I understand a charge controller isn't necessary with panels less than 15W but does it provide any benefits for the smaller panels? And what about the desulfator?
Once you understand what solar charge controllers do you'll have a better understanding of why you may or may not want one with a 15 watt panel.
A solar charge controller does several things that help keep your battery charged and in good health.
The first, and most basic, function of a solar charge controller is to prevent your hard-won battery charge from bleeding out backward through your solar panels after the sun goes down. Most solar panels come with "blocking diodes" (also called "blocking resistors") that do the same thing, but, because "most" is no "all," quality charge controllers have blocking diodes as well.
The next, basic function is a charge controller works like a simple on/off switch that connects the solar panel to the battery when the battery voltage falls below a pre-set voltage, then disconnects it when the circuit voltage rises back up to 14.2 or 14.5 volts. In other words, it disconnects the solar panels when the battery is near full charge and reconnects them once the battery has lost some of its charge.
Cutting off the charging process at 14.2 to 14.5 volts is important when you're pushing a lot of watts into the battery because once the battery approaches its fully charged state much of the extra energy going into the battery turns into heat. That heat, in turn, boils off the battery electrolyte (battery water) and causes other chemical changes that reduces the battery's lifetime and ability to hold a charge.
That said, letting a fully-charged battery sit isn't a good idea, either. Batteries store and concentrate energy through chemistry, and chemical energy doesn't like to sit still. Leave a fully charged battery on a shelf and it will gradually loose charge and start to "sulphate," or damage itself. You can combat this loss of charge and sulfation process by using a "trickle-charger" that sends just a trickle -- ideally 1-2% or so of the battery's Amp-Hour rating -- of electricity into the battery. THat trickle of energy pushes the chemicals inside the battery to stay in their full-charged configuration rather than going downhill like nature and physics expects it to.
And that's the answer to your question: Can you connect a 15-watt panel to your battery without a charge controller? Yes! For an 80 amp-hour an input of 8-16 watts is ideal for maintaining the battery, as long as it has a blocking resistor . . . or one 15w panel's worth. Before you go out and buy a bare panel, what good can having a charge controller -- particularly a BatteryMinder controller -- do?
By now you're guessing that better-quality solar charge controllers are also trickle-chargers that keep the battery fully charged and prevent sulphation when you're not using the battery. If you have more than 15 watts of solar this is the first, most valuable feature of an advanced solar charge controller.
Another thing advanced solar charge controllers do is squeeze more battery charge into your battery. Remember I said that basic solar charge controllers disconnect the solar panels when the battery reaches "near full charge" at 14.2 - 14.5 volts to stop the battery electrolyte from boiling? When batteries run down they tend to accept a fresh charge very easily, but as the battery starts to fill up and reach capacity it resists accepting more charge, and if you push more power at it than it can handle the excess electricity once again dissipates as battery-damaging heat.
Advanced controllers recognize this problem, but instead of just shutting of the juice at 14.2 - 14.5 volts they reduce the amount of electricity they send to the battery as it reaches capacity. It's a half-way point between fully engaging the panels and trickle-charging that allows the battery to reach its fully charged state without overheating.
There are two ways to figure out how much power to send the battery as it gets full. One is to monitor the charging circuit and make an educated guess at when the battery is starting to get full based on voltage. The other is to mount a sensor on the battery and use that monitor battery temperature and slow the charge rate down when the battery gets too warm. Both methods will get your battery fully charged when you have ample solar panel capacity and sunlight, and neither will get you fully charged if you don't. The difference between one and the other is what happens when you have just
enough sunlight to top your batteries off.
The temperature method controllers will squeeze that last little bit of charge into you battery on days that there's just exactly enough sunlight to top the battery off. The voltage-method chargers tend to work more conservatively to protect your battery, and will leave the battery a few percent (say two or three percent) below its max charge when there's just enough sunlight to do the job.
My take on this is that I'm more worried about days when I don' get enough sunlight to charge my battery in the first place than I am about getting my battery to the absolute top of its charge curve, and since temperature-sensitive chargers cost well over $100 where voltage-sensitive chargers cost $40-$75, I'd rather spend that extra money on a few more watts of solar panels than the more complicated system.
Now, what about BatteryMinder controllers? BatteryMinder controllers are a variation of the voltage-sensitive charger concept that have a special, and very cool, trickle charger feature. When a BatteryMinder charger is working in trickle-mode it emits higher-voltage pulses at the precise frequency that makes sulfate crystals vibrate and fall
apart. What that means is a BatteryMinder controller will keep your batteries from sulphating, so they'll last longer and hold more charge.
Spending money on a BateryMinder solar charger will do you no good if you plug your trailer in and keep the battery charged when you store it. (For that I'd suggest a traditional plug-in BatteryMinder battery charger.) It'll only do the job right if you leave your trailer out in the sun when you store it and don't a second battery charger or converter in.
Other good choices (IMHO) in Solar Controllers are the Sunsei and Morningstar Sunsaver products.
The Sunsei CC-10000 is a 10-amp (120 watt) controller that has the three-level charging capability of an advanced controller. It's similar in capability to the BatteryMinder controller, but doesn't have the desulfation feature. They cost $40-60 and work really well for those with smaller solar systems who boondock. It's what I use with my 105-watt solar system, and I have no complaints.
The Morningstar SunSaver doesn't have desulfation, either. What it does have is an internal switch that disconnects your battery from your trailer electrics when its charging voltage reaches 11.5 volts, and is 75% discharged. This is good because after reaching 11.5 volts you can do serious and irreversible damage to your battery, but once in a while the dim lights
in your trailer will suddenly go out when the furnace
tries to start up and your circuit voltage drops to 11.5 volts. Then you'll find yourself in a suddenly cold, dark trailer. The SunSaver automatically re-connects when the battery charging voltage gets back to 12.6 volt, a safe point to start drawing power again.