My 2001 2.7L eat batteries, about once every 18 months, after a few years I had a warning light and when the garage checked the output from the alternator was low.
After changing the alternator, even an old battery that appeared to be on the way out lasted another 2.5 years. Basically the current draw when standing still is quite high and unless you use your Boxster frequently the battery discharges.
If I leave it and go on holiday the battery will last about 4 to 5 weeks, so bought a solar panel which helps, keep a bit of charge in the battery. My car sits on the drive and so not really practical to leave plugged in to a Ctek charger.
During lockdown last year I did buy a Ctek Battery Sense monitor, these just connect to the battery and use Bluetooth to your phone, it gives graphs of voltage, battery charge state and temperature, so you can see when it is getting down.
I got a second one for my other car when that failed to start one morning.
Finally, I bought a jump start battery, it will go in a glove box, but mine is higher capacity than most, you just clip it on the battery or terminals inside the cabin and it will open the front and jump start the car.
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