Many Smart-UPS have either multiple internal battery cartridges and/or the ability to add external batteries.
Resolution:
No, you should not mix old and new battery cartridges within the same Smart-UPS system.
As VRLA (Valve Regulated Lead Acid) batteries age, their capacity is reduced. Your Smart-UPS batteries are connected in parallel and all batteries are charged at the same rate. Mixing batteries of different ages can result in some batteries being under charged while others are over charged, which would lead to reduced runtime.
When uninstalling and installing the battery cartridges, you should keep the battery cartridges in their original location in the system.
When replacing the batteries in a Smart-UPS system, you should replace all battery cartridges(internal and external) at the same time.
When replacing battery cartridges that are comprised of more than one module, you should keep the modules together during the installation.
For example, an APC Smart-UPS SRT 5000 system with an external battery pack would take one battery cartridge for the UPS and two battery cartridges for the external battery pack. Each battery cartridge has two battery modules. When replacing the battery cartridges in this APC Smart-UPS SRT 5000 system, the two battery modules for the UPS should be kept together and the four battery modules for the external battery pack should be kept together.
When adding additional external battery packs to a Smart-UPS system, you should first replace the existing battery cartridges (internal and external) if they are more than 1 year old.
You should not mix battery cartridges with more than a 1 year age difference in a single installation.
For detailed information on how to replace battery modules or add additional external battery packs, refer to respective user manuals