I agree with dbrick — you don't really know if your alternator is charging or not. And the BATT light is working as it should, which also means that the alternator D+ wire (thin red) and ground wire are OK.

The alternator could be putting out good voltage that isn't making it into the battery due to a corroded battery terminal — so the battery dies after repeated starts without ever getting "pumped up" again while driving.

The fact that the BATT light doesn't come on while running is a further sign that the alternator is probably OK. Because it comes on only when the alternator voltage is LOWER than the battery voltage.

I would disconnect and scrape/clean:

1) both battery terminals (inner surface and posts)
and while they are off,
2) the starter terminal (to/from alternator and battery)
and,
3) the B+ terminal at the alternator (big red wire going to starter terminal)

Then with engine running, measure voltage at both Alt B+ and Battery Positive terminal. Battery should be within 0.5 volts of the Alt B+ output.
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Bruce Young
'93 940-NA (current) — 240s (one V8) — 140s — 122s — since '63.

Why do you think the alternator isn't charging? The idiot light's supposed to come with the key, before engine start...and it does. And it's supposed to go out when the motor starts...and it does.

The battery could just as well be going flat because there's a high-resistance electrical fault somewhere else in the car. Turn everything (including door lights) off, and pull the fueses one by one, and see if one sparks as it's removed. That circuit is passing current, even though things are "off."

You could have the alternator load-tested by an electric motor shop, or try this: run the car for a half-hour or so, so that the battery would be fully charged. Then disconnect one battery pole, and let the car sit overnight. In the morning, reconnect the battery and try to start the car. If it cranks and starts, you know that the alternator was charging correctly, the battery's OK, and that the problem's an electrical leak somewhere in the car. If it doesn't crank, you know that either the alternator didn't charge the battery or that the battery's dead.
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David Brick, Santa Cruz CA, 1988 245

wire to battery goes to idiot light - no idiot light means connection to battery is poor, and you won't get proper charge.
Measure the voltage across the battery with no accessories, and with lights and wipers on.
If you aren't seeing around 14v with accessories, (you won't) you're not charging and will drain the battery.
You can either route new known good wiring to the battery from the alternator, or fix the existing stuff.
Hope you have your old alternator, it's guaranteed to be fine...

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Stef (81 245 B21A SU M46 3.91) 325000km my volvo pages