Minimal electronics to interface a solar cell to a battery powered system
The minimum requirement is a diode from the PV (solar) panel to the battery.
This will clamp the PV panel voltage to a diode drop above the battery voltage.
Panel output will be > Imp and &lt; Isc. It will effectively be close to being a constant current source. This arrangement will give you about Vbat/Vmp of the panel maximum wattage or about 11/18 or or around 60%.

You can get much closer to full panel output wattage with a simple buck converter. This could be a discrete design using a few transistors and an inductor - but it's easier and almost as cheap tp use something like a MC34063. These are very old, lower max frequency than most modern SMPS IC's, but very flexible, available and low cost.
A MC34063 buck converter can be built with the IC, an inductor, a Schottky diode and a few resistors and capacitors. Efficiency can probably approach 90%. Using an external MOSFET will help efficiency.  

You can get close to MPPT performance by using the panel loaded voltage as your controlled voltage - the converter functions to try and keep Vpanel at Vmp. This can get within a few percent of MPPT power out over much of the range of solar input.