Solar panels may make power at very high voltage and current. If all that is fed directly to the battery then it may get overcharged or blast (in extreme cases).A solar charge controller prevents this from happening. It is is placed in between the solar panels and the battery bank to regulate the charging current and voltage of the batteries through solar. It also prevents the reverse flow of current from batteries to the solar panels during night-time.Basically, without a charge controller it is not possible to safely use solar energy for charging batteries.