15W solar and a group 24 battery should have a controller unless you are watching the voltage often - in which case you are the controller.
A little about solar controllers. There are two types MPPT and PWM.
The simpler and less expensive controllers are usually PWM and these basically just use an electronic switch to connect the panel to the battery. When the controller wants to cut back on the solar charging, it starts to modulate the switch so that the panel is not connected to the battery all the time. The PWM controller can still have all the normal charging phases of bulk, absorption and float so will do a good job of charging the battery.
The more complicated type of solar controllers are maximum power point tracking (MPPT). This type of controller will actually search for the best voltage to operate the panel at to get the maximum power from the panel. These pretty much also have smart battery charging of bulk, absorption and float so do a good job of battery charging. The solar panel output voltage is temperature dependent and on a cool day, you might get 20% even up to 30% (perfect conditions) more power out of the panel than a PWM controller would. But on a hot day, you might only get 5%. On average, you might get 10% more power out of the panel by using a MPPT controller.
The two controllers that I have used in the last few years have both been MPPT.. and neither is completely waterproof (used them on a sailboat). Both of these are for lead acid wet cell batteries. I thought both of these performed well. Ive also used PWM type controllers in the past and they also worked well.. you just get slightly less battery charging for a given solar panel size.
I used a Genasun GV-5 for panels between 10W and 40W Its 5 amps so will actually handle up to maybe even an 80 watt panel but has some nice specs for very low power panels
https://www.amazon.com/Genasun-GV-5-Pb- ... 9161&psc=1A couple years ago I put an electric fridge on the sailboat and these are big power hogs so I went to a 160 watt solar panel. I changed the solar controller to a Victron 75 15 (up to 75 volts solar voltage, up to 15 amps current). I bought one that does not have the bluetooth feature but a friend did get this feature and it shows a lot of cool information with an ap for a smart phone.
https://www.amazon.com/Victron-SmartSol ... dpSrc=srchFYI, as mentioned, nothing at all wrong with a less expensive PWM type controller, you will just get slightly less output - like maybe 10% on average. If the panel is always cool, MPPT gain goes up. If you are in a hot place, MPPT gain goes down.
One final thing I have learned the hard way.. NEVER short the solar panel side of the controller when there is sun on the panel. Even with fusing, it almost always burns out the controller.