12volt battery charging from a 48volt battery bank

[Beanyboy57]

How would one charge a 12 volt battery from a 48volt battery bank?

Rather than discharge my good batteries at night just to run some LED lights I thought it might be better to keep a 12 volt battery charged on the side and let that one cycle instead. All my heavy work, when needed, would be done during the day when the sun is out.. Clothes washer, dish washer, etc. If my 48volt bank is topped off and I have no work to do, and there is still sun available, how can I use that power to charge a 12volt battery to run some LED lights at night?

I know I could down convert the 48v to 12v but I was trying to figure out a way to bypass the 48v all together for this purpose.

Is this possible, stupid concept, etc.?

Is it possible to plug in a normal battery charger to your inverter output and charge the 12volt battery during the day by using a timer to switch the charger on during the time when you PV array is functional?

[johngalt]

John you are really dragging me deeper than I really like to go on a forum with details, and raising all kind of RED FLAGS. You say this is your house right? I can only assume you have services like commercial AC, TV, CATV Phone. All those require a common EARTH GROUND. You cannot have isolated Earth Grounds. All GROUND ELECTRODES (rods in your case) MUST be bonded together to form a COMMON GROUND ELECTRODE SYSTEM. Understand?

Got it.

Thanks

[Sunking]

From what I'm reading they are expecting a negative conductor ground somewhere else in the system other than at the controller. I currently do not have a GFPD or a negative conductor ground in the current design.

OK lets deal with this part.

What they are saying is what I told you. The Negative Polarity is bonded to the chassis of the CC, but there equipment is not certified or capable of passing external fault currents to clear them. They have no idea of what size batteries you intend to use. That being said, you have to provide the means. The best way for you to do that is to provide a Ground Buss Bar where you connect all the negative polarity cables at the CC output.

[Sunking]

I can bond the two ground rods together with more cable. If I use just one rod I have to run more cable anyway to get from the panels to the rod at the house or from the components at the house to the rod at the panels.

John you are really dragging me deeper than I really like to go on a forum with details, and raising all kind of RED FLAGS. You say this is your house right? I can only assume you have services like commercial AC, TV, CATV Phone. All those require a common EARTH GROUND. You cannot have isolated Earth Grounds. All GROUND ELECTRODES (rods in your case) MUST be bonded together to form a COMMON GROUND ELECTRODE SYSTEM. Understand?

[johngalt]

From my diagram, perhaps I should replace those 30 amp disconnect breakers with a GFPD instead?

The controller manual said to do it in the solar circuit or battery circuit. The combiner box seems to be the easiest way to meet this requirement.

[johngalt]

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.

I can bond the two ground rods together with more cable. If I use just one rod I have to run more cable anyway to get from the panels to the rod at the house or from the components at the house to the rod at the panels.

From the Charge Controller manual it says to ground the controller box to earth ground where indicated and there is a warning to NOT bond system electrical negative to earth ground at the controller. Since they are telling me not to do it I can only assume they didn't either?

From what I'm reading they are expecting a negative conductor ground somewhere else in the system other than at the controller. I currently do not have a GFPD or a negative conductor ground in 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.

[johngalt]

Here is a copy of the current design.

[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]

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.

[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]

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]

Tap the brakes John.

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

[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]

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.