How to test which battery is bad (internal or external?) on a Smart-Ups XL. You can test the external batteries simply by performing a self-test on the UPS.
Smart-UPS Replace Battery Self Test Procedure. Technical Description: The replace battery indicator is designed as a early warning signal to the Smart-UPS .
APC (American Power Conversion) has been producing AC- and DC-based back-up power products including surge suppressors, uninterruptible power supplies (UPS). Use a voltmeter and ammeter, or a multimeter, to test your APC battery.