You need to remove the battery and install a good known replacement. Connecting a good battery in parallel with your existing (questionable) battery doesn't always work as a poor performing battery can compromise the donor.
With your multimeter, check the battery voltage across the terminals - you should see at least 12.5v without the engine running. To check the alternator output, with engine running you should see at least 13.5v at the battery terminals.
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2001 Boxster S (triple black). Sleeping easier with LN Engineering/Flat 6 IMS upgrade, low temp thermostat & underspeed pulley.
2001 MV Agusta F4.
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