Battery 12.6 V resting, 13.8-14.6 V running with accessories off. Below 13.5 = alternator weak; above 15 = regulator failed.
Charging system test with a digital multimeter: engine off, fully charged battery should read 12.6 V at rest (12.4 V = 75% charged, 12.0 V = nearly dead). Start engine, with no accessories on, voltage at the battery should be 13.8-14.6 V; healthy is right around 14.2 V. Below 13.5 V running = alternator output insufficient (failed diode, brushes, regulator). Above 15.0 V = stuck regulator overcharging (will boil the battery). Turn on headlights, blower high, and rear defroster - voltage should still hold above 13.5 V. If it drops below 13.0 V under load, the alternator is undersized or weak. Stock XJ alternator is 81-117 A depending on year and options. Upgraded 136A alternators (Powermaster, Mechman) are popular for winch and lighting builds. Battery posts should test within 0.5 V of the alternator output; a higher discrepancy points to corroded cables or a bad ground (often the engine-to-firewall ground strap).
Written and maintained by an AZ wheeler and driveway wrencher. Always cross-reference your factory service manual — modifications affect vehicle safety and warranty. Work at your own risk.