Coastal Telecom Tower Backup in Mozambique

Scenario

A 4G/LTE telecom tower near Inhambane, Mozambique powers radio units, baseband processing, and backhaul microwave links. It relies on hybrid solar-diesel operation but requires 72-hour battery autonomy during grid outages and fuel delivery delays. Site constraints include high humidity (>85% RH), salt-laden coastal air, 35°C average ambient temperature, and strict weight limits on tower-mounted equipment (<120 kg for battery assembly).

Given Data

• Daily energy consumption: 6,800 Wh (measured at DC bus—includes 2,100 Wh for active cooling fans)
• Days of autonomy: 3 (72 hours, per SLA with mobile operator)
• Temperature derating factor: 0.85 (manufacturer-provided derating for AGM batteries at 35°C; higher temp reduces usable capacity and accelerates sulfation)
• Depth of discharge: 0.5 (conservative DoD to extend AGM cycle life in hot, high-cycling environment)
• System voltage: 24 V (legacy DC plant architecture; upgrading to 48 V would require full rectifier replacement)
• Inverter efficiency: 0.9 (inverter not used—load is DC-native; this input reflects DC/DC converter losses between battery and radios)

Calculation

Using the same formula:

Battery Capacity (Ah) = (Daily Energy Consumption × Days of Autonomy) / (System Voltage × Depth of Discharge × Inverter Efficiency × Temperature Derating Factor)

Substituting values:

• Numerator = 6,800 Wh × 3 = 20,400 Wh·days
• Denominator = 24 V × 0.5 × 0.9 × 0.85 = 24 × 0.3825 = 9.18 V·dimensionless
• Capacity = 20,400 / 9.18 ≈ 2,222.2 Ah

Accounting for weight limit: 2,222 Ah @ 24 V using flooded lead-acid would exceed 120 kg. Switched to high-density AGM: selected eight 280 Ah, 12 V AGM batteries (4S2P configuration = 24 V, 560 Ah total). Wait—this yields only 560 Ah. Re-run calculation with realistic battery options: required Ah must be ≥2,222 → minimum 8 × 280 Ah = 2,240 Ah (4S2P yields 24 V, 560 Ah — incorrect wiring). Correction: Use 2S8P — two 12 V strings in series (24 V), each string with eight 280 Ah batteries in parallel → 24 V × (8 × 280 Ah) = 24 V × 2,240 Ah. Total mass ≈ 118 kg (per datasheet).

Result and Decision

A 24 V, 2,240 Ah AGM battery bank (16 × 280 Ah, 12 V units in 2S8P) was installed. Ventilation was enhanced with corrosion-resistant exhaust fans, and battery room humidity controlled via desiccant packs. Remote voltage/temperature telemetry confirmed <0.5% DoD variance across units after 6 months.

Lesson

In hot, corrosive environments, conservative DoD and aggressive temperature derating often demand larger physical battery banks than intuition suggests—always cross-check calculated Ah against real-world packaging, weight, and ventilation constraints before finalizing topology.