As peter said you can overcharge a battery if you don t have a solar controller between the solar panel and the battery.
How to keep a solar panel from overcharging a battery.
Once you ve attached the input wires to the charge connector you should have 2 loose wires each ending in an mc4 connector.
A cheap pwm controller will do this ok though they are less efficient for the charging process.
Match up the male and female connectors with the opposite ones coming away from the solar panel and snap the mc4 connectors into place.
It safeguards the deep cycle batteries from being overcharged during the day.
It is around 14v at 40c.
If you want to get more efficient charge for your solar power you can use one of the many solar fed switch mode battery charger chips like the ti bq24650 this also gives you more flexibility for use of lifepo4 which is starting to replace lead acid.
Even a small solar panel hooked directly to a fully charged battery can overcharge it.
Connect the wires from the controller to the solar panel.
Using a solar controller limits the charging voltage to a safe 14 5v or so and protects the battery.
A solar charge controller is a component that helps manage the power that is going into the battery store from the solar panel.
Here i m using it to charge a 12 volt car battery as quickly as possible without overcharging.