The PWM (Pulse Width Modulation) solar controller is a traditional controller. The PWM controller cannot limit its current output. They just use array current. The nominal voltage of the solar cell array must match the battery-level voltage.
Therefore, if the solar cell array can generate 40A current, and the charge controller you use is only rated at 30A, it may damage the controller. It is important to ensure that the charging controller matches the panel, is compatible, and has the right size.
The solar controller (often called a regulator) is similar to a conventional battery charger, regulating the current flowing from the solar panel into the battery pack to avoid overcharging the battery. Like the battery charger of conventional quality, it can accommodate various types of batteries, the absorption voltage, the floating voltage can be selected, and sometimes the time period and/or tail current can also be selected.
Think of a PWM (Pulse Width Modulation) controller as an (electronic) switch between the solar panel and the battery(automatic voltage regulator suppliers):
When the charger mode is large capacity charging mode, this switch is ON
Flick the switch on and off as needed (modulate the pulse width) to keep the battery voltage at the absorption voltage
At the end of absorption, the switch is turned off and the battery voltage drops to the float voltage
The switch "flicks" ON and OFF again as needed (modulated pulse width) to keep the battery voltage at a floating voltage
Please note that when the switch is OFF, the panel voltage will be the open-circuit voltage (Voc); when the switch is ON, the panel voltage will be the battery voltage + the voltage drop between the panel and the controller.
The best panel matching of the PWM controller is the panel whose voltage is just higher than the battery charging and the temperature is taken into account. Generally, the panel with Vmp (maximum power supply voltage) about 18V can be a 12V rechargeable battery. Even though their Vmp is about 18V, they are often referred to as 12V panels.