Quote:
Originally posted by xoxoxoBruce
Even a dead short should not cause an explosion because of the internal resistance of the battery becomes current limiting.
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Battery explodes because internal battery resistance dissipates the entire short circuit. In a car battery, that resistance may be on the order of hundreths of ohms. In a short circuit, all battery electrical power would be dissipated by that internal resistor - resulting in a battery explosion.
However if shorted by a circuit that also includes a battery cable, then the short circuit would have more fractional ohms to also dissipate the power - outside the battery. Granted, that cable is still too low to provide full protection. But the additional resistance of battery cable will at least slow down the 'time to explosion'. Gives a human time to correct his mistake.
Returning to the original question - probably the most important reason to ground to chassis - one cannot confuse which is positive and negative. Therefore someone will not blow out car's entire electrical system. In can't tell you how many times the black electric cable was replaced by a red one. Mechanic was more concerned with what he had rather than doing it right. Then when someone jumpered the battery, they hooked positive cable to the red wire. Sparking is well after damage has been done.