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This is to help see some problems.
Solar panels act like a current source once the voltage is less than the maximum power point. For a three watt 36 cell solar panel (e.g. SLP003-12, because I have it in hand) the current source is set at 180mA and is in parallel with a 20V Zener which gives the open circuit voltage. The 16.5 Ohm is the panels internal resistance. Once the panels output voltage drops bellow 17V the Zener will stop conducting.
No need for a dummy load because the solar panel is happy with open circuit voltage.
Notice that as the battery charges up the power produced is less than the panels rated power, which requires it to be operated at 17V.
Taking the Battery to 15V is for flood type lead acid, but that high a voltage will cause hydrogen gassing, good chargers will do some gassing to stir up the electrolyte, and then hold an absorption voltage which is temperature dependent.
Absorption is time-consuming, it is named because a chemical reaction is taking place and gradually depleting (or absorbing) the last remaining lead sulfate (some is even in solution). Absorption is important in lead-acid because the lead sulfide can deposit in places that prevent it from providing power storage (sulfation). When all the lead is converted into its metallic form the electrolyte can dissolve some of the sulfation, and reverse the damage, but this is tricky business, of cycling gassing and absorption voltages without allowing the cell to run out of water or build up to much pressure. And as you might expect the power used during absorption and gassing is mostly not recoverable.
As shown this charger will not do a proper absorption cycle, so sulfation will build up and quickly rob the battery of its storage capacity. Once the battery has been discharged even a little sulfation starts to build up, at first the deposits are tiny seeds that are easy to absorb, but if it sets for too long the seeds grow into crystals that can become a nightmare to recover.
Letting a 12V lead acid voltage drop to 9V is not a good idea, it may not take a charge afterward.
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