Why did the new IFIXIT battery swell up in less than a year?

Sounds like something you can take up as a guarantee case with Ifixit as batteries are usually not supposed to fail this easily. Did you doublecheck everything at the time of installation? One minor thing like a tiny shard of glass or a tiny bit of metal shavings under the battery can cause these issues over time if it was not doublechecked and installed properly. More details about how it was stored etc would help people here give a better diagnosis.

Thanks Markus - as a retired electronics engineer I am usually quite careful about working in a sterile environment and avoiding foreign objects getting into the innards of anything I'm operating on, so I hope that was not the issue.

As to storage, it has been at room temperature since the new battery was fitted, with as previously mentioned, some active use and then left powered down for a few weeks or maybe a month or more, then put on charge and left unused.

I haven't yet seen any obvious way to claim under guarantee but will poke around more on the IFIXIT website.

in friendship

Rowland