I have experienced the same problems with my ST4000+ Head unit, which is installed on my recently purchased Catalina 36 Mk II. The 'Low Battery warning would kick in after a short period, forcing you to switch back on to 'Standby'.
Fortunately I had a the original ST4000 unit, which I replaced. This cured the problem. The ST4000+ was returned to Raymarine
UK, along with the drive unit for examination. Raymarine
told me that the processor in the head unit was faulty and that as these units are now obsolete, they do not carry the parts to replace it, and if they could, it would cost around £400.00. They did offer me a discount on a new wheel pilot for around £1100.00.
So for now I shall be using the original ST4000 Head.