Basically, using a car battery is fine. It's just 12v to 12v. It's when the car's running that the problem occurs. The cause of the problem is not the battery (voltage), but the car's charging system. It's putting out much more current (amperes) than the small bike battery can handle, so it's liable to fry the bike's battery. Think of it this way - you can use a high current to charge a car battery, with no problems, but only around a maximum of 2 amps (Suzuki recommend 0.75 amps) to charge the bike battery.
