I was having trouble keeping my battery charged. I suspected a voltage regulator problem since I was seeing 15.7-15.8V across the battery while idling. I tested the alternator at the parts store and it tested bad, so I have replaced the alternator in with a Bosch reman.
While the old battery tested 'good' at the parts store, my charger would not charge it past 60%. So I went ahead and replaced it "just in case" (it was 6 years old, so not inconceivable it would give up the ghost from old age soon anyway). The new battery takes a charge and holds it.
HOWEVER! The new alternator still shows 15.7V at the battery. I've since learned that the ECU is actually controlling the voltage regulation. I had a spare (but possibly not legal) ECU, so swapped it and saw 15.2V at the battery. Hurray - I concluded that in fact the voltage regulator in the first ECU was bad, so I bought a third one. I installed it today and... I'm seeing 15.8V at the battery. What are the odds of two ECU's with bad voltage regulators? I'm wondering what else I'm missing.
Also: The ECU in the car is coded BP4W. A friend has a spare coded BP5R which I'll swap and see if I get better regulation. I can't find any info on the interwebs about differences in those codes - any reason to avoid the swap if I see better voltage with it?