Solar Panel to Battery Question
The simplest way is to use a FET in series between panel and battery. Add some circuitry to sense battery voltage (VBATT). When VBATT > VBATT_MAX, you turn off the FET. When VBATT &lt; (VBATT_MAX - V_HYST) you turn the FET on.

VBATT_MAX depends on the battery chemistry. V_HYST could be maybe 1 to 3V, depending on many factors. The reason for using V_HYST is to avoid switching fast. When you disconnect the charger from the battery, the battery voltage will fall a certain amount rapidly, and you don't want to instantly turn it back on.

Using a MPPT solar charge controller is a decent idea, though, so give it some consideration. An MPPT can let you use a smaller panel and get the same amount of power out of it as a larger one, utilized less efficiently.

"HYST" is meant to stand for "hysteresis," which you can look up if you are not already familiar with it.