how to reduce certain value of a DC voltage bot not the other values?

Hi there,

i have a DC device wich working on -48VDC (-41~-59 Volts). it's supplied by a rectifier (floating voltage of the rectifier is -54V).

when the AC power got interrupted, the rectifier get interrupted too. but the DC voltage still supplied by the battery strings.
the problem is my equipment is limited to 5V/ms voltage variation. when the rectifier go down the batteries start immediately supplying ~-49V.
of course they will start discharge (slowly or rapidly according to the load.
when the voltage of the batteries goes to something like -48 or -47 and the AC power retrieved (and hence the rectifier is retrieved) the voltage jumps from -48 or -47 to -54Volts, which is not matching my equipment standard of 5V/ms change at worst.

i have no authority to change anything with the rectifier.
so is there anything that allows me to reduce the -54V to sth like -51V but keep the lower voltage as it is.
in other words, what i want is to let the voltage goes from -47 to -51 (to not get interrupted).


i hope you got my point, if there is anything not clear please let me know.



kind regards,,