Battery Charge Controller Question

[BartAtRanch]

I have a set of 24 volt solar panels charging 12 volt batteries.  The power is used for a 12volt water circulation pump on a stock tank.

I am looking for a new charge controller.  Are there any that can run off the power from the solar panels, along with power from the batteries.  The installation is in a remote field on our ranch.  We got several big snow storms each winter.  The weather can be cloudy for several days and the batteries can discharge during that time.  As the batteries are discharged, the charge controller will not function.

I have tried the approach of having a relay controller to control a relay that shut off the load when the battery is low on charge, so to have power for the charge controller.  But this setup has not always worked and the relays I have tried fail in the extreme cold.

I am looking for a simpler setup that eliminates the relay controller and the relay.  But this would require a charge controller that can run off the energy from the solar panels, as well as from the batteries.  I cannot find such.  Do these even exist?

Thanks for any help you can give.

[bigrockcandymountain]

Usually a charge controller will have 10a load control terminals.  The pump can run directly off these if it is less than a 10a pump, or can be wired through a relay.  The load terminals shut off the pump when the batteries get low, then reconnect it when they charge back up.  The charge controller should always function, even with low batteries.

What are the specs on the panels? 24v panels aren't ideal for charging 12v batteries unless you use a mppt controller.

[BartAtRanch]

I have been using a Morningstar TriStar TS-45 controller. Attached is the manual for the controller.  It may not be an MPPT type.

What is the advantage of a MPPT controller for 12 volt batteries?

The pumps I have tried are usually 3 amp or 5 amp.  But I can't see in the manual that there are load control terminals, but maybe I don't know what to look for.

Always having to have even a low charge level in the batteries seems a flaw in the design of the controllers.  There is always a power source from the panels, when the sun is shining.  Batteries can lose all of their charge no matter how carefully their charge levels are controlled. Then the system is dead and needs an outside source to recharge the batteries to get the system back working.  This does not strike me as a self-healing, sustainable configuration
 
Thanks for any help you can give. 

[DamonHD]

If you let lead-acid batteries get low enough (voltage) that the controller's electronics can't run (assuming a decent controller) then you have already destroyed the batteries!

Rgds

Damon

[mab]

That particular controller looks as though it can do charge control or load control, or diversion control but not charge and load control together - if you want to do low voltage load disconnect you would need two; one between the panel and battery for charge control, and the other between battery and load to do LVD.

Have you set the dip switches on the controller for 12v battery? By default they are set to autodetect, but if the battery voltage is outside the expected voltage range the controller can't determine what voltage it should be charging to. This might cure your problem of starting with a dead battery but I'm not sure - most controllers assume you never let the battery drop too low (as damon said, it kills the batteries quickly).

One possible solution:- This controller can be setup as a diversion controller. For this you need a dump load (big resistor/heating element) that can absorb all the current your solar panels can produce. You then connect the panels directly to the battery, and the diversion controller regulates the battery voltage by turning on the dump load when the battery is full. In this setup, even if the battery is dead, the solar brings it back up, and the charge controller doesn't have to do anything until the battery reaches 14v.

Is your battery lead acid?  or lithium which might have its own management system and disconnect itself to prevent overdischarge?

[Mary B]

Instead of a mechanical relay use a solid state relay. No moving parts...

[bigrockcandymountain]

The smaller controllers like morningstar sunsaver, etc have separate load control terminals. Give us some specifics on how many panels and what the specs of the panels are and we can definitely help you.  Most of the controllers with load control are smaller controllers.  If you don't need 45a and could get away with 30a, there are tons of controller options that should work. 

A 30a controller at 12v will hanlde 360w of panels.  If that isn't enough, you can run 2 controllers.  Your Ts-45 is one of my favorite controllers. As mab pointed out though, it doesn't do load control and charge control at the same time. 

The load control terminals turn off the load when the battery gets low.  Usually around 11.5v they don't reconnect until the battery gets a decent charge.  Usually 12.8v or so.  This prevents damaging expensive batteries.