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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?

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TL DR: No.

Throw everything you wrote about the voltages and such (in your second paragraph), because really it's kind of moot. The reason is, when you put the jumper pack on, you are putting power to the donor vehicle in parallel. You've added amperage, but not voltage.

If the battery is worn down enough to quit (while running without the alternator), the battery is most likely going to be much lower in voltage than 9v. A car will continue to run on a low battery for quite some time as long as there isn't a huge parasitic draw from some other part of the car (like the lights, stereo, or some other). You have to have enough power to run the computer and the fuel pump. Both can survive on pretty low voltage (for a time).

The jumper pack will not boil over the battery which is in place because it doesn't have enough power (juice, amps, watts, volts, what have you) to do it. Alternators with bad regulators can produce enough voltage to boil a battery, but that's in the neighborhood of 16+vdc, when applied continuously. Anything up to 15-15.5vdc could possibly do it if left on the battery long enough. The thing is, your jumper pack probably only has around 13vdc at maximum charge. Then, once attached to the dead battery, starts to lose some of its charge immediately. There's basically no way for the jumper pack itself to cause the issue in the first place.

Really, for the safety of the person's car you are trying to move, you use whatever field expedient means are available to get it to safety. You aren't causing the battery any damage by using your battery pack in the manner you're talking about. In effect you are basically replacing the battery which is there with the portable one (well, at least the power is being replaced). There's no real worry of it catching fire unless there's a physical reason for this to happen (jumper pack isn't secured enough and falls into the moving parts of the engine, type deal).

Really, there should be no worry about this at all.

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