If you have a multi-meter, check to see if the alternator is indeed charging the battery. You should see in the neighbourhood of 13.6-14.2 volts if it is charging while running. If it is not charging, there is a good chance the alternator has gone bad. Graham has a good point that it would be a good idea to check your cables as well. If cables are fine, you could remove the alternator and possibly install a new voltage regulator. I just did this on my 944 Turbo after purchasing a regulator from Autopartsway.com for $17. If this does not fix it, you alternator will need to be rebuilt or you’ll need to buy a new one.