What kind of battery was it? Just a standard automotive FLA battery? Or something else (deep cycle, AGM, etc.)?

In general, when FLA batteries fail, the plates become sulfated and/or corroded. This tends to "increase" the battery's internal resistance, and reduce the surge current for starting.

I have seen "old/failing" car batteries fail after an hour of running a radio, kids playing video games, etc. The sulfates make less "active material" available for producing current. Effectively reducing the battery AH capacity. If the remaining active plate area was enough to provide starter current, and otherwise very little AH capacity, a "smaller load" running for minutes (or a couple hours) could effectively drain the battery.

Of course, there are 6 cells--So you could have 4-5 "good cells" and 1 or 2 bad/weak cells that degrade overall battery performance.

Looking at the "math", starting is not that much AH capacity vs other loads. Making up numbers:

• 80 amps starting current * 5 seconds cranking * 1/3,600 seconds per hour = 0.11 AH of battery "usage"
• 5 amp radio current * 10 minutes / 60 minutes per hour = 0.83 AH of battery usage

Give that a "typical" car battery may be 80 AH, neither of the above loads use significant battery capacity (of a good battery).

However, you can see that 10 minutes of a radio can use almost 8x more AH capacity than a typical cranking load does... So the radio could "kill" the battery capacity faster than an a typical starting cycle.

Depending on lots of over variables, Lead Acid batteries can have a "surface" charge where their "resting voltage" is relatively high--And after a few hours or days, the resting voltage could fall. So if you were running the Jeep every day--The battery could be "surface charged" enough to start the vehicle, but other loads (radio, lights, etc.) could take the battery "flat" enough to prevent starting/running the electronics.

-Bill "my guesses" B.

Another experience I had with car batteries... If the batteries are over 1+ years old, and the battery has been taken dead (lights left on, cranking the non-starting engine until dead, etc.)--I have found that once the battery is recharged and any loads have been "fixed" (starting problem fixed, etc.)--The battery may work fine for a few days to a couple of weeks, and then die. This even happens when a car has been driven and parked for a few hours (driven/properly charging, etc.).

These days, if I want a reliable car--Any older batteries that have been taken dead--I just replace them.

Other quick test... Every once in a while I start the car with the headlights on (typically evening or night) and notice how much they dim. When the battery gets weak (near replacement time), I note the headlights dim quite a bit on the older/soon to be failing battery. Another indication that a new battery is due.

-Bill