PWM Controllers:
a FET switch is used, under control of some algorithm, to
rapidly connect a power source, to a battery. The switch "makes" and "breaks"
many times a second, to control the charge. (Pulse Width Modulation)
When the battery is quite low, the FET switch is ON, and the
panels are connected directly to the battery, and since PC panels are a
"current source" their voltage drops (or is pulled low to the battery
voltage) and some power is lost, depending on the difference of the Vpmax
of the solar, and the present voltage of the batteries.
PV voltage - Battery voltage = Difference x Amps = Lost wattage

Note if the array was a 200W array, amps would be 11.11Aa
18V @ 11.11A = 200watts
18V PV - 12.2V battery = 5.8V @ 11.11amps = 64.438 w lost

MPPT Controllers: [Maximum power point tracking]
a more sophisticated circuit is used, much like a "DC
Transformer" to dynamically convert the power source (water, wind generator
or PV panels) via an efficient DC-DC conversion process. This can also
down convert high voltage DC, to suitable charging voltage for the battery,
with very low losses, about 95% efficient. This is a good thing if you
have a remote array - you can run high voltage DC to the controller and avoid
expensive high amp wire, and incur lower losses.
The MPPT advantage vanishes when the controller switches
from the BULK charging mode, to ABSORB, and does not need the extra power
recovered from the conversion process, and falls back to PWM for ABSORB and
FLOAT stages. This can be managed in different ways for individual
systems, as many MPPT controllers have user programmable settings.

this is the simple gist of it, the actual workings are much deeper than
I can go into here.

As the system gets larger, the cost trade off between the
simple PWM controller and the expensive MPPT (with possibly shorter
lifetime with more complex innards) and PV panel & rack costs, shifts, and
it's left to each user to decide which controller is most appropriate for
their situation. The tipping point is around 600 - 1,000 watts

old post might be here: