Over Charging

A couple of years ago, I fried a battery.... literally. On a long trip, it got hot, swelled up and spewed some acid. The replacement is swollen and getting hot now. I think the alt may be overcharging. Any experience with a problem like this???

What year? As far back as at least a 96, they have a battery temperature sensor on the battery tray that contacts the bottom of the battery and sends a input to the engine computer that controls the charge rate. It is spring loaded to contact the bottom of the battery. Maybe it's lost it's "spring".

JoBo

Normal charging voltage is between 14 and 14.5 volts. Much more voltage will fry the battery. It's simple to check it with a volt meter.

Actually, the charging voltage is usually between 13-14.5 volts. The temp sensor near the battery is used by the ECM to change the charging voltage according to the temp. The lower the temp the higher the voltage and vice-a-versa.

I don't know on the Jeeps, but other vehicles the battery temp sensor is just in the near vicinity of the battery, it isn't actually touching it. It might be different in different vehicles.

Your battery/electric warning light should come on if your charging system is out whack. The light is suppossed to come on when voltage is too high or low, or the field control is out of whack for the alternator. Thats on the later models.

What year and configuration for your XJ?

It's a 93 Country package... electric windows, locks, etc. When it blew the first time, I attributed it to the battery being possibly old. It was on there when I got it. I had to get the replacement in a Wal Mart in the middle of Kansas, but it was one of their better (more expensive) ones.

Now that this battery is swelling and getting hot, I realize it wasn't the battery. I haven't tested the voltage yet, but my gauge always read high. i've never had any kind of warning light though.

Do these alternators have internal voltage regulators like most other vehicles? Could that be the culprit?

I'm not positive, but most Chrysler, and other manufacturers, voltage regulation is done by the PCM (engine controller) and it uses the battery temp sensor to vary the voltage for a better and/or safer charging voltage.

Theres a wire that runs from the PCM to the Alternator and controls the fields. The actual rectifing is done internal to the alternator, technically making it a GENERATOR, since only DC current comes out of it.

At warmer temps the charge voltage is lower towards 13 volts, in colder temps the charge voltage is raised all the way up to 14.5 volts in freezing temps.

The "early" XJ's (to 1990) used a Delco internally-regulated alternator - Delco SI 1984-1986, and Delco CS 1987-1990.

The "later" XJ's (ChryCo production, 1991-2001) use a Nippondenso externally-regulated alternator, with the regulator provided as a circuit in the PCM. The regulators rarely (but not never!) go bad in those. Replacing the regulator entails replacing the entire PCM (which is why I'm not fond of the idea) or using an early external regulator (and ending up with a persistent alternator code, due to OBD,) or converting to a "self-exciting" Delco unit (also ending up with a persistent alternator code.)

Second that you'll want to check for alternator output voltage at the battery terminals before you suspect anything - ideal charge voltage it 13-14.5VDC, with up to 15.2 being acceptable. However, battery life is shortened if the charge voltage is much over 14.25VDC.

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