Battery Backup Sizing for a Basement BDA
A basement BDA for an 800 MHz public-safety system is powered from a 120 VAC circuit and backed by a 12 V lead-acid battery string rated 100 Ah at the 8-hour rate. The BDA draws 18 A at 12 V during normal operation. During a utility outage, the system dropped after about 45 minutes. The owner claims the battery is new and fully charged. The charger is a 10 A float charger, and the battery room is at 25 degrees C. The AHJ adopted a 12-hour standby requirement for emergency responder radio coverage systems and requires a load test at installation. The building has no generator. The BDA manufacturer specifies a minimum input voltage of 11 V and a maximum charge current of 20 A.
1. Calculate the battery capacity required for 12 hours of standby and compare it with the installed battery.
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Required capacity: load 18 A for 12 h equals 216 Ah. Apply a usable depth of discharge of 80%: 216/0.8 = 270 Ah. Apply an aging/design margin of 20%: 270 x 1.2 = 324 Ah. At 25 degrees C, no temperature derate is needed. The installed 100 Ah battery is far undersized. Even at 100% depth it would last about 5.5 h at 18 A; at 80% usable it would last about 4.4 h. The observed 45 minutes suggests additional problems such as voltage sag, a bad battery, or the charger not supporting the load. The 10 A charger is also below the 18 A load, so the battery drains even while on charger. The required charger should be sized for the load plus recharge current, within the manufacturer's 20 A maximum.
2. Diagnose why the system failed after 45 minutes even though the battery is new.
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Check battery open-circuit voltage and perform a load test. Verify charger output and that it is connected and operating. Measure voltage at the BDA terminals during discharge. The 45-minute failure could be a battery that is not fully charged, a charger that cannot carry 18 A, undersized conductors causing voltage drop below the 11 V minimum, or a battery whose 100 Ah rating at the 8-hour rate does not translate to 18 A discharge because of Peukert effect. The 10 A charger cannot both run the load and recharge the battery. The installation therefore does not meet the 12-hour standby requirement even if the battery is new.
3. Specify corrective measures and the acceptance test needed.
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Replace the battery bank with one sized at least 324 Ah, or follow the manufacturer's derating for the actual discharge rate. Install a charger rated for the load plus recharge current and within the 20 A maximum. Use proper overcurrent protection, disconnects, and conductors sized for voltage drop. Add low-voltage alarm and remote monitoring. Perform a 12-hour load test with the actual BDA load, recording battery voltage at the start, midpoint, and end; the system must stay above 11 V for the full duration. Document the capacity calculation, charger rating, and test results for the AHJ. Verify ventilation and battery-room conditions. If the AHJ permits a generator, an automatic transfer switch could supplement the battery, but the adopted requirement here is 12-hour battery standby.