These make tools run, but they're dangerous because they break the safety battery safety features which are built into the tool. With the exceptions of Ryobi an

These make tools run, but they're dangerous because they break the safety battery safety features which are built into the tool. With the exceptions of Ryobi and Makita, most power tool battery safety features are built in to the tools, and they communicate to the battery. At minimum, they check a thermistor. More sophisticated schemes can query the battery for shutoff voltage and maximum current.