1999 225 Tach/Voltage Problem

1999 225 Johnson. Model # J225PLEEC. Serial # G04534887.

Hummingbird 997C wired direct to cranking battery. After fishing for a few hours "Low Voltage" warning comes on the bird, even ran the battery dead once. Also had problems with Tach reading irradically.
Changed battery. Checked connections on Tach and battery, all tight and clean. Still having the same problem.
Could this be a rectifier going out?

sure sounds like it. Of course you should check the rest of the components too.