My friend has a 2001 Volvo C70 that drains the battery constantly. She has replaced 2 batteries and is on her second alternator. It runs when boosted but turn it off and its dead. What can the problem be? I know its an open circut but no lights are on in the interior. None! I've read post about disconnecting all the bulbs and replacing them one at a time on a new battery (if it holds), but to be honest I don't have that kind of time. Drained batteries seem to be a common problem with this make and model.

Had same problem check the driver or pass Sun visor light it stays lit even when flipped in close position leave open when leaving !

The unfortunate thing is diagnostics DOES TAKE TIME. You need to set aside the time to be accurate.

Something for sure is draining the battery and methodical diagnosis is the only way to get it figured out. IF there were an open in the alternator to battery circuit, the new alternator would be damaged.

Pull the interior light and radio fuses, disconnect the negative battery and put a test light or volt meter in series with the cable and battery terminal. I expect the light will be lit or the voltage to be above .2 volts draw.... Disconnect the battery, remove one component, connect the battery test setup, and see if voltage drops or light goes out..... when it does, you have the offending system, so investigate that.

Be sure that you can garantee good ground between battery and everything....especially the engine.