The BT521 is an advanced battery analyzer from the Fluke 500 Series designed for comprehensive testing and maintenance of stationary batteries and battery banks in critical backup applications. It measures internal battery resistance (3 mO to 3000 mO range), DC voltage (6 V to 1000 V), AC voltage (600 V), AC current (400 A with accessory i410), ripple voltage (600 mV to 6000 mV), frequency (500 Hz), and temperature (0°C to 60°C) with integrated infrared temperature measurement via the BTL21 intelligent test probe. The BT521 features wireless communication for remote data download and display via the dedicated Fluke Battery Analyzer mobile app, automatic sequence testing of battery strings with storage of up to 100 profiles (each storing up to 450 batteries), and a 7.4 V 3000 mAh lithium-ion battery providing over eight hours of continuous operation. Its ergonomic design includes rugged coaxial two-pole Kelvin test pins with remote SAVE button, threshold comparison capabilities for resistance and voltage, and comprehensive data logging with visual and audio feedback, making it ideal for maintenance, troubleshooting, and performance testing of individual stationary batteries and battery banks.