4.2 UPS Topologies & Batteries
Key Takeaways
- Double-conversion (online, VFI) UPS provides zero transfer time and full isolation, with normal-mode efficiency of about 94-97%.
- Eco-mode raises UPS efficiency to roughly 99% by routing the load through static bypass during good utility conditions, trading some isolation and transfer time.
- UPS batteries are typically sized for 5-15 minutes of autonomy at full load - enough for the standby generator to start and accept load.
- Lithium-ion offers about 60-70% smaller footprint and a 10-15 year life versus VRLA's 3-5 years, at higher up-front cost.
- A flywheel or rotary UPS stores energy kinetically, giving roughly 10-20 seconds of ride-through to bridge generator start.
Why the UPS Exists
The Uninterruptible Power Supply (UPS) is the heart of data centre power protection. Its job is twofold: first, to bridge the gap between a utility failure and the standby generator accepting load (typically 5-15 minutes of stored energy); second, to condition the power so the IT load never sees a sag, swell, surge, or harmonic. The CDCP exam tests three static (rectifier/inverter) topologies plus rotary/flywheel alternatives, and the battery chemistries that store the energy.
The three static UPS topologies
Learn these cold - they appear on essentially every CDCP exam.
| Topology | Normal-mode path | Protection | Typical use |
|---|---|---|---|
| Offline / standby | Load on raw utility; inverter idle | Lowest - 4-10 ms break on transfer | Office and desktop, non-critical |
| Line-interactive | Utility through an autotransformer (AVR) | Medium - trims sags/swells, no battery use | Small server rooms, network closets |
| Double-conversion (online) | Utility to rectifier to DC to inverter to load, always | Highest - full isolation, zero transfer time | Data centre white space (Tier III/IV) |
Offline (standby) UPS: the load runs on raw utility power and the inverter only switches on after an outage is detected, creating a short 4-10 ms break. IT power supplies can usually ride through this, but the load is exposed to poor power quality the rest of the time, so it is unsuitable for white space.
Line-interactive UPS: adds an autotransformer / buck-boost (automatic voltage regulation, AVR) so it can correct minor sags and swells without draining the battery. Better than offline, but the load still sees utility power quality during normal operation - it is mid-tier protection.
Double-conversion (online) UPS: continuously converts incoming AC to DC and back to AC. The load is completely isolated from the utility, transfer time is zero, and the output is the cleanest available. This is why it is standard for Tier III/IV. In IEC 62040-3 terms it is classified VFI (Voltage and Frequency Independent). Normal-mode efficiency is typically 94-97%.
Eco-mode
Because double-conversion continuously runs power through the rectifier and inverter, it incurs conversion losses. Eco-mode (economy / bypass mode) routes the load through the static bypass line during good utility conditions - pushing efficiency to about 99% - and snaps back to double-conversion when a disturbance appears. The trade-off is a small transfer time and reduced isolation, so operators reserve classic eco-mode for less critical loads. Newer multi-mode / advanced-eco designs shrink the transfer to near-zero while keeping most of the efficiency gain.
Static versus rotary/flywheel
Most UPS are static (solid-state rectifier, inverter, and batteries). An alternative stores energy kinetically:
- A rotary UPS uses a motor-generator with a heavy flywheel; a Diesel Rotary UPS (DRUPS) couples the flywheel to a diesel engine and clutch so the flywheel bridges the seconds needed to start the diesel, eliminating batteries entirely.
- A flywheel typically provides only 10-20 seconds of ride-through, which is plenty to start a generator but not for long outages. Flywheels are prized for very high cycle life, small footprint, tolerance of heat, and no chemical disposal, and are often paired with batteries to absorb the first few seconds of every discharge.
Batteries: VRLA versus lithium-ion
The exam contrasts the two dominant chemistries:
| Attribute | VRLA (lead-acid) | Lithium-ion (Li-ion) |
|---|---|---|
| Footprint / weight | Large, heavy | About 60-70% smaller and lighter |
| Cycle life | About 200-500 cycles | Thousands of cycles |
| Design life | 3-5 years (up to 10) | 10-15 years |
| Temperature tolerance | Sensitive; life roughly halves per +8-10 C | More tolerant |
| Up-front cost | Lower | Higher (lower total cost of ownership) |
| Safety | Off-gassing, spills | BMS-managed; thermal-runaway concern |
VRLA (Valve-Regulated Lead-Acid) is the traditional, sealed, maintenance-light choice. Lithium-ion (usually LFP - lithium iron phosphate - for safety) is displacing it thanks to a smaller footprint, longer life, and an integrated Battery Management System (BMS). Both are typically sized for 5-15 minutes of autonomy at full load, long enough for the generator to start and accept load.
Runtime, autonomy, and monitoring
Autonomy (runtime) is how long the battery can carry the load; it scales with the number and size of cells and inversely with load. Because battery cost and footprint grow roughly linearly with runtime, designers rarely exceed 15 minutes when a generator is present. As a worked intuition, a battery string sized for 10 minutes at 100% load will support roughly 25-30 minutes at 40% load, because a lightly loaded string discharges more slowly - a useful check when a hall is only partly populated. Battery monitoring systems track cell voltage, internal resistance/impedance, and temperature to predict a failing string before it fails during an outage - a weak cell is a hidden single point of failure. On the exam, remember that batteries are the most failure-prone part of a UPS, which is why monitoring, periodic load-bank testing, and scheduled replacement (executed under a formal MOP) are emphasised.
Modular and parallel UPS
Modern white-space UPS are often modular: several hot-swappable power modules sit in one frame, so capacity can be added in steps and a failed module replaced without dropping the load. Multiple UPS units can also be paralleled to reach N+1 redundancy at the system level (see Section 4.3) - for example four 500 kW units where only three are needed. The exam expects you to connect topology (how a single UPS conditions power) with redundancy (how many UPS are provided): a double-conversion UPS delivers clean power, but only an N+1 or 2N arrangement of those UPS delivers availability. A single online UPS, however clean its output, is still a single point of failure.
Which UPS topology continuously converts AC to DC and back to AC so the load is fully isolated from the utility with zero transfer time?
A facility engineer wants to raise UPS efficiency toward 99% during periods of clean utility power, accepting a small transfer time and reduced isolation. Which feature does this describe?
Compared with VRLA batteries, what is a primary advantage of lithium-ion batteries in a modern UPS?