12volt battery charging from a 48volt battery bank

[johngalt]

I would just hook a small DC-DC converter to 48V and get 12V out. Simple. Make it just big
enough for the (lights ?) load, could be turned off days by a photocell control. I'd use one with
an isolated output, at slight loss of efficiency. Bruce Roe

What I may do in the future is add a second, smaller, MPPT controller to charge a 24V battery bank (2 x 12v in series) instead of 12v.

My current array has two strings of 5 panels each with 725 watts per string. Without adding more panels or reconfiguring them I could, through a series of disconnects, take one string away from the 48volt system and move it over to charge the 24v system. The current string configuration would still be compatible with the smaller charger. During the day if I need to do some work with AC I could swing the string back over to the 48V side.

I could then use the 24v batteries to run 24v LED lights at night or other 24v applications like DC fans and save my 48v bank for daytime use with the inverter for AC jobs. If I also needed 12volts I could then use a DC to DC converter off the 24v battery instead of tapping into the 48v bank.

This is just food for thought for the future. I have to finish the 48v system now and run with it for awhile. Having a secondary 24v system instead of 12v would be the most efficient use of my current setup.

[Sunking]

What I may do in the future is add a second, smaller, MPPT controller to charge a 24V battery bank (2 x 12v in series) instead of 12v.

John it will need to be a second system. You cannot parallel controllers. Does not work that way.

[johngalt]

John it will need to be a second system. You cannot parallel controllers. Does not work that way.

I was not going to parallel the controllers. If I do this it would be a separate system. Separate controller, separate batteries, cables, etc.

The only thing that would get reused would be one string of panels. If I used the string to charge the 24v battery it would be totally cut off from the 48v system. If I used the string to charge the 48v system it would be totally cut off from the 24v system.

I can foresee a time when the 48v bank is full with nothing to do and that would be a good time to divert one of the strings over to the the 24v system and do some charging.

I don't even know if I'll go this route but it does seem plausible. The only common piece is one string of panels that if not being used for the 48v bank is just wasted energy I could store elsewhere.

Since I'm not living off the system it would not be an inconvenience to switch the string back and forth as needed. Right now I don't envision putting such a high load on the 48v bank that I could not borrow a string for a day or two after the batteries are full and sitting idle.

Of course, I haven't started using it yet so all my assumptions may change. If I use the 48v system with the inverter so frequently that I need every hour of sunlight to keep the batteries full then the idea of borrowing a string to use on the 24v system immediately falls apart.

If so, I could build out another string for the 24v system if and when I decided to build it. I have two extra panels so I would only need three more. Finding another good spot to put them up would be the hard part.

[johngalt]

Part of the confusions is the title of this thread.

At first I was trying to figure out if there was a way to charge a 12v battery from a 48v battery bank. I abandoned that idea and replaced it with the idea of just building another system at 24v instead.

First I was thinking to build a separate 12v system until I realized my current string configuration would not support it. That's when I came up with the bright idea of building a separate 24v system by using one of the existing strings as configured.

I think I need to finish working on what I have instead of looking for more work to do.

[bcroe]

Multiple MPPTs

John,

Switching panel high voltage DC output is not a trivial matter; the
AC equivalent switches & relays will tend to arc over on DC.
As pointed out, connecting 2 MPPT controllers & loads to the same
panel will cause them to fight & confuse each other. They will both
want all the available power. Each operating voltage adjustment
made by one, will cause adjustments by the other, never reaching
a stable operation.

With multiple size battery capacities, you might find the smaller system
reaching full charge, while excess PV output that could have charged
the larger system is lost. And in my opinion, using a diversion load
on a PV system means its time to step back and reconsider the whole
configuration. At the phone co we used one big 48V plant of sufficient
capacity. Then other voltages can easily be supplied as needed, with
efficient, regulated, isolted DC-DC converters. Some attempts to use
multiple primary battery voltages a half century ago, were phased out.

Bruce Roe

[FloridaSun]

I can foresee a time when the 48v bank is full with nothing to do and that would be a good time to divert one of the strings over to the the 24v system and do some charging.

Since I'm not living off the system it would not be an inconvenience to switch the string back and forth as needed. Right now I don't envision putting such a high load on the 48v bank that I could not borrow a string for a day or two after the batteries are full and sitting idle.

If so, I could build out another string for the 24v system if and when I decided to build it. I have two extra panels so I would only need three more. Finding another good spot to put them up would be the hard part.

IF you have extra 48V capacity you would have little prob with using a 48:12 or 48:24 DCC converter. Good idea to start there as it's very little cost as compared to setting up a 24V system. With the converter you could get an idea just how much lower voltage you might use, build another lower voltage system in time, if wanted. If you've run out of roof space you might consider a pole mount in a sunny area.
yeah, I know what you mean starting too many projects at one time. I usually have too many going at once myself. The vegie garden needs weeding, still have not finished my improved solar panel rack, need to have a new roof put on front house, finishing up building a new pumphouse, always clearing more of the jungle, etc etc etc....not to mention all the projects planned I haven't even started yet.

edit, oops, didn't realize ': D' would make a smiley face that should be DC : DC

[Sunking]

John I am thinking out loud here and not sure I have all the details but here goes.

Assuming you have a MPPT controllers that can operate 12.24, 48 volts, and assuming the panel wattage does not exceed the CC 12 volt current rating. all you have to do is switch the output of the CC to either the 12 volt or 48 volt battery.

[johngalt]

John I am thinking out loud here and not sure I have all the details but here goes.

