Hey guys so im just curious because someone told me this could be bad for my ECU. When i disconnect my battery terminal in the back it tends to let out a good spark. Granted my car has a lot of electrical modifications so it is probably pulling a little more when reconnected. But is this bad for my ECU or car in general? Any thoughts would be great.

 

ANY time DC power is interrupted/connected, it makes a spark....
That is why relays get a buildup on the contacts, etc. From an engineering point of view, a High amp DC switch/Fuse is pretty expensive.... leading to part of the reason AC power is more common in many uses.....High amp DC switches/fuses etc are used in Aviation..think 100 amp+, but are costly, and still need regular maintenance...
While I would not sit there purposely making a spark, it is a fact of life......so no worries....sure a little surge goes through anything anytime you turn it on or off...but how can you avoid it? The battery is disconnected for a reason....so the lesser of 2 evils if you get my drift!