I have a 2006 Honda Civic SI.
It was slow to crank when starting up, and while driving, electrical systems progressively got worse (radio would turn off and on, power steering would stop, and then car would eventually just turn off completely and turn back on).
The symptoms were worse when there was a heavy electric load (windshield wipers and lights on, etc.)
Because it was having issues while running, I assumed this would make it an alternator problem. With my multimeter, the battery tested at 12.6V while off, and between 13V and 14V while on.
At Autozone, their tester said it had a bad cell.
Once I replaced the battery, all the symptoms were gone. What did the battery have anything to do with the car having those symptoms while turned on?