Patent ID: 7385795

Claim:
A cordless power tool comprising: a battery pack comprising at least one battery cell and a discharge switch coupled to said at least one battery cell, said discharge switch comprising a field effect transistor (FET), wherein said FET has a drain to source resistance of about 3.0 milliohms at a junction temperature of about 30 degrees Celsius, and wherein said drain to source resistance increases as said junction temperature increases; a motor to drive an element of said cordless power tool, said motor being powered by said at least one battery cell; a protected weak link element configured to fail from an overload condition before one of said at least one battery cell and said motor fails from said overload condition wherein said protected weak link element comprises said discharge switch; and monitoring circuitry configured to monitor a power condition of said protected weak link element wherein said power condition comprises a voltage drop across a source and a drain terminal of said FET, said monitoring circuitry further configured to protect said protected weak link element from said overload condition in response to said monitored power condition, and to thereby also protect said at least one battery cell and said motor from said overload, said monitoring circuitry comprising a sense amplifier configured to receive at least one signal from said protected weak link element and to generate a first output signal to a comparator, said comparator configured to receive said first output signal and a threshold level and to generate a second output signal to a driver if said first output signal is greater than or equal to said threshold level, said driver configured to protect said protected weak link element from a failure by opening said discharge switch if said voltage drop is greater than or equal to said threshold level, said threshold level set so that said monitoring circuitry is configured to open said discharge switch before a failure of said discharge switch.