"As the battery gets close to full, it needs higher voltage to push in the remaining charge."
Er, no. First, solar panel
output will always be in the same, fairly limited voltage range. (This is classical physics, Einstein's photoelectric effect.) The only exception to this rule is when the panel isn't getting a lot of sun and the panel isn't producing much power. So, even when your 100w solar panel
is only producing 50, 25, or even 10w, it's still producing that power at about 17.5 volts.
Beyond that, batteries have a charging cycle that runs through three or four phases: bulk charging, absorbtion, equalization (the optional phase), and float, and none of these charging phases ever requires more than the 17.5 volt output from a solar panel.
During the initial phase of battery charging, when the battery is run down, it'll take in the full output -- volts and amps -- of a battery charger or single-panel solar panel system without over-heating or damaging the battery. This is the "bulk" charging part of the cycle, when the battery's resistance to charging is almost zero and the charging system's full output is converted almost directly into battery charge.
Once the battery gets to about 60-75% charge, the battery's internal resistance to charge increases and, unless the charging voltage drops to 14.5-14.9 volts, the excess energy will go into creating battery-damaging heat. This is the "absorbtion" phase, when the battery's resistance gradually climbs and the number of amps the battery can accept as charge declines until the battery reaches around 95% charge. It's this part of the charging cycle where an MPPT charger can make a big difference, increasing the actual current passed to the battery by 15% or so. More on that later.
When the battery's resistance flattens out at about 95% of full charge, batteries (AGM batteries in particular) can still accept just a little bit more charge, but the charger really has to work to make this happen quickly. Some charging controllers oblige them by boosting their output voltage just a bit, by a couple volts or so, for a short period of time, packing that last little bit of juice into the system in the shortest period of time. This is the equalization phase, when the charge stored in the battery's cells is "equalized," and spread evenly over the entire surface of the battery's lead plates. This charge phase has to be kept short, or the battery will overheat and become damaged.
The last phase is the "float" phase, where the charger pushes around 13.6 volts into the battery to prevent its gradual self-discharge, but, like the equalization phase, it'll also pack more charge into the battery if it has remaining storage capacity. It'll just happen much more slowly.
If you're a tech type, you can read details on how this works on the BatteryTender website, here: http://batterytender.com/resources/battery-basics.htm
Here's the difference between a less expensive Pulse-Wave Modulation (PWM) solar charge controller and an Maximum Power Point Tracking (MPPT) controller:
When the battery charge hits 60-75% and it's time to enter the absorbtion charging phase and reduce the charging voltage, PWM controllers reduce the voltage by rapidly switching the power coming from the panels on and off, so they're only fully "on" about 85% of the time, so the average voltage passed to the battery is about 14.6 volts. This means you lose that last 15% of the panel's charging capacity. When the battery charge hits 95% and it's time to enter the float phase, PWM controllers reduce the "on" part of the cycle to about 78% of the time, so 22% of panel capacity is lost.
I do not know of any PWM controller that performs an equalization step.
MPPT controllers take the full output of the solar panels and run it through a special inverter/converter circuit that pumps out that same 14.6 volts, this time without turning the panels on and off, so you get the full charging potential of the panels. They do this same thing throughout the absorbtion, equalization, and float charging phases.
So, here's the question: Is an MPPT controller worth it? Here's my take:
your expected daily solar panel output comfortably exceeds your daily energy consumption, no. You already have excess charging capacity and don't need an ultra-efficient charger.
your expected average solar panel output is much (25%) less than your daily energy consumption, no. Your solar system will only rarely enter the absorbtion part of the charging cycle, and you're just throwing money at buying charge controller features you'll never need.