What is wrong with the car?

So my gfs car just got the car stolen off of it. So I got a
straight pipe welded on and it did just fine. The battery
started losing charge so I replaced it with one of of my
other car which I know is new. It did fine up until I took a
long drive and it started losing electrical power
(gauges,radio, and headlights)  so I managed to get it home
with the help of a jump box. The battery was completely
dead last night. I wake up this morning and put the key in it
starts right up no issue has all power.  Wtf is happening