Depending on the conditions, a starter can pull up to 500 amps. These levels of amperage stress ALL components in the circuit including cables, terminals, batteries, and more. Problems with these components can be difficult to diagnose as they appear fine at rest and under a light load, but generate high resistance under heavy amperage draws. This results in low voltage to the starter while cranking, causing heavier amperage draw, and increased heat leading to starter failure. Batteries, cables and additional components should be checked and serviced regularly. Voltage measured at the starter during cranking should always be above 11V DC. Starter heat shields are not required, but are an excellent idea for heat protection.