.

A charge controller will limit the current but only if there is current available from the solar panels. What you are missing is solar panels are current sources, not voltage. So if there is only 30 amps available from the panels, 30 is all you can get with a PWM controller.

Assuming these 60 watt panels are made with 36 cells and the panels are wired in parallel, the most current they can supply is roughly 600 watts / 18 volts = 33 amps. The only way you can increase the charge current is with more panel wattage. To get 90 amps you are looking at 1600 watts and 2-60 amp charge controllers.

In the mean time turn the voltage up on the charge controller as high as it will go, or in Equalize.

[aerodyn]

No it is not correct, at least in the terms you are thinking.

A charge controller will limit the current but only if there is current available from the solar panels. What you are missing is solar panels are current sources, not voltage. So if there is only 30 amps available from the panels, 30 is all you can get with a PWM controller.

Assuming these 60 watt panels are made with 36 cells and the panels are wired in parallel, the most current they can supply is roughly 600 watts / 18 volts = 33 amps. The only way you can increase the charge current is with more panel wattage. To get 90 amps you are looking at 1600 watts and 2-60 amp charge controllers.

In the mean time turn the voltage up on the charge controller as high as it will go, or in Equalize.

Ok, so the consensus here is that I have enough battery storage (480 amp hrs). I am in the process of putting up more panels, number 11 is in my shop. I plan to max out at 15, which is 900 watts max. and should bring about 45 amps at peak sunlight. My original reasoning was to increase the panels so that I would get sufficient charge in dim light. We don't get a lot of long sunny days here. I figured if I could get 10 amps in overcast weather, that could probably be enough for my lights. Now, I am not so sure about that. Some days I can only get the batteries up to 1.250, some days up to 1.260, never up to 100%.

Sorry if I made you think that I think the charger would charge if there was no current coming from the panels. Not sure how you got there, but I actually do grasp the idea that current has to come into the charger before it can pass it on to the batteries.

[Sunking]

Some days I can only get the batteries up to 1.250, some days up to 1.260, never up to 100%.

OK what is the temperature of the pilot cell? Fully charged range is 1.255 to 1.275 across a wide temperature range. You need to apply a temperature correction factor.

For temperature correction use the following equations. Below 70

[aerodyn]

[QUOTE=Sunking;46897]OK what is the temperature of the pilot cell? Fully charged range is 1.255 to 1.275 across a wide temperature range. You need to apply a temperature correction factor.

For temperature correction use the following equations. Below 70

[Sunking]

I take the specific gravity reading with a temperature-corrected hydrometer. I got it from a guy who sells Trojan batteries and he said to me "I don't actually know how it works, but it is what I use."

OK then are you applying the correction factor after you take the reading? If not your readings are in error. In fact the batteries maybe fully charged and you just do not know it. Or they could be really low.

[aerodyn]

OK then are you applying the correction factor after you take the reading? If not your readings are in error. In fact the batteries maybe fully charged and you just do not know it. Or they could be really low.

Sunking, the hydrometer is "temperature-corrected". No separate factor needs to be applied. I have another hydrometer that has both the specific gravity reading and the temperature reading and that hydrometer reading has to have the factor added or subtracted from its reading. "Temperature-corrected". Already adjusted. Automatically. My guess is that somehow the temperature of the fluid influences the float and the reading. As the guy told me, he doesn't actually know how it works, but he uses it. Without adjustment.
Additionally, what do you think about my using a Xantrex meter that is advertised as "a fuel gauge for the battery bank"? They offer a Linkpro and a Linklite. I don't know if they work or the difference between them. Do you have any experience with them?

[Sunking]

Additionally, what do you think about my using a Xantrex meter that is advertised as "a fuel gauge for the battery bank"? They offer a Linkpro and a Linklite. I don't know if they work or the difference between them. Do you have any experience with them?

Plenty of experience with battery monitors, just not the model you mention. They all work the same by measuring voltage to measure the State of Charge (SOC).

Problem is SOC is only half arse accurate on a rested battery meaning it has not load or charge current for several hours. That is not practical on a working system. The only accurate real time measurement is the temp corrected SPG.

However there is a method to build your own battery monitor using nothing more than a simple volt meter. It requires some work but you may find useful if you know what you are doing. It is not as accurate as SPG, but close enough to give you an applicable measure.

Rather than me printing again click here and it will tell you how and answer a lot of the questions you have been asking

[aerodyn]

Plenty of experience with battery monitors, just not the model you mention. They all work the same by measuring voltage to measure the State of Charge (SOC).

Problem is SOC is only half arse accurate on a rested battery meaning it has not load or charge current for several hours. That is not practical on a working system. The only accurate real time measurement is the temp corrected SPG.

However there is a method to build your own battery monitor using nothing more than a simple volt meter. It requires some work but you may find useful if you know what you are doing. It is not as accurate as SPG, but close enough to give you an applicable measure.

Rather than me printing again click here and it will tell you how and answer a lot of the questions you have been asking

Thanks, that is good information, although as you say it requires some work and time. My system is relatively simple compared to others which are going into inverters and running all kinds of things which are variable. Mine is simple in that it simply runs some outdoor led lights which draw a fixed current. In the morning when I turn off the lights, I take a look at the battery bank voltage. The "load" has been steady for several hours and it is either 4 amps, 6 amps, or 7.5 amps, depending on which lights were on. Can I tell anything from that morning voltage? Also, at that time there is no charge coming in from the panels and has not been since the previous afternoon.

[Sunking]

My system is relatively simple compared to others which are going into inverters and running all kinds of things which are variable.

That is really irrelevant because it is past the batteries.

Mine is simple in that it simply runs some outdoor led lights which draw a fixed current. In the morning when I turn off the lights, I take a look at the battery bank voltage. The "load" has been steady for several hours and it is either 4 amps, 6 amps, or 7.5 amps, depending on which lights were on. Can I tell anything from that morning voltage? Also, at that time there is no charge coming in from the panels and has not been since the previous afternoon.

Well you could do it with your lights but it is painfully slow.

I am too lazy and tired right now to go back and look at your battery capacity but for demonstration sake let' say it is 400 Amp hours. To make the SOC chart you need to start with a fully charged battery, and discharge it down to 50%. OK with a 7 amp load it would take roughly 200 AH / 7 A = 28.5 hours. I do not know about you but the last thing I would want to do is set around 28 hours taking SPG reading every 15 minutes. I got better things to do with my time.

So the point here is you want to use a large load to shorten the time element, but not so much so it causes excessive voltage drop on the battery. Ideally you would want something like a C/8 load. That would cut the time down to around 4 hours. So if you had a 400 AH battery C/8 = 400 Ah / 8 Hours = 50 amps. 12 volts x 50 amps = 600 watts.

What you can do if you want is get yourself some cheap car headlights for a dummy load to test with. The pros use a computerized load box like this one made by Alber. But you can get decent results with MacGyver style.