Assuming you have a MPPT controllers that can operate 12.24, 48 volts, and assuming the panel wattage does not exceed the CC 12 volt current rating. all you have to do is switch the output of the CC to either the 12 volt or 48 volt battery.

Ah, another good idea. I had not considered that. I'll have to do a little home work on that one. It may be much easier and less expensive to just divert the CC output than to buy another controller and try to divert the panel input. I'm pretty sure I can do that for 24v. Not sure if I can for 12v but I'll check that out too. The CC has an auto discover feature for the battery bank so I may not even have to change it if I use that setting. I may be able to disconnect from the CC, switch over to the other bank, and reconnect. That would be very cool if I can get it to work.

Thanks again, I'll let you know how it goes.

[Sunking]

Tap the brakes John.

What panel wattage are you running and what size charge controller?

[johngalt]

Tap the brakes John.

What panel wattage are you running and what size charge controller?

I have 1450 watts. (10 x 145W 12v Panels) configured as two strings of 725 watts and a TriStar MPPT-60 charger.

I could use the 1450 watts to charge the 48v bank. (As I am setting up now) Or I could use both strings at 24volts, but most likely would turn off one string and just use 725watts. With 12 volt batteries I could only use one of the 725W strings.

I have checked the Morningstar string calculator and for all three voltages the 5 panel strings I have are doable as-is without rewiring them. The only difference is whether I can use both strings at the same time. For 48v yes, for 24v either 1 or 2 strings, for 12v just one string max.

1450W / 48V = 30.2A
1450W / 24V = 60.4A
725W / 24V = 30.2A
1450W / 12V = 120.8A (Too high for the CC)
725W / 12V = 60.4A

If I do this I think my best options would be 1450W with 48V battery and 725W with 24V battery. If I needed 12v for something I could just use a DC to DC converter off the 24v batteries saving the 48v setup just for AC

[Sunking]

John again I am thinking out loud.

If you are using some 12 volt power, would that not take or shift demand off the 48 or 24 volt system? Why not just rob a panel or two from what you have, buy a another controller for the 12 volt system. All you are doing is shifting load around.

[johngalt]

John again I am thinking out loud.

If you are using some 12 volt power, would that not take or shift demand off the 48 or 24 volt system? Why not just rob a panel or two from what you have, buy a another controller for the 12 volt system. All you are doing is shifting load around.

I currently don't have any load. 12v or 24v. I am thinking about the future. I do have a 12V prototype lamp that I built and was going to replicate it for lighting but that could easily be redone with 24v LEDs instead. I was also looking at a couple DC ceiling fans. They come in either 12v or 24v. Other than that I can't think of many other DC uses except for maybe charging phones or tools.

I just thought it would be a good idea to use the 48v setup with the inverter during the day and use a separate setup for either 12v or 24v for DC stuff anytime. If I were to buy or make 24v devices then I really wouldn't have a need for any 12v at all.

I could rob from what I have but then I cut into the available wattage for the 48v system. Since the 2 strings are compatible with either 48V or 24v they way they are currently configured I just thought it would be easier to flip a switch and re-purpose them as needed.

I'll figure something out, right now I just need to finish the 48v setup. It's pretty modular the way I have things laid out so switching things around later would not be a major issue.

I still like your idea of using the current CC and switching between battery banks. I found several 48V battery switches online. I haven't committed to this idea yet but started checking in to it. If I added a 24v set of batteries, battery fuses, breaker box and a switch between them coming out of the controller it would be an easy setup. I have a diagram I left at the office. When I get a chance I'll post a copy and show you what I'm talking about.

[johngalt]

IF you have extra 48V capacity you would have little prob with using a 48:12 or 48:24 DCC converter. Good idea to start there as it's very little cost as compared to setting up a 24V system. With the converter you could get an idea just how much lower voltage you might use, build another lower voltage system in time, if wanted. If you've run out of roof space you might consider a pole mount in a sunny area.
yeah, I know what you mean starting too many projects at one time. I usually have too many going at once myself. The vegie garden needs weeding, still have not finished my improved solar panel rack, need to have a new roof put on front house, finishing up building a new pumphouse, always clearing more of the jungle, etc etc etc....not to mention all the projects planned I haven't even started yet.

edit, oops, didn't realize ': D' would make a smiley face that should be DC : DC

I thought about just using a DC to DC converter but I was trying to dedicate the 48v batteries to just AC use during the day for running the dish washer, washing machine, etc. and not suck off them at night. My inverter is 1100 watts continuous and the array is 1450 watts. If I only use is during prime time I was thinking I would have minimal draw from the battery bank thereby making them last longer.

With that in mind I thought setting up a cheaper set of 24v batteries on the side I could then use for LED lighting at night. The draw should not be much and they could quickly be recharged.

I hear you about too many projects. I just finished remodeling the kitchen, have a new wood stove sitting in the living room I have yet to finish installing, a half completed solar project, compost mountains that need turning, a dirt road that needs scraping to fill in the pot holes from all the rain, a tool-shed that needs siding, and a lot of carpentry work to finish around the house. I would also like to setup a DC pump system for my well but that will have to wait until some of these other things are put to bed. I don't want to even think about all the yard work that needs to be done......

[johngalt]

Here is a copy of the current design.

[Sunking]

John you have a huge No-No in your diagram and very dangerous. Bond the two ground rods together or just use one rod. In addition you might have another big problem. I see you have all the controllers ect bonded to earth. That is a good thing, but you are using OCPD on each polarity at the battery. You need to check your charge controllers because I bet money they bond the negative polarity to the chassis which means the system is grounded. If that is the case you will need to remove all OCP devices on the negative polarities.