Typhoon Yangliu crossed Guangzhou on 24 August and knocked out power across much of Panyu district. A dental clinic called us that evening. The outage lasted 12 minutes, and when the power returned the clinic's HIS system, which holds registration, medical records and billing, would not open. The server was a Lenovo ThinkSystem ST250, and SQL Server was reporting a database consistency error.
The clinic manager could not understand it. "I bought a UPS five years ago, over RMB 10,000. The indicator light has been green the whole time. How can there have been a power failure?"
We were on site on the morning of 25 August. The database came first, then the UPS, because the UPS is the root cause of this accident and a hazard plenty of companies share.
A green light lies: it only means the mains is fine
The unit on site was a Santak C3KS, a 3 kVA online model, with its panel light steadily green. Most users misread that light:
| Indicator | What it actually means | Common misreading |
|---|---|---|
| Mains light steady green | Input power is normal and the UPS is charging the battery. The charging circuit works, which says nothing about whether the battery holds a charge | "The UPS is fine" |
| Battery or fault light on | Battery voltage is abnormal or protection has tripped | Most units only light it after total failure |
| Runtime shown on the panel | On most models an estimate calculated from the rated capacity of a new battery; it does not reflect degradation | "The panel says 30 minutes, so it will last 30" |
Put plainly: a UPS with a completely dead battery still shows a green light every day, as long as it is plugged into the mains. The only reliable checks are a real discharge test with the mains disconnected, or an internal resistance meter on the cells.
Measured: resistance 3 to 7 times over spec, runtime under 3 minutes
We measured the pack cell by cell with an internal resistance tester. The C3KS carries six 12V 9Ah sealed lead-acid batteries:
| Battery | Float voltage | Internal resistance (mΩ) | Verdict |
|---|---|---|---|
| #1 | 13.5V | 48 | Over spec, against about 15 mΩ new |
| #2 | 13.4V | 62 | Badly over |
| #3 | 12.9V | 105 | Scrap |
| #4 | 13.3V | 71 | Badly over |
| #5 | 13.5V | 55 | Over spec |
| #6 | 13.1V | 88 | Scrap |
Lead-acid cells should be replaced once internal resistance reaches 1.5 to 2 times the factory value. This pack averaged 3 to 7 times. We also ran a real discharge: mains disconnected, carrying the server and switch at a measured 780W, the UPS alarmed and shut down after 2 minutes 50 seconds. A 3 kVA unit with a new battery should hold 780W for 20 to 25 minutes.
The pack was five years old, in a Guangzhou server room that reaches 32-35°C in summer, and heat shortens lead-acid life. The industry rule is forced replacement at three to five years. This one was overdue and had never been tested.
The repair: database first, then batteries
SQL Server was reporting page 823/824 errors, a transaction page corrupted while it was being written as the power dropped. DBCC CHECKDB located three corrupt pages. We rolled forward from the last full backup, taken the previous Sunday, plus the transaction log, and recovered to 15 minutes before the incident. The 15 minutes of registration records from that night were lost and re-entered by hand. Six hours of work, RMB 2,800.
Along the way we found that the clinic's automatic backup job had been failing for three months because the backup target disk was full and nobody was watching. Had the damage been worse, the restore point would have been three months back. Backup jobs need failure alerts, or "we have backups" is only psychological comfort.
For the batteries, six 12V 9Ah maintenance-free units from Santak, including replacement, discharge testing and recycling of the old cells, quoted at RMB 1,180. We also recommended an SNMP card at RMB 680 with email alerts for mains failure, battery faults and overload. That card is what turns a UPS from dumb equipment into something that tells you it is in trouble.
Sizing a UPS: not too small, not too big
A simple method for an SME:
- Add up the load you need to protect. Server nameplate power times the number of servers, at roughly 60% of nameplate in real draw, plus switches or routers and any essential display. Example: two servers rated 550W, about 330W each in practice, plus an 80W switch, gives 740W.
- Size the UPS. Load divided by power factor, 0