Volvo Repair: I replaced my ABS unit and it did not fix it., gross polluter, abs brakes

Question
Hi, Thanks for being here. I love my Volvo. Earlier this year I couldn't pass smog- it said I was a gross polluter. My engine light was on soI went to Autozone and they  used a OBD-ll to diagnose it. I replaced the part a sensor under my car I believe it was at one end of the catalytic converter.  The check engine light was gone and I passed registration.A few weeks later the check engine light came on again. and then the SRS light too.  I didn't jhave money for gas there for a while let alone andy repairs when my car was running OK. Last week the radiator cracked and I replaced it, then a few days later the battery kept dying so I bought a die hard for 150.00 at Sears.  It was running so well for a week then yesterday i was driving and I hit the wipers for a minute and noticed they were really slow then within moments the engine was pulling the dash warning lights for ABS-BRAKES-TRAC-and the other light had an S-STC I think. It felt stuck to the road when I stepped on the gas like it didn't want to go. Then the speedometer went limp and my mileage was just lines!?
I got it home I was close. I turned it off and then tried to start it but could not.
The next day it started right up and those new warning lights did not come on? I did not drive it.
Online I investigated the problems I had had the day before.  The same answer kept comming up- it said I needed to replace my ABS Unit. Today I bought one used for 200.00 and we replaced it today.  I drove around the block and it was running great. I came home parked it and got ready for work.  Then I started to drive it to work.  All of the same warning lights came on and the mileage was just a line and the speedometer went limp.  Just like what happpened the day before happened again.  I took it home and I can't start it again.  How do you test anABS Unit ?  Could I have replaced a bad one with another bad used one? Or could it be something else? Could it need anobd 11 test to tell me something else related to the lights SRS and Check engine that were on for months before all of this happened?  Please give me your professional thoughts. I am so confused, and I am so broke, and my husband is out of work, and it is Christmas!   Thank you Karyn  

Answer
  Karyn, wherever you are getting your advice from, tell them I said to go back to being a plumber and leave cars alone. VBG
  Thank you for the excellent detail in your posting. Your first clue was the battery dieng. Your old battery is probably fine. Did anyone test the battery and charging system before replacing it? I bet not. Your problem is the alternator or a loose/worn belt. It is charging intermittently and probably quits hot and when under heavy load. The ABS warning light will display when the ABS computer sees low incoming voltage. Same for the odometer lines and limp speedometer. Also explains low power and slow wipers. All symptoms of low voltage. There is nothing wrong with your ABS unit.
  If you are capable and to save money, replace the voltage regulator/brush assembly on the alternator. (Little black box that the small wires go to. Two screws and you're there.)That should extend it's life. A $ 20-30 part. If that doesn't cure it. Replace the alternator.

Merry Christmas,

Mark