Don't do this, it'll ruin your car's battery!
Automotive batteries are designed just for start the car's engine, their specs are lower than the specs of the so called "stationary" batteries, or "deep-cycle" batteries.
If you compare the price of replacing the batteries of a huge computer "no-break" (UPS - Uninterruptible Power System) and the battery of a car, you'll see that, even if the batteries have similar Ah specs, the UPS' batteries cost much more, they're expensiver because they have better specs.
You should avoid depletting the battery of a car at lower than, say, 90% of their full capacity. If you need to power many electronic devices in your car, for a long time, you should either keep the engine on, or use another kind of battery.
"A deep-cycle battery is a lead-acid battery designed to be regularly deeply discharged using most of its capacity. In contrast, starter batteries (e.g. most automotive batteries) are designed to deliver short, high-current bursts for cranking the engine, thus frequently discharging only a small part of their capacity.
While a deep-cycle battery can be used as a starting battery, the lower "cranking current" implies that an oversized battery may be required.
A deep-cycle battery is designed to discharge between 45% and 75% of its capacity, depending on the manufacturer and the construction of the battery. Although these batteries can be cycled down to 20% charge, the best lifespan vs cost method is to keep the average cycle at about 45% discharge.1 There is an indirect correlation between the depth of discharge (DOD) of the battery, and the number of charge and discharge cycles it can perform.
And also wikipedia:
An automotive battery is a rechargeable battery that supplies electrical energy to a motor vehicle. It is also known as an SLI battery (starting-lighting-ignition) and its main purpose is to start the engine. Once the engine is running, power for the car's electrical systems is supplied by the alternator. Typically, starting discharges less than three per cent of the battery capacity. SLI batteries are designed to release a high burst of current and then be quickly recharged. They are not designed for deep discharge, and a full discharge can reduce the battery's lifespan.