With car running its more amps that the bike system was designed for...Can also destroy the ECU....(Seen it happen more than once)
At a set voltage you can't force excess electricity into something, the load determines the current. Your bike battery alone is capable of over 100A. A car/truck works on a 12V system same as the bike, both systems are charged from 14.2-14.4V charging systems. Standard alternators output less than 100A, a high output alternator is capable of 100-150A output and so is your bike battery. If you charge off a non-running vehicle or just a bare auto battery it might be capable of providing over 1000A from a full size battery. Doesn't matter as the bike (the load) determines how much current it will use. You could have a dual battery system capable of over 2000A and it still isn't going to hurt the bike. You can't force excess current thru a load while holding the voltage constant, electrical laws prevent it.
At home you have a 200A service with multiple 20A receptacles, plug in a .004A (.5W) night light and it doesn't blow because 20A is available. It uses what it needs and you can't force more through it.
A running diesel truck with dual batteries can supply over 2000A from the batteries and additionally 150A from the alternator. Nothing electrical is turned on, you open the door and the little inside cab light comes on. It is not damaged even though over 2150A are available, because the voltage is held at 14V and the current (load) is determined by the light itself, constant voltage and constant load equals constant current.