I'm a service employee for a highway assistance program. I came across a vehicle where the alternator had failed, and the vehicle drew all available power from the onboard battery, causing the operator of the vehicle to be stuck on the side of the road. One of the tricks I've learned is it's possible to temporarily connect in a battery jump pack to the battery, secure the hood somehow, and limp the vehicle off the road to a safer location. However, one of my peers mentioned that they thought running a vehicle solely off a battery pack could cause damage, fire, or boil over of the battery on the target vehicle.
In this case, we can assume the target vehicle's 75 Ah 12v battery is sitting at about 9v surface charge (not showing signs of a deep discharge, yet.) We can also assume we are using a reputable jump pack, such as a Jump-N-Carry JNC660 1700 Peak Amp 12V Jump Starter which shows an internal voltage of 12.5 volts unconnected. There is no damage or short to the existing batteries at this time. My thoughts are that this implicates the voltage delta between battery system and jump pack, and whatever the safe charge rate of a standard lead acid flooded car battery may be (I think C/5 for ~10% state of charge?)
Will hooking a jump pack up to a vehicle with discharged batteries and a broken alternator, starting the vehicle, and driving the vehicle for a minimal distance cause any harm to the vehicle's onboard battery in the way of boil over, overheating, or overcharging?