The battery voltage in a modern car sits at 12.65V+-0.1V while the car is parked with nothing turned on; it is at 14.2 to 14.7V while the engine is running fast. A 12V SLA battery should get 13.7V+-0.1V while being "floated"; and 14.7V+-0.1V while being "charged". If the SLA is simply paralleled across the car's battery, then it will be discharged while the car is parked, and will likely become overcharged if left connected if the car is driven for more than two hours per day.
Using a relay to connect the SLA to the car's battery only when the car is running will solve the back-discharge problem, but it still leaves the potential overcharge problem if the car is driven a lot. A circuit consisting of a voltage reference, a comparator, with hysteresis, which pulls-in the relay at SLA<=13.6V, and turns off the relay at >=14.4V, would prevent the overcharge.
Another simple way is to use a Schottky rectifier diode between the car battery positive and the SLA positive. That prevents the back discharge, and drops the charge voltage by about 0.35V (from 14.7V to 14.35V) which the SLA will tolerate for a couple of hours per day.