Fried battery while towing small trailer through the mountains

I have a 2008 Jeep Liberty 4X4 Sport. I pulled a small trailer with a motorcycle on it through the mountains in southern Colorado. Was running at pretty high rpm’s at times (4000 - 5000) to get over the mountain passes and overcharged my battery, rotten egg smell. Did this happen because, at high rpm’s the alternator charges the battery and I just overdid it and need to take it slower, or is there another reason for this?
Thanks for the help.

I don’t believe that I would be looking at high RPM as the problem.

I am not all that familiar with Jeeps and there electrical design,

The rotten egg smell is caused from the battery being over-charged. So the battery is shot.

Your vehicle has a generator, and the output from the generator to the battery is contolled thru the Powertrain Control Module or computer. So either the battery was bad and PCM kept trying to charge the bad battery, or PCM was no longer controlling the output from the generator to the battery.


“Your vehicle has a generator”

Not an alternator?

Here’s a wiring diagram for 2003 Liberty charging system.

As you can see, there is no voltage regulator, rectifiers, or diodes. The generator output is controlled thru the PCM.


Whether your Jeep has an alternator or generator is immaterial, although I believe it would have an alternator. Your battery has been over-charged. I am not familiar with how the output of the charging unit is regulated, but tester is probably correct that it is controlled by the PCM. Older cars either had an external voltage regulator or had a regulator built into the alternator. At any rate, the battery shouldn’t overcharge no matter how high the rpm. When you replace the battery, with the engine running, measure the voltage at the battery. It should not exceed 14.5 volts. If it does, the battery is being overcharged.

The schematic is excellent, @Tester. It clearly indicates that the computer has the voltage regulator function built in. But the alternator does have diodes/rectifiers and is labeled ‘generator.’ I think the nomenclature is a common holdover from 50 years ago and may be an acceptable description technically. For previous years a remote voltage regulator was available in the aftermarket to eliminate needing to replace the computer when that function failed and it may still be available but my source has gone out of business. Maybe I can locate a source and some specific information.

Just some food for thought, but maybe this rotten egg smell is caused by the catalytic converters. Loaded vehicle with a trailer, pulling mountain passes in summer heat, high RPMs, and type of gasoline may have something to do with it.

That is a possibility that needs to be checked out, @ok4450. You may have saved the OP a chunk of change and a lot of hassle. The battery and charge rate need checking and if OK there may not be a problem other than pushing the engine to its limits and overheating the cat.

I agree the schematic is confusing - since it shows the circuitry for something that is clearly an alternator, but is labeled as a generator.

I have searched my usual sources without any luck but an after market remote regulator was a commonly installed part on Mopars that used the computer to regulate the alternator. It would most likely still be available and over the years I installed dozens with none ever returning with a complaint.

Vehicle manufacturers begain calling the alternator a generator about ten years ago as it is the proper SAE-J1930 term.

Chrysler vehicles have used the PCM to control the generator field since the eighties.

That charging system is quite reliable, if there was a malfunction there would be a fault stored in the PCM. The most common problem is a shorted cell in the battery, charging 14 volts into a 10 volt battery gives the effect of overcharging and the smell.

OP’s battery was at least 5 years old, I’m guessing

That was an acceptable life span

I thought (to get SAT on you), “Alternator is to Generator, as Square is to Rectangle.”

Meaning, all alternators are generators, but not all generators are alternators. Colloquially, though, “rectangle” is understood to be “not a square”, and “generator” is understood to be “not an alternator.”

I think @Nevada_545 brings up a good point about the battery possibly having a shorted cell causing this trouble. If the regulator seems to be working okay I would have to think that is what really happened here. After installing a new battery the charging system should be checked out for any issues.

At this point there is missing information though. The OP says the battery is fried.

What they have not stated is whether they’re basing that opinion on a smell or if the battery has actually been tested and found to be bad.

I kind of read this as the problem has gone away now that the mountain climbing is over.

Maybe the smell is a transmission slipping… :frowning:

Basically alternators are generators…they generate voltage and current to supply the vehicle when running. .They convert ac to dc through the diode pack. . However…back in the golden days, cars were equipped with actual generators that produced only dc current…The problem with these were, at idle produced very little current compared to an alternator. I remember my dad had a 53 chevy with a generator and we got stuck in a traffic accident jam for over 30 minutes ( I was a tyke then ) until the road could be cleared…I remember him turning off the headlights and just running the parking lights as at idle it was discharging from the battery on the ammeter gauge with the true DC generator as it was not spinning fast enough to supply the demand at idle. Most of these generators had a maximum output of about 30 amps or so when the engine was running at 2000 rpm, and weighed about 25 lbs or more.

If you took one apart was all heavy gauge copper wire and actually had to be polarized .Looked like a starter motor. Back in those days we did not have all the accessories that we have today on vehicles.