The Red Bladder wrote:It sounds as if your old UPS just had a rectifier or similar (cheap component) failure. A buzzing noise prior to total failure is typical and I would just get the damn thing fixed.
Whilst I am almost always in favour of fixing not flinging gear battery technology has moved on apace in the last 5 years or so. The modern Lithium battery are smaller, lighter and far less fussy about charge rates than the Sealed Lead Acids normally used in a UPS and do not suffer the "memory effect" of NiCads.
I also suspect the fault is more extensive than simply a punched through rectifier? A filter cap may have shorted, that borkerd the bridge and there might be blown print all over the shop. We are talking serious electrical energies here.
I don't think OP needs a continuous converting supply? "Modified Sine" with rapid switch over seems to suit most people ok. Computer power supplies "modify sines" very well as it is!