Charge the battery at a slow rate for a few hours with a portable charger. You have a pretty good jumper pack if it will crank an engine with a dead battery. Once running, the engine should continue to run on the alternator, however, a couple of things could have happened. First, there is going to be a rather large fuse bolted into the under-hood fuse box. That is for the very high-current alternator output circuit. Without looking at the diagram, that fuse will be in the order of a 125-amp. You can visually inspect it through the clear plastic cover.
Second, when a battery is totally discharged, it will take some time for the acid to become electrically conductive and START to take a charge. If you use a small portable charger with an amp meter, you'll see charging current stays near 0 amps for five to fifteen minutes, then it will slowly start to climb. Ideal charging current is ten amps or less for a few hours. I consider a battery done charging when current drops down to five amps.
This is for a small portable charger. Don't use a larger wheel-type charger unless it has a very low setting. Battery chargers vibrate the battery's plates. On older lead-acid batteries, that accelerates the natural flaking off of lead from the plates, which eventually is what shorts a cell and kills the battery. On newer "absorbed glass mat", (AGM) batteries, charging rate is monitored in the vehicle to keep the charge rate low. All batteries give off explosive hydrogen gas. It's harder for those gas bubbles to dissipate when there's no liquid acid. Reducing the charge rate gives the battery time to expel that gas gradually. I have a 2014 Ram and a 2014 Grand Caravan. Both came with AGM batteries. It's a good bet your Jeep did too.
Also be aware a real lot of computer damage can be done by disconnecting a battery cable while the engine is running. Running the engine with a totally discharged battery is close to doing the same thing. All AC generators develop three-phase output. That is very efficient but includes "ripple" voltage. That is small variations in output voltage. The battery is responsible for smoothing that out. When the battery is dead, depending on the charging system circuitry, the voltage regulator in particular, some will respond to the lower voltages and try to bump that up. As long as at least a dead battery is there, the engine typically keeps running. Some regulators respond to the small voltage rises and try to cut back on that. Often they cut back so far that the engine stalls due to a slowed fuel pump or weak injector voltage pulses. That alone doesn't cause damage, but it explains why an engine will stall even though the charging system is working just fine.
All of this can be overcome by just charging the battery for a few hours. If it won't crank the engine after that without the jumper pack, it has probably failed. Chrysler used to say a good, fully charged battery will be strong enough to crank an engine fast enough to start after sitting for three weeks. Today that is the industry standard unless specified otherwise by the vehicle manufacturer. That is due to all of the computer memory circuits drawing a very small current to keep their memories alive. What I found two winters in a row while my Caravan was in storage, is after about two months, the battery was totally dead, then they froze and were ruined. Now I put a small battery maintainer on them.
Friday, April 3rd, 2026 AT 10:48 AM
