Introduction #

Reliable UPS runtime depends on correct battery sizing and disciplined maintenance. This guide shows how to calculate required battery capacity, derate for real-world conditions, and run a preventative maintenance program.

Key Factors That Drive Runtime #

  • Load power (kW) and power factor – determines kVA demand
  • Battery type – VRLA, lithium-ion, NiCd have different energy density and life
  • Temperature – every 10°C rise roughly halves VRLA life
  • Age and cycle history – capacity fades over time
  • UPS efficiency – online vs line-interactive vs standby

Sizing Batteries for Target Runtime #

Quick Formula #

Battery Wh = (Load W × Runtime hours) ÷ Efficiency

Example: 6 kW load, 30 minutes, 92% efficiency

Battery Wh = 6000 × 0.5 ÷ 0.92 ≈ 3260 Wh

Series/Parallel Planning #

  • Choose DC bus voltage per UPS design (e.g., 48V, 192V).
  • Determine strings in series to meet voltage, then parallel strings to meet Ah.
  • Keep string count balanced; avoid mixing old and new strings.

Age and Temperature Derating #

  • Age derating (VRLA):
    Year 0–1: ~100% - Year 2: ~85–90% - Year 3: ~70% - Year 4+: plan replacement
  • **Temperature:** Use 20–25°C as reference; add margin if room runs warmer.
## Chemistry Choices
  • VRLA (AGM/GEL): Lower cost, 3–5 year life, heavier, temp-sensitive
  • Lithium-ion: Higher cost, lighter, 2–3× life, better high-temp tolerance
  • NiCd: Rugged, good for harsh temps, but expensive and regulated

Maintenance Playbook (VRLA Focus) #

  • Monthly: Visual inspection, check ambient temperature, alarm review
  • Quarterly: String voltage checks, IR scan of terminations
  • Annually: Load test or conductance test; clean terminals; verify torque
  • Replacement: Plan at 3–5 years; avoid mixing ages in parallel strings

Runtime Testing Procedure #

  1. Charge batteries fully; note float voltage.
  2. Disconnect utility (or use test mode).
  3. Apply representative load; record runtime until threshold.
  4. Reconnect utility; confirm recharge current within spec.
  5. Log results and compare to previous tests.

Common Pitfalls #

  • Using nameplate capacity without derating for age/temperature.
  • Mixing old and new batteries in the same string.
  • Ignoring power factor—runtime is based on kVA demand.
  • Poor airflow; heat kills VRLA quickly.

Integration With Calculators #

Replacement Strategy and Budgeting #

  • Track in-service dates per string; stagger replacements to spread cost.
  • For critical sites, keep at least one spare string or rapid procurement plan.
  • Consider lithium retrofit where high temp or space constraints exist.

Conclusion #

Accurate battery sizing plus disciplined maintenance prevents surprise runtime failures. Derate for age and heat, test quarterly, and plan replacements before capacity falls below requirements.