I got the "low battery" reading as well. I had the Raymarine repair guy on my boat. He took all sorts of readings checked my connections and called Raymarine. They said the unit has issues and could be sent in for repair but they could not guarentee they still even had parts. They offered a reconditioned one or a new model at a discount. I'm waiting for prices.
This unit and issue has come up on a nimber of blogs I've read. Ths consensus is to replace it. The new unit comes in two parts and is supposed to be better built since they did not try to cram all the necessary stuff into that 1 small box.