Remote BTS Battery Runtime Collapse
A 48 V DC telecom site has a 50 A rectifier, a 100 Ah VRLA battery, and a measured load of 22 A. During a grid failure, the BTS resets after about 25 minutes even though the battery was on float at 54.0 V before the outage. At the moment of reset, the battery terminal voltage at the battery disconnect is 43.8 V, while the voltage at the BTS input is 41.9 V. The battery is two years old, but during a load test one block in the 24-cell string reads 1.5 V while the others read 1.95-2.00 V. The site is remote, and the technician has a multimeter, a clamp meter, and a hydrometer intended only for flooded cells, not VRLA. The operations centre wants the site restored before the next outage and asks whether the battery can be kept in service until the next maintenance window.
1. Estimate the healthy battery's nominal discharge time at 22 A and explain why 25 minutes is evidence of a failed battery, not just a short outage. State your assumptions.
Read the worked answer
Nominal arithmetic: 100 Ah / 22 A = 4.5 h. A telecom VRLA string is normally discharged to about 1.75 V/cell, which is 42 V for 24 cells, and capacity is rated at a slower rate, so even with aging and Peukert derating a two-year-old healthy string should give well over 1.5 h at 22 A. The observed 25 min = 0.42 h, delivering only about 22 x 0.42 = 9.2 Ah, roughly 9% of nominal. That is not a normal end-of-discharge; it matches a string with one failed block limiting capacity. The 1.5 V block under load is the direct evidence. Assumptions: load constant at 22 A, no rectifier contribution during the outage, and end voltage 42 V. Even if the load rises somewhat, the gap is too large to explain by load alone.
2. Which measurements would separate a failed battery block from a high-resistance connection? Give the order of checks and the safety controls.
Read the worked answer
First, with the charger isolated and the load stable, measure each block voltage under load and clamp the string current. A failed block shows a low on-load voltage (1.5 V vs 1.95-2.00 V) while the others are normal; open-circuit voltage may look normal, so load testing is essential. Next measure the voltage drop from the battery terminals to the BTS input under load: 43.8 - 41.9 = 1.9 V at 22 A, about 86 mOhm, which is far too high for a healthy connection. That points to a loose terminal, fuse, breaker, or cable, but it can coexist with the bad block. Inspect for heat, discoloration, and torque. Order: visual and thermal check, block voltages, terminal-drop measurement, then isolation and repair. Safety: treat 48 V DC as an arc-flash and short-circuit hazard; remove jewellery, use insulated tools and PPE, isolate and lock off the rectifier and battery, and never use the flooded-cell hydrometer on VRLA. Replace the failed block or string with matched cells and retest before restoring load.