Battery negative cable sparks on hookup

[toothpic125]

2016 Equinox. Pulled engine did timing chain rear main seal. All fixed. Went to hook up negative battery cable and it sparked. Didn't hook up but testing from neg terminal to neg cable I have 12v. Disconnect positive cable at alt. Everything is ok. Can I have short in alt?

 

[RIT333]

To me, it sounds like you have a parasitic load.