Power & UPS
Battery Backup Reference
DOD limits by chemistry, temperature derating curves, DC bus voltage standards, battery type comparison, typical runtimes and IEEE 485 / EN 50272-2 rules. Use alongside the Battery Backup Estimator.
Last updated: June 2026
Terms
- IECInternational Electrotechnical Commission
- The international body that writes electrical safety standards used across most of the world outside North America. When a chart cites an IEC number, it's pointing to the official rulebook behind a calculation.
- UPSUninterruptible Power Supply
- A battery backup that keeps equipment running for a short time if mains power cuts out, giving generators time to start or equipment time to shut down safely.
Battery Chemistry Comparison
Common rechargeable battery types used in UPS, data centre and critical infrastructure backup systems.
| Chemistry | Typical DOD | Cycle life | Energy density | Self-discharge / month | Temperature range | Typical DC voltage per unit | Notes |
|---|---|---|---|---|---|---|---|
| VRLA / AGM (lead-acid) | 50 % | 200–500 cycles | 30–50 Wh/kg | 2–3 % | −20 to +50 °C | 2 V / 6 V / 12 V | Most common UPS battery; replace every 3–5 years |
| Gel (lead-acid) | 50–60 % | 300–700 cycles | 30–50 Wh/kg | 1–3 % | −20 to +50 °C | 2 V / 6 V / 12 V | Better deep-cycle tolerance than AGM; no spill risk |
| Li-Ion (NMC) | 80 % | 1 000–2 000 cycles | 100–200 Wh/kg | 1–2 % | 0 to +45 °C | 3.6–3.7 V / module | Emerging in data centre UPS; higher upfront cost |
| LiFePO₄ (lithium iron) | 80–90 % | 2 000–6 000 cycles | 80–160 Wh/kg | < 1 % | −20 to +60 °C | 3.2 V / module | Safest Li chemistry; wide temp range; home and telecom |
| Nickel-Cadmium (NiCd) | 60–80 % | 1 000–2 000 cycles | 40–60 Wh/kg | 10–20 % | −40 to +50 °C | 1.2 V / cell | Extreme temperature tolerance; restricted in EU (REACH) |
DOD vs. Cycle Life — Lead-Acid
Depth of discharge (DOD) has a dramatic effect on total cycle life for lead-acid batteries. Design for the shallowest practical DOD consistent with the required backup time.
| DOD | Approx. cycle life (VRLA/AGM) | Design recommendation |
|---|---|---|
| 20 % | 1 500 – 2 000+ cycles | Ideal for daily cycling; size bank large |
| 30 % | 1 000 – 1 500 cycles | Good for daily cycling with moderate bank size |
| 50 % | 500 – 800 cycles | Standard for standby UPS applications (IEEE 485) |
| 80 % | 200 – 300 cycles | Acceptable only for occasional emergency use |
| 100 % | < 100 cycles | Avoid — severe damage; voids manufacturer warranty |
Temperature Derating — Lead-Acid
Battery capacity decreases significantly at low temperatures. The values below are approximate — always check the manufacturer datasheet for the actual capacity-vs-temperature curve.
| Temperature | Capacity factor (VRLA/AGM) | Capacity at 100 Ah rated | Notes |
|---|---|---|---|
| 40 °C | 1.05 – 1.10 | ~105 Ah | Capacity slightly higher; lifespan severely reduced |
| 25 °C | 1.00 (reference) | 100 Ah | Standard rating temperature (IEEE 485) |
| 20 °C | 0.96 – 0.98 | ~97 Ah | Minimal derating; common indoor UPS room |
| 10 °C | 0.85 – 0.90 | ~87 Ah | 15 % derating — apply in unheated spaces |
| 0 °C | 0.70 – 0.75 | ~72 Ah | 25–30 % derating — outdoor enclosures in winter |
| −10 °C | 0.55 – 0.65 | ~60 Ah | 40 % derating — consider heated enclosure |
| −20 °C | 0.40 – 0.50 | ~45 Ah | Severe derating; AGM may not start at all |
Common DC Bus Voltages
| DC bus voltage | Typical application | Battery string (12 V cells) | Standard reference |
|---|---|---|---|
| 12 V | Consumer UPS, home backup | 1 × 12 V | Manufacturer spec |
| 24 V | Small UPS, solar systems | 2 × 12 V in series | Manufacturer spec |
| 48 V | Telecom, small DC UPS, home energy storage | 4 × 12 V in series | ETSI EN 300 132-2; ITU-T L.12 |
| −48 V | Telecom central offices (negative ground) | 4 × 12 V (negative ground) | ETSI EN 300 132-2 |
| 120 V DC | Utility / industrial UPS, switchgear control | 10 × 12 V in series | IEEE 485; NFPA 70 |
| 240 V DC | Large data centre UPS, high-power systems | 20 × 12 V in series | IEC 60896-2; IEEE 1188 |
| 380 V DC | Data centre DC distribution (emerging) | Module-based (Li-Ion) | IEC 60906-3; ETSI EN 300 132-3 |
Worked Example — Server Room Battery Sizing
Critical load: 5 000 W. Target runtime: 1 hour. DC bus: 48 V. Battery: 12 V / 100 Ah AGM. Inverter efficiency: 92 %. DOD: 50 %. Temperature derating: 90 %. Aging factor: 80 %.
