12volt battery charging from a 48volt battery bank

[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.

[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.

[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?

[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.

[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

[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?