The controller will stop it overcharging once its full, but the amount of power you put in at once can damage the battery

Both discharging too fast and charging too fast cause problems.   

When charging, the battery will create gas - the casing is sealed to prevent that gas escaping. Eventually the pressure causes it to condense back into liquid.  
If you charge it too fast then it creates too much pressure and can damage the battery, as well as makes the plates less efficient. 

The controller doesnt know the capacity of the battery and maximum rate its designed to accept charge- it just puts as much power as it can in, until it reaches the "full" voltage level by stopping the charge and doing occasional voltage tests , then it stops the charging when the voltage reads full. 

More expensive controllers will do multistage charging etc but they are beyond the basic requirements of this project. They too though dont know the capacity of the battery. 

Maximum charge rate is typically C10 for a sealed lead acid battery. That means total capacity over 10 hours or in this case 0.7amps for a 7ah battery. 

Discharging too fast also affects the battery. I use much bigger batteries such as 120ah capacity. 
A 120ah battery discharged at C10 (12amps per hour over 10 hours) will have a much bigger capacity if discharged slower at C100 (discharged over 100 hours) where it is rated for 150ah of capacity. 

Unsealed batteries like car batteries are designed for cranking amps where they deliver more amps immediately - and can be recharged faster because the gas exits the vent on top. You then refill a car battery with a small amount of water twice a year to replace the lost gas. They are discharged only a shallow amount and are recharged in a couple of minutes quite quickly. 

This also makes them unsuitable for solar or deep cycle usage where they are better to be sealed.