One way to deal with a battery designed for starting a car and put into constant current drain service (not recommended), is to derate it by 90 percent. Thus what may be the equivalent of a 75ah battery in your car, should be treated like it is only a 7.5 ah battery. Start the car, or immediately recharge after your session. You can get away with more, but you will be hurting the battery.
If your car battery is rated only in CCA, see if the manufacturer provides a 20-hour rating to get a better idea, and then derate that by 90 percent.
"Marine" batteries are hybrids, but are still primarily designed for starting services, with plates that are a little bit thicker for occasional constant current draw. You can choose these, but given the choice, opt for the TRUE deep-cycle types. Your wallet will thank you in the long run as you will be replacing marine batteries much faster than if you chose a true deep-cycle in the first place. Choosing Sealed lead-acid AGM types instead also means that you can use them indoors and not have to worry about spilling/splashing electrolyte etc.
Conversely, deep-cycle batteries can also be put into service for vehicle starting services, but you should figure in an additional 20 percent for the ah rating.
In the end, choose the right battery for the right job.
