ORIGINAL: Black80XSA
With the engine running a multimeter across the battery terminals should read around 13.4-13.8V - its the alternator working flat out to recharge the battery. Switch the engine off and you will probably see 11V across the terminals and this will fall slowly even without any consumers on (ie lights, radio etc). You have the luxury of a voltmeter in the dashboard, the Boxster doesn't
I'd advise you to change the battery fairly quickly - running the alternator flat out for hundreds of miles will do it no favours at all.
Fitting a non Porsche battery may/will invalidate the new warranty scheme if this applies to you - Porsche want about £160 for the battery for DIY fit, about £280 fitted. Its a heavy item to lift out on your own - i changed mine using a bar through the handles and a person either side to lift the bar.