With a 30 amp fuse used by those devices, you can see that it is physically impossible for them to be really 40 amp units. If you watch them on a gauge, you can see the real output from them. Most i have seen is 23.3 amps at full power @17volts.. The Race ones at @20volts still only put out about 24amps at full dial setting. Putting in a 40amp fuse makes no change to that output.