The 90 days limit is a Let's Encrypt limit. I don't think TLS limits that length in and of itself. Except that it might need to be finite. But I'm not sure that the `Not After` field is required.
That's also not a hard technical limit in the TLS spec, you can easily generate a self signed cert that expires in the year 3000 if you want, but it's voluntarily abided by from all the Certificate Authorities that want to have their root certs included in almost every web browser. It's _possible_ that EV chargers don't use common Certificate Authorities, but it feels unlikely to me.