Before I start, just a quick relevant note about me the author: I have electronics qualifications and live off grid.

One of the first things a complete novice to solar panels learn, is that you are not simply going to put solar panels on batteries and have everything going smoothly without some kind of voltage regulator or charge controller. Charge controllers are more sophisticated than simple regulators.

Solar panels designed to charge 12V  Lead Acid batteries usually have an open circuit voltage of around 21V in good sunlight. 12V lead acid batteries, when under charge, starts to overcharge at about 13.8V@25°C ( depending on the charge method ). You can put the 21V solar panel straight on a  discharged 12V battery and the voltage will drop to well below 13.8 V. The voltage will rise as the battery becomes charged. When it  reaches 13.8V@25°C the voltage must be controlled so that the battery does not overcharge and gas. A battery might run dry if left gassing. Uncontrolled overcharging like this can lead to the destruction of the battery. If the temperature is elevated from 25°C the Voltage at which the battery starts to gas is lowered. A wet lead acid battery cell has a temperature coefficient of about -2.7mV/°C. The coefficient differs slightly from manufacturer to manufacturer, and it is not linear. If the temperature rose to 35°C the new gassing voltage would be approximately 13.8+(-2.7*6/1000)*10) = 13.63 Volt.

For minor drops and rises in temperature, voltage compensation is not crucial. A bit of overcharging can in fact have a desulfating effect under some conditions. In a  valve regulated battery  lead acid ( VRLA ) battery most of the hydrogen and oxygen gas will recombine to form water in the case of mild overcharging. Simple solar panel regulators can be made at low cost. However, off the shelf smart charge controllers usually have an abundance of features and are not always very expensive.