Don't bother; at that point just plug them into 200~300 V DC right to the battery (with just fuse and meter between, no need for any conversion, the washer motor controller that'd be needed to utilize a capacitor for smoothing these spikes would do just fine on a reasonable battery charge/discharge voltage spread.
Most electronic power supplies for computers and similar devices (like e.g. modern TV) have zero complaints about being fed such DC.
Sadly only those that try to sell to Alirack applications (i.e., Chinese server PSU's like for example Lenovo Servers) bother testing in QA and doing enough design review to officially claim that support on their data plate.
You can tear down a sacrifice from a series of identical consumer PSUs though, and reverse engineer the front end schematic to check that the AC side of the active power factor correction design that you'd find in e.g. a decent gaming PSU doesn't do anything that would react badly to a lack of AC.