Step 1 — Energy required: 5 000 W × 1 h ÷ 0.92 = 5 435 Wh
Step 2 — Combined derating: 0.50 (DOD) × 0.90 (temp) × 0.80 (aging) = 0.36 effective
Step 3 — Required Ah: 5 435 Wh ÷ (48 V × 0.36) = 315 Ah
Step 4 — Batteries in series: 48 V ÷ 12 V = 4 in series per string
Step 5 — Strings in parallel: 315 Ah ÷ 100 Ah = ⌈3.15⌉ = 4 strings
Step 6 — Total batteries: 4 series × 4 parallel = 16 batteries (12 V / 100 Ah AGM)
Actual bank capacity: 4 × 100 Ah = 400 Ah at 48 V = 19 200 Wh gross
Typical Runtime by Application
| Application | Typical backup requirement | Purpose |
|---|---|---|
| Server room UPS (IT shutdown) | 5 – 15 minutes | Graceful shutdown or generator transfer |
| Data centre UPS (with generator) | 15 – 30 minutes | Generator start and transfer sequence |
| Data centre UPS (no generator) | 2 – 4 hours | Full work-day bridge until power restored |
| Telecom central office | 4 – 8 hours | Regulatory minimum; typically 8 h in EU (ETSI) |
| Critical medical / life-safety | 2 – 24 hours | Regulatory requirement; varies by jurisdiction |
| Home / SOHO backup | 30 min – 4 hours | Bridging typical local outage duration |
| Remote / off-grid site | 24 – 72 hours | Between generator fill cycles or solar recharge |
Frequently Asked Questions
What is the correct C-rate for sizing a UPS battery?
Battery Ah ratings are stated at the C/10 (10-hour) rate. A UPS discharging in 15 minutes operates at C/4, and in 5 minutes at C/12 but in reverse (high current). At high discharge rates, available capacity is lower than the nameplate rating — use the manufacturer's discharge table for the actual runtime at your load. For example, a 100 Ah AGM battery may only deliver 65–70 Ah at C/2 rate. The estimator uses nameplate Ah; add a 20 % margin to compensate.
What does IEEE 485 say about battery sizing?
IEEE 485 (Recommended Practice for Sizing Lead-Acid Batteries for Stationary Applications) specifies sizing to 25 °C with specific gravity correction, a design margin of at least 10 % over calculated Ah, an aging factor of 1.25 (inverse of 80 % capacity at end-of-life), and DOD not to exceed 80 % of nameplate capacity. It is the primary reference for industrial UPS and utility battery rooms in North America.
When do I need a separate charger study?
The charger must restore the battery bank to 95 % charge within the site's specified recharge time (typically 8–12 hours). Charger current = Bank Ah × 1.15 ÷ recharge time (hours). For large banks (> 500 Ah) or fast recharge (< 4 h), the charger current can exceed 50–100 A — cable sizing, protection fusing and heat dissipation all require engineering review.
How do I handle ventilation for lead-acid batteries?
Lead-acid batteries off-gas hydrogen during charging. IEC 60896-2 and IEEE 1187 specify ventilation requirements to keep hydrogen below 1 % (LEL is 4 %). The minimum ventilation rate (m³/h) depends on battery count and charging current. Sealed VRLA/AGM batteries produce far less gas than flooded cells, but still require adequate ventilation in enclosed spaces. Never install in sealed, unventilated cabinets.
Can I add more batteries to extend runtime later?
Only if you add whole strings of identical batteries at the same age and state of health. Adding new batteries to an existing bank causes the new cells to be charged/discharged unequally — they will fail prematurely. Plan your battery room for the full future bank from day one, or plan a full bank replacement cycle.