alternator die

Hi, I have a couple of questions in trying to repair my '91 900 Turbo died a while ago The car wouldn't keep a charge and on the day the car died the tach dropped to zero and showed no signs of life. I have just installed a new alternator and bought a new battery. The old battery was dead...

I just got back from deployment about a month ago, plugged in the battery and everything seemed wor just fine. Over the last few weeks it has seemed a little tedious at times o start the car ie it seems like there was low power. I figures the baytery just decided to have an issue from sittingso...