Can i leave the car battery in situ still connected and use one of these to charge it. I remember asking an electrically minded friend of mine a few years ago (when i had a normal charger without safety cut offs, trickle maitainer) and he said he wouldn't because if the battery is faulty the current from the mains could fry the ecu etc.

I assumed i could but after reading the intruction manual its says to disconnect it.

So what your opinions