CdnoilRAM nailed it in one sentence.
The down-side to installing a Deep Cycle [DC] Battery as your main battery in a vehicle is, by its very nature as far as what it does, it's not the battery's primary purpose.
Like mentioned in the second reply, DC batteries have a lot less CCA than your standard car battery. CCA are a pre-requisite to start your vehicle as that sudden high current energy bursts are what starts your vehicle. The flip side is the have exceptional Reseve Capacity [RC] to provide a steady flow of power while the engine is parked.
Also, DC batteries are designed to be discharged to as low as almost 85% of their maximum capacity. Their optimal operating range and longevity is dependant on this; although no greater than about 50% is what I was always taught. Your standard automotive battery will die a quick death under similar conditions as they are designed for rapid burts of energy and, typically, do not like to be drained more than about 15% of their capacity.
DC batteries can be discharged and re-charged several times over the course of their life; they thriven it. Your standard lead/acid battery will be MIA after about one-half at best] discharge/charge cycles.
Yes, a DC battery can most certainly be used as a main vehicle starting battery, however, this environment is NOT what it was designed for nor is it working under its optimal conditions.
I'd pass.
Good luck ultimately in your decision.
HowStuffWorks explained it well:
People who have recreational vehicles (RVs) and boats are familiar with deep cycle batteries. These batteries are also common in golf carts and large solar power systems (the
sun produces power during the day and the batteries store some of the power for use at night).
Both car batteries and deep cycle batteries are
lead-acid batteries that use exactly the same chemistry for their operation. The difference is in the way that the batteries
optimize their design:
- A car's battery is designed to provide a very large amount of current for a short period of time. This surge of current is needed to turn the engine over during starting. Once the engine starts, the alternator provides all the power that the car needs, so a car battery may go through its entire life without ever being drained more than 20 percent of its total capacity. Used in this way, a car battery can last a number of years. To achieve a large amount of current, a car battery uses thin plates in order to increase its surface area.
- A deep cycle battery is designed to provide a steady amount of current over a long period of time. A deep cycle battery can provide a surge when needed, but nothing like the surge a car battery can. A deep cycle battery is also designed to be deeply discharged over and over again (something that would ruin a car battery very quickly). To accomplish this, a deep cycle battery uses thicker plates.
A car battery typically has two ratings:
- CCA (Cold Cranking Amps) - The number of amps that the battery can produce at 32 degrees F (0 degrees C) for 30 seconds
- RC (Reserve Capacity) - The number of minutes that the battery can deliver 25 amps while keeping its voltage above 10.5 volts
Typically, a deep cycle battery will have two or three times the RC of a car battery, but will deliver one-half or three-quarters the CCAs. In addition, a deep cycle battery can withstand several hundred total discharge/recharge cycles, while a car battery is not designed to be totally discharged.
