multiple battery banks

[Oly]

Hi all, hoping to get some help here.  My cabin is wired separately for both 12v and AC.  I'm currently running only the 12V stuff from 6 new T-105s charging from solar panel and charge controller. The cabin is also wired for AC, but as of now I have only generator input to the AC panel and we don't much like running the generator. I also don't want to power high-load AC stuff from this battery bank as it's probably insufficient. I'd like to take a separate bank of 6 T-105s (an older set that I won't mind trashing a little bit) and run an inverter off that bank to supply AC to the panel.

My question is, can I use a single solar panel/controller to charge both banks of batteries, all the while keeping the two battery banks separate from one another? I asked someone at an on-line solar supplier, and he told me the RV folks have a transfer switch that does exactly what I want: says it will take the charge from the charge controller and direct it to whichever bank is lower in voltage, automatically switching between the two of them as needed.  However, I've called a few RV guys and they say such a thing does not and cannot exist. One suggested using a 90amp isolator, but he couldn't really explain how it would keep the banks separate from one another.

I've been told not to run more than 3 parallel strings of batteries, and not to mix new/old batteries, that's why I would like to keep the banks separate.

Learning as I go, and appreciate any input here. Thanks, Oly.

[veewee77]

You can actually do this with simple diodes (big ones!!!).

You likely have blocking diodes between the solar panels and the battery banks.  Just use a separate blocking diode for each bank of batteries.  The lower set will pull the most current until they are equal in charge.  The only drawback with this is that the better batteries will have more capacity and likely a bit higher voltage so it may try to cook the lower (older) set trying to bring up the weaker, older set.  I would not be afraid of it unless you have serious solar panels on there, though. . .

The blocking diodes will keep one bank from discharging the other, but allow the solar panels to charge both banks simultaneously.

Doug

[nanotech]

And just as a by the way....

That switch thingy you were talking about IS an isolator.

JC Whitney sell them.

[johnagony]

There are some on ebay, search  'battery isolator'..you will also drop 0.5V across each forward conducting diode..so if you have 2 diodes in series, 1 volt will be lost from solar array output to battery positive. Do you meter the V and A of each bank, both input (charge) and output (discharge)?

[pyrocasto]

I am thinking of doing this later as well, since scrounge batteries are not all the same. The problem though, is you will need a seperate charge controller for each bank.

[ghurd]

The isolator will have the drop, but some controllers can be adjusted for that.

Other controllers will go into regulation before the batteries are full.

Some may not even work if one battery is full and the other is low.

Maybe you could make a dump load for the primary bank, that was just a relay to switch the solar to the secondary bank.

G-

[Oly]

Thanks all for your input. Now that I know it is an isolator I'm after, I've done a little reading about them. Quite a range of them available, still not sure how I want to do it. The folks at backwoodssolar put together a nice summary of how to do it 2 different ways. http://www.backwoodshome.com/articles2/yago93.html. The first method

uses an isolator, but I'm still not clear whether that would provide feedback to my charge controller. (My charge controller can be adjusted, so I could add 0.5-1 V to the charge limit if I lose that voltage across the isolator)  The second method

uses 2 charge controllers instead of an isolator. This one is easier for me to understand, but will require more $$$.  

Will the isolator method in the first diagram give voltage feedback to my charge controller so it knows when to charge the banks and when to stop?

Oly

[Experimental]

    Hello Oly

    Didn,t want to get to involved in this discussion, as I know, what the guys are saying -- is the right way to do this -- BUT, I have been running my cabin  (somewhat like you have described)

     For nearly two years now, I have been charging the batts, in both my cabin, and a 24' trailer -- off the same solar panel, and controller !!

    I just came off the controller, with two seperate sets of wires (12 Gage) and have a 10 amp fuse on each positive line !!  That fuse, guards against shorts, but I put it in the line, actually, to protect the wire from burning up, if one bank of batteries "bleeds" back to the other, while in use !!

    There are two batts, in the trailer  (12 volt about 110 amps each) and about the same in the cabin -- both sets, are different types, and the one set is about a year older than the other!!

    I only use 12 volt in the trailer, but have two inverters ( one on standby, in case the other dies !!) in the cabin, for lighting and light stuff -- TV, drills and small saw, Etc...  The fridge, is propane, and only needs 12 volt, for the starting and temp control and a water pressure pump!!

    BUT, I never run the microwave on the inverter -- for that, I start the generator -- but future plans are to improve the battery bank, with some "Trojan"

L-16,s 350 Amp -- then we might use that microwave !!

    I also have a wind generator, but I have operated on only the solar panel for the past two years (65 watts)and have had no problems, as is !!

   The only other thing I might add is -- obviously, I don,t use much power and shut off, everything I,m not useing -- AND, that cabin is in Northern california, top of a hill, with LOTs of sunlight !!

   The trailer, will be taken away, when all work is done on the cabin, so I didn,t want to spend a lot of $$$ on the hook ups--- But, the "guys" are right about how to do this properly  -- other than that, IT works for me and the one thing I have found out --- there is no substitute, for good batteries !!    Bill H...

[richhagen]

It sounds like you essentially have your two banks connected in parallel by connecting the positive and negative leads together at the charge controller.   This should keep them charged to the same voltage as each other in the long run, which could be disadvantageous if they reach full charge at slightly different voltages because of age or battery type.  If you had really heavy draws which dropped the voltage in one bank or other and the 12guage wire was short enough, then you would blow those fuses.   Rich

[Experimental]

     This is true Rich, but in two years, I have never blown those fuses and I.m absolutely sure, there have been many times, the power has blead from one set of batts to the other -- such as when i have been working in the cabin, at night !!

     I,m not suggesting that this is the way to do it -- If I were going to have this as a permanent system, I would certainly "refine" the thing !!BUT, as yet, it hasen,t been a problem and I,ve been surprised to find my batts in both banks often test at 13.5 to 13.8 volts !!

     I only told Oly, because it has worked for me , for a long time and if need be, I could go to 30 amps on those fuses, as #12 wire is good for 30 -- but, I haven,t needed to -- That surprises me too !!!

     As experimenters -- we often do unorthidox things -- sometimes it works, other times ---SMOKE !!!  As long as I don,t burn down that cabin, I can save up money, for those "Trojan, L16,s" --- AND, if it "ain,t broke", I,m not going to fix it!!

    Keep working, and inventing Guys,and have a pleasant evening ..  Bill H....

[ghurd]

They look like SS controllers, and the site says the same.

I will disagree with the first diagram.

The Morningstar SS-10 is not adjustable, except for sealed or flooded.

The isolator drop will cause it to stop charging before the battery is full.

The controller will see 13.8V, 13.1 from the battery + 0.7 from the diode = 13.8.

See this.

http://www.morningstarcorp.com/products/FAQ/Isobanks_one_controller.pdf

The second diagram is not quite right either.

See this.

http://www.morningstarcorp.com/products/FAQ/iso_banks.pdf

Hope this helps.

G-