If the starter battery is not excessively low... you can jumper it to the car battery and boost its charge. Make the connection with the engine OFF. (and be sure to get + to +... or something will explode)

I hooked up a bannana plug connection in the back of my Suburban and plugged my 7 amp starter battery into that when driving to the field. Worked fine. 30 min drive and the starter battery was fresh every time I flew. (tapped it off the trailer wiring harness which had the RV style plug... which can charge the travel trailer's battery)