The FP-30 is an internal switching power supply for the FT-897 and FT-897D radios. The FP-30 provides 13.8 VDC at up to 22 Amps for an extended periods of time. It is used in place of the internal batteries. For myself, I am not using the batteries so I decided to buy this PSU and I have not been disappointed.
I have heard a few people complain about noise, but mine is extremely quiet. And when I do take my rig to other locations, I do not use the batteries because I'm always around power. Having an internal PSU makes carrying this radio much easier than also having to lug around a power supply.