Upgrading system - 24 to 48 Volt

[Sandlaker]

I am in the process of upgrading our system from 24 to 48 Volts and I came across a post from a year or so ago where someone was discussing the same type of thing. One post that caught my eye was a post about the amount of panels needed to properly charge a system and I need some clarification on this.
This was posted by SUNKING if configured at 24 volts the current out from the charge controller is 1410 watts / 24 volts = 58.75 amps. With 6 strings of batteries means the current is divided by 6 right. So we have 58.75 amps / 6 strings = 9.79 amps /string.

I currently have a 24Volt system using 6 panels @ 235V, a 60Amp MPPT solar controller and 16 - 6Volt batteries hooked up in four rows of 24Volts each for a total of 1600AH.
Now SUNKING suggest that the current solar panels are not enough to properly charge, maintain the batteries
He posts; The minimum charge current to prevent sulfation is C/12 where C = the battery amp hour capacity. Your batteries are 225 AH batteries correct? So C/12 for your batteries is 225 AH / 12 hours = 18.75 amps minimum charge current required.
In my case the batteries are 400AH but it would mean that my batteries would require 400/12 = 33.33Amps and I am only getting 1410/24 = 58.75 / 4 = 14.69Amps so also not enough to prevent sulfation?
If I go to 48 Volts I have the batteries hooked up in two strings of 8 and double the panels to 12 I still only going to get 2820 / 48 = 58.75 / 2 = 29.38Amps. Still not enough. Somehow this doesn't sound right to me and I am missing something.

Any thoughts?

[Sunking]

Yep you are missing something. If you have a 400 AH battery it needs a minimum of 400 AH / 12 hours = 33 amps, 40 is the sweet spot. To get 33 amps on a 24 volt battery requires a panel wattage of 33 amps x 24 volts = 792 watts just call it 800 watts.

Using the same batteries configured at 48 volts you have 200 AH @ 48 volts. Minimum charge current is 200 AH / !2 hours = 16.5 amps. To get 16.5 amps on a 48 volt battery requires a panel wattage of 48 volts x 16.5 amps = 792 amps just call it 800 watts.

Reconfigure the batteries for 12 volts you have a 12 volt 800 AH battery. Minimum charge current is 800 AH / 1 hours = 67 amps. To get 67 amps on a 12 volt battery the minimum panel wattage is 12 volts x 67 amps = 800 watts.

No matter how you spin it with your batteries you need a minimum panel wattage of 800 watts.

[paulcheung]

I currently have a 24Volt system using 6 panels @ 235V, a 60Amp MPPT solar controller and 16 - 6Volt batteries hooked up in four rows of 24Volts each for a total of 1600AH.
Any thoughts?

You have 800 amperes @48 volts. you need at least 16 235watts panels to charge the two bank 48 volts batteries without any load at all for C10 charging rate. So you need 8 panels per battery bank plus any load to add more panels.

Cheers

[Sandlaker]

Upgrading system from 24 to 48 Volts

No matter how you spin it with your batteries you need a minimum panel wattage of 800 watts.

Thank you for your response SUNKING, however your reply did not solve my confusion. I am not quite understanding the 400AH thing.
According to you I need a minimum of 800 Watt panel capacity, which I have, I have 1410Watts but I also have 16 - 400AH batteries.
So, do you mean that I need 800 Watts per battery??? or do I need the capacity per my set of 4 (4 x 6Volt batteries to get 24Volts)
As I explained I currently have 16 - 400AH batteries (four banks of 24 Volts to get 1600AH capacity overall)
The reason I wanted to add panels is because I feel that the current 6 panels don't give me enough power to regenerate the batteries so I am going to add 6 more to get 12 panels.
I keep good track of the way the batteries behave and when they regenerate and found over time they went into absorb less than 50% of the time, hence the need for more panels.
At the same time it was suggested to change/upgrade the system to 48 Volts as I needed to either add another solar controller or buy another inverter.
The inverter is a little more money but it gives me the opportunity to run a 240Volt run to power a deep-well pump.
I think this is a good solution but I am a little confused about being able to charge this system with the 12 panels.
I will have 2 runs of 8 - 6Volt/400AH batteries (Surrette S-530) to get 48 Volts and have 1600AH capacity. Now, when installing 12 - 235Watt panels..... do I have enough panels????

[Sunking]

Thank you for your response SUNKING, however your reply did not solve my confusion.
I will have 2 runs of 8 - 6Volt/400AH batteries (Surrette S-530) to get 48 Volts and have 1600AH capacity. Now, when installing 12 - 235Watt panels..... do I have enough panels????

OK my bad I was under the impression you had a 24 volt 400 AH battery.

Lets go through the math right quick. For FLA you need a minimum charge current of C/12 where C = the AH capacity of the battery. So you are going to have now a 1600 AH battery right? Do the math and find the minimum charge current. 1600 AH / 12 Hours = 134 amps. Right off the bat means you are going to need two charge controllers of 80 amps.

Now to find panel wattage Watts = Battery Voltage x Amps. so 48 volts x 134 amps = 6432 watts of panel power required.

