The designation is RBP0051 according to the data sheet. But look what it says on the battery.
You can find it here: https://www.battery-direct.de/cyberpower...

Battery for CyberPower PFC SINEWAVE series CP1500EPFCLCD
For UPS models: CP1500EPFCLCD
Manufacturer: battery-direct©
Product group: Replacement battery for CyberPower UPS
Product type: UPS battery set (cables and attachments are to be used from the old battery)
Battery type: Maintenance-free, sealed brand lead battery with suspended electrolyte (AGM technology). Best high current capability for UPS systems.
Directive: APC UPS 2002/95/EC, RoHS compliant
Warranty: 2 years (optionally 3 years)
Lifetime: Up to 6 years at 25°C
Length: 151 mm
Width: 65 mm
Height: 94 mm
Weight: 4.70 kg


Search for a compatible battery at CH-shops, e.g. Conrad or distrelec.ch.

Is not very widespread, only found a link for Ebay: https://www.ebay.de/itm...