M30 Battery / Alternator / Starter Question

[jlin89]

Please help! Owned a 1990 M30 with 135K miles.

My battery drains in a few days even if I don't turn on lights, radio, or ac/heater while driving during the day. I've put in a new battery 6 months ago & it worked ok for the initial few months, but the battery is now drained after sitting in the garage for a week. Even after I recharge the battery, it takes only a couple of days before the problem comes back. Another related problem is when I try to turn the engine over, sometimes it won't turn at all...sort of feel like the battery is dead (but I just charged it). I then tap on the positive battery cable a couple of times, and the electricity would come back on & I'm now able to turn on the engine. This problem comes up intermittently.

Took it to a friend & he told me the battery isn't recharging unless the engine is rev'd to above 2500 RPM. He thinks it's the alternator, but I hope it's the positive battery cable.

Does anyone have any idea what the problem is? How much does it normally cost to replace the alternator?

Any feedback is much appreciated...

Thanks!