In your case, it is probably the battery ground wire that needs tightening. Or, the alternator was already dying when your X replaced the dead battery.

You battery will only let you run for a little while without a charge. Please, turn off the headlights, radio, cabin fan, and anything you can think of. The daytime running lights can be turned off by using a small screwdriver or fingernail file to turn the small screw in the headlamp switch. Push in and move it to 11:00 o'clock.

Klaus
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Proud owner of a 220S. If I had more room, I would have more Volvos.

Right...double-check that the cables are tight, and that there is a secure ground connection, although if the car is starting, the battery itself is fine.

With the car running, grab a voltmeter (or drop by Advance Auto or other parts store and they can test it for you - they're usually pretty friendly about doing this, and its free!), but you should be seeing >12.8 V across the battery terminals, indicating that the system is charging.

If you have a bad alternator, you can still drive home, but don't do a whole lot of driving...once the battery runs out of juice, the car will outright die in the middle of the road - not fun.