12volt battery charging from a 48volt battery bank

[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

[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

[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.

[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.

[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]

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.

[bcroe]

DC to DC

getting back to johngalt's original question... seems to me a 48v to 12v reducer would be the best way to go unless you have room for a separate 12v system, panel and all. If dependent on excess amps from 48v panels to maintain a 12v battery, what would you do if there was no excess and that 12v was not topped off? Run the 48 to 12 reducer anyway.

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

[Naptown]

According to Morningstar as a diversion control it can be used to charge another battery.

http://www.morningstarcorp.com/en/support/library/TriStar%20Datasheet%20English.pdf

This may also require yet another controller to prevent overcharging the alternate battery.
Again Check with Morningstar for exact configuration.

[FloridaSun]

getting back to johngalt's original question... seems to me a 48v to 12v reducer would be the best way to go unless you have room for a separate 12v system, panel and all. If dependent on excess amps from 48v panels to maintain a 12v battery, what would you do if there was no excess and that 12v was not topped off? Run the 48 to 12 reducer anyway.

[Sunking]

Well someone correct me if i am wrong here, but doesn't a diversion load just connect the input to output? If so. does not matter what voltage the battery is as long as the input voltage is greater than the battery voltage. Solar panels are current source and when operated below MPPT voltage is just a plain ole everyday current source all the way down to 0 volts.. If the IMP is 6 amps, that is what you get assuming 100% iradiance.

Of course doing so is real risky as you can easily over charge the battery with no control. Diversion loads are just places to dump heat, and to make a dummy load.

[FloridaSun]

Suggestion
Contact Morningstar for their direction. I have read about this capability but have no direct experience with this and how it is configured.

No immediate plans to try any of this out but always interested in concepts.

My intended use was to use as a diversion load from the ARRAY not the batteries.

This sounds more like stacking a PWM with an MPPT, directing outputs to two dif batteries, not a diversion load.

[Naptown]

Suggestion
Contact Morningstar for their direction. I have read about this capability but have no direct experience with this and how it is configured.

[FloridaSun]

Solar is different from wind and hydro as the use of a diversion load for wind and hydro is to keep the turbine from spinning to too high an RPM. With solar if there is excess energy it just sits there. The only reason to be able to divert 100% of the source with solar would be with an AC coupled system where the solar is not fed through a charge controller and batteries first (the internal battery charger is used)

So on my 24V system would a tristar hooked up to a 12V batt produce 12v, with the 12V battery connected before diversion control input from solar panels/24V battery as per manual diagram. You say you would connect the diversion control to the array? not the battery? Do you mean before the primary CC? But doesn't diversion depend on a max voltage setting of the battery? LOL! I love this brain exercise. Please excuse my density at times. Takes me awhile to get this fully into my head.

[Naptown]

I was wondering about Naptown's response after reading this from the Tristar manual;

6.4.5 Minimum Diversion Load
The diversion load must be large enough to divert all the current produced by the
source (wind, hydro, etc.). This value is the maximum battery voltage times the
maximum source current.
For example, if a hydro source can generate up to 30 amps of current in a
nominal 48 volt system (60V maximum), the minimum diversion load size =
60V x 30A = 1,800 watts (for loads rated at 60 volts).

The diversion load must be large enough to divert ALL the current produced? Indeed that could tend to overload a 12V battery at times.

Solar is different from wind and hydro as the use of a diversion load for wind and hydro is to keep the turbine from spinning to too high an RPM. With solar if there is excess energy it just sits there. The only reason to be able to divert 100% of the source with solar would be with an AC coupled system where the solar is not fed through a charge controller and batteries first (the internal battery charger is used)

[Naptown]

I'll propose that ANY MPPT controller with 60VDC solar input, would be fine to charge a 12V battery with. You must match the charger size to the battery & load size. PWM controllers "short" the input to the output, and a 48V battery has no inherent current limit that a PV panel has, so I would only expect the PWM controller to work for a few minutes before it fries something.
Sorry Naptown, I disagree with you here.

My intended use was to use as a diversion load from the ARRAY not the batteries.