Question

The MPPT charge controller will generally only get warm/hot if it is passing lots of current to the battery bank. As the charging current is reduced, the controller should cool down. Should not hurt your controller as long as you don't exceed its specifications (Vpanel max input voltage, etc.). Also, you should have good ventilation around the controller (do not put controller into a closet/under a shelf/in direct sun behind a window/etc.).

Solar panels are perfectly OK to draw less than full power from them. Technically, they do get a little bit hotter because you are not drawing the power out--But it is nothing to worry about with standard silicon panels.

So, regarding your batteries. More or less, the controller typically has two voltage set points. One is Absorb and it is roughly 14.75 volts (for a 12 volt battery bank). If your battery is discharged, the controller will output maximum available current (power) from the solar array to the battery bank (bulk mode).

Once the battery hits the Absorb set point, the controller will start cutting back on the current to hold the battery voltage at 14.75 volts for 2-6 hours (depending on lots of things, and different controllers may choose to end absorb charge differently than others).

Once absorb mode is ended, the charge controller will cut back to float voltage setpoint of ~13.4-13.6 volts or so... At this point the controller will hold this voltage until the sun goes down (or your DC loads exceed the charging current from the array).

For you to determine if everything is OK, you are looking for Absorb and Float voltage. Also, if this a flooded cell battery bank, you will check specific gravity of each cell and make sure they are balanced and >90% state of charge in the evening (you do not need 100% SoC at the end of every day--100% SoC charging is actually a bit hard on the battery bank).

Another thing to check for Flooded Cell Lead Acid batteries is the electrolyte level. You typically need to add water around every 1-3 months. If you need to add water more often than 1x per month, you may be over charging the bank. If you don't add water after 6 months, you probably are under charging your FLA bank.

The above is a very general answer--And the details do matter. What is it you are concerned about? It sounds like your system may be operating "differently" than one would normally expect (such as MPPT controller getting hot when the battery is fully charged and not drawing current).

Telling us more about your system and what you are seeing at different times of a sunny day (array voltage/current. Battery charging voltage/current @ 9 amp, noon, 3pm, etc.) would be helpful.

-Bill

With off grid systems, you really only use ~50% to 65% or so of your daily predicted loads on average/easily. To use more, you have to manage your loads very closely based on weather/season/needs for the day--Which some people really enjoy, and others do not.

One way to use more of your system capacity is to have some load that can be run automatically... For example running an electric water heater, pumping water to a cistern/pond when the batteries hit float. For smaller systems, you may not be able to heat or pump enough water to make it worthwhile. And it may not be worth the money to make for a larger system to do these tasks.

Add that most people can harvest 2-3x as much energy in the summer vs winter--Most systems end up being "oversized" to generate enough energy in the winter (and not have to run the genset every day), that they do have lots of extra energy during the summer.

And if you use the "free" energy at night (say to heat water for a shower), that extra cycling on the battery bank will (usually) shorten the life of the batteries... Not really free.

Energy is a highly personal set of choices. It is not "wrong" to do what you are asking about. But there are usually trade offs with any choices.

-Bill