But be warned here that is the minimum and does not mean it is enough to replace what you use everyday. You may need 8000 watts. I have no idea because you have not stated your daily watt hour use or location which is needed to calculate everything.

[paulcheung]

Sandlaker,

I have the same amount of the battery like yours, 750 amps just a little shy of your 800 amps at 48 volt in two banks as well. My house load is between 1500 watts to 3000 watts during the sun hours. I use one Outback FM60 and one midnight classic 150 charge controller to charge these batteries and supply the house. I have 28 panels total 6300 watts. so if your load is similar to mine, then you need around same amount of the panels. you need 16 panels at least if you don't have any load at all.

Cheers.

[Sandlaker]

Lets go through the math right quick. For FLA you need a minimum charge current of C/12 where C = the AH capacity of the battery. So you are going to have now a 1600 AH battery right? Do the math and find the minimum charge current. 1600 AH / 12 Hours = 134 amps. Right off the bat means you are going to need two charge controllers of 80 amps.

Now to find panel wattage Watts = Battery Voltage x Amps. so 48 volts x 134 amps = 6432 watts of panel power required.

I was a bit confused once again when you brought up the 48Volts in your calculation but I think I may have it figured out now.... I hope.
Why I was confused was because no matter what voltage your system runs at, your battery bank stays the same size, whether I have 16 batteries on 24 Volts or 48 Volts the storage stays the same. That is why the calculation was confusing. The 1600AH capacity is because I have 4 banks of 400AH @ 24Volts (4 batteries of 6 Volts per bank). Now if I go to 48 Volts that becomes 2 banks of 400AH (8 batteries of 6 Volts per bank), right? So the calculation than goes, 800AH / 12 Hours = 66.67 Amps. x 48 Volts = 3200 Watts. So I would need 14 - 235 Watt panels MINIMUM to charge this system. Now, that makes a lot more sense. I was going to increase from 6 to 12 panels, but now I may consider adding a few more. So thank you for the info SUNKING. I do have it right this time, right????

By the way, how do you know / measure your daily power usage? I know we don't use much, a fridge, TV and lights mostly. Just our water-pump is the biggest user right now and pretty well kills my capacity. That is the main reason to go bigger and get 240 Volts to run a deep well pump - now I have a 1/2 HP Jet-pump pulling 50+ Amps on my 24 Volt system for 10 to 15 minutes every time it comes on.

[Sandlaker]

Sandlaker,

I have the same amount of the battery like yours, 750 amps just a little shy of your 800 amps at 48 volt in two banks as well. My house load is between 1500 watts to 3000 watts during the sun hours. I use one Outback FM60 and one midnight classic 150 charge controller to charge these batteries and supply the house. I have 28 panels total 6300 watts. so if your load is similar to mine, then you need around same amount of the panels. you need 16 panels at least if you don't have any load at all.

Cheers.

Thank you for your response and input as well Paul, I did response to SUNKING as you may be able to see and I hope I have calculated the right amount of panels.
But thank you non-de-less. I hope you can agree with my calculations.

[paulcheung]

Thank you for your response and input as well Paul, I did response to SUNKING as you may be able to see and I hope I have calculated the right amount of panels.
But thank you non-de-less. I hope you can agree with my calculations.

I think 14 panels are still too small for the battery banks even no load as the 235 watts rating is lab figure, in real life you rarely get that maybe 10 minute in a day. If were me. I would get 12 more panels total 18 panels, and one more 60 amp charge controller since you have one 60 amps already.

Cheers.

[Mike90250]

.... Just our water-pump is the biggest user right now and pretty well kills my capacity. That is the main reason to go bigger and get 240 Volts to run a deep well pump - now I have a 1/2 HP Jet-pump pulling 50+ Amps on my 24 Volt system for 10 to 15 minutes every time it comes on.

Water pumps are nasty loads. I went and installed several thousand feet of pipe, to have elevated storage, and just run the pump when the sun is up.
Get the pump curves, and size the pump rotors and motor for your specific need (lift, pressure in P-tank, GPM required) 1/3 hp pump may serve you well.

Charge controller inputs should not be paralleled with each other. You could have 2 pole mounts on 1 controller, aim them a bit east. The other 2 pole mounts, aim a bit west, on the 2nd controller. The panels on each controller should be identical in mfg and orientation. This spreads your power harvest curve a bit wider over the day.

[Sandlaker]

I think 14 panels are still too small for the battery banks even no load as the 235 watts rating is lab figure, in real life you rarely get that maybe 10 minute in a day. If were me. I would get 12 more panels total 18 panels, and one more 60 amp charge controller since you have one 60 amps already.

Cheers.

And you're right about that. That is why I indicated "MINIMUM" in my post. I am actually planning to go to 18 panels - just glad I don't have to buy and extra 30 - but probably will buy an FM80 solar controller as it can handle all the panels through the one controller. Then I can sell my FM60 and will be better off economically that way. After that I will be running out of room on my roof anyway, so hopefully it will work.

Thanks

[paulcheung]

The midnight classic 150 is rated 96 amps and is same price as the FM80. it give you more room for one controller, I never like the idea to max out the controller, 18 panels at 235watts is 89 amps at 48volts. but I do like the outback controller's menu.
Cheers