The Solar Controller:
The job of the controller is to regulate the voltage from the panels as it goes to the batteries. Batteries require very specific charge voltages at specific times in the charging cycle. As the battery becomes full, it acts somewhat like a tire being inflated.

In the early (or bulk stage,) large volumes of air (or electricity) are easy to push into the tire (or battery). The effort gradually becomes greater and greater until it takes a good deal of effort to push in a small amount of air. If you watch the pressure gauge it goes up more slowly and the pressure is much higher. This is also what happens when you charge a battery. At first the amperage will be high and the voltage low. The amperage will gradually decrease and the voltage will rise. It is this rising voltage that we need to be concerned with.

There are 3 stages to the charge cycle.
Bulk Charge: This stage dumps as much power into the battery as possible. Most standard wet cell batteries (Car batteries are wet cell batteries also called flooded cell batteries) can accept charges to 10% of their amp hour capacity. Other types like AGM (Absorbed Glass Mat – described later) can take much more current. In either event unless you have a Reallllllly big solar array, you won't hurt the batteries at this point in the charge cycle.

Acceptance Charge: To get that last bit of air in the tire requires a higher pressure for a longer time. With the battery, the controller holds the voltage at a specific high voltage for a specific time. The actual voltages and time varies with the battery type and the battery manufacturer ratings should be used, not the ratings that may come with the controller.

Float Charge: Finally the voltage is reduced to a lower level which is used to hold the battery at the “full” point as it sits unused, or with a small load. Most batteries (with some exceptions) gradually loose their charge over time due to internal resistance. The float charge is designed to replace this internal loss.

Do not use a controller unless it has these 3 stages and you must be able to control the voltage at the acceptance level and the float charge level. These settings are critical to good longevity of your expensive batteries. Again, use the battery manufacturer recommendations for your exact battery model.

Solar controller mumbo-jumbo is second only to battery mumbo-jumbo. Be very careful to read and understand what the vendor is saying. Compare the various types and consider your application, not some hypothetical case. Seek good advice, which is not usually from either the manufacturer or vendor.
I will get into the specific numbers later when we discuss batteries.

Another feature of the charge controller, is how it manages the voltage. Here comes the mumbo-jumbo part. Some manufacturers use special circuits that are advertised to increase the efficiency of the controller by up to 20%. I have yet to see any real conclusive test results that verify that claim. These controllers are much more expensive and it appears to me that they contribute that additional 20% only under very specific conditions. But again, I have not seen any specific test data so you are on your own here.

Mounting the controller has a few tricks. It is common to mount the controller someplace convenient for the installer, or the user if the controller has a built in display. I frequently see these controllers mounted above the refrigerator. It provides a convenient location for the display.

However, remember that the controllers job is to provide a precise voltage to the batteries. If the controller is located a long distance from the batteries, it may be sending 14.4 volts out of its terminals, but the wire resistance (remember that from the section about wire?) will reduce that voltage to some extent. The problem is that we need to control that voltage to within 1/10 of a volt. So the controller may be sending 14.4 but the batteries see only 13.9. As a result, the batteries never get fully charged. Batteries must be periodically fully charged or they will fail very early.

Therefore the controller should be mounted as close to the batteries as possible. However be careful. Remember that flooded cell batteries give of hydrogen which is explosive. You don't want the controller in the same air space as the batteries. Also the fumes in the battery compartment are somewhat corrosive. I have seen controllers that have failed because of corrosion.

Remember that we are working within a very small voltage range. Using my batteries as an example (yours will be different) the full charge voltage is 12.8. The 50% charge point is 12.2. It is best to limit your discharge rate to 50% of the batteries capacity. So called Deep Discharge Batteries are no exception. This leaves us with a working range of only 12.8-12.2 = 0.6 volts between full and (functionally) discharged.

A small digression. The life of a battery is directly related to the amount and depth of discharge. If you only discharge your battery a little bit, it will last significantly longer than a deeper discharge. We will get into that more in the battery section.

So one can see why putting the controller at a distance where there might be significant line loss is a bad thing. Even one or two tenths of a volt line loss between the controller and the battery is significant..

But there is another consideration.

Temperature Compensation.
Anyone that has tried to start a vehicle in winter has experienced the effect that cold weather has on batteries. Heat has an effect as well.

Remember we are dealing with only 0.6 volts from full to empty. During cold weather batteries need a higher charge voltage and during warm weather a lower charge voltage. I use a special type of battery called AGM (Absorbed Glass Mat). We will discuss what that is all about later, but they have a slightly different temperature compensation need than the typical flooded (golf cart or deep discharge) battery.

Here is data for my Lifeline 31T AGM batteries. Your system will have different values, but the concept will be similar. ( Data lifted from a Lifeline datasheet that came with my batteries).



Note the variation in float voltage for the manual system. From freezing to hot is a range of 0.8 volts. Greater than the voltage range from full to empty!! A temperature compensated controller is a must have item.

Many dealers will tell you that their controller is temperature compensated, but the compensation is monitored at the controller. It must be monitored at the battery. My controller is a Trace C-40 and it is about 14 years old. It has a phone type socket into which one plugs a cable ending in a small temperature sensing module. This module is glued to the side of one of the batteries.

This leads us to the next item... Batteries.
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...........From Nomad.........Been "on the road" since '97