Sounds like you need to replace the battery given its age but would be best to use a volt meter on the battery terminals and see what charge the alternator is putting into the battery with the engine running. You should see in excess of 14volts, if its less then that will be an indication that the alternator is failing to charge the battery and in need of replacement. I had a problem with my Z3, if run at night using headlights the next day the battery was flat. Checked the alternator output and it was below 13volts, running with headlights drained the battery hence the no start next day. It provided enough voltage for daylight running but using the headlights was too much for it. Following this I fitted a small usb socket with built in volt meter in the dash so I can monitor the state of the battery and the charging rate the alternator is producing. Fitted a similar volt meter to my Z4 as well.
https://www.ebay.co.uk/itm/5-12V-Car...cAAOSwn4hfUz89
If you don't want to fit a permenant voltmeter buy one that plugs into the cig lighter socket like this
https://www.ebay.co.uk/itm/Dual-USB-...kAAOSwAE9fhBw6
Good idea to put a battery tender on the car as suggested earlier given the infrequent use the car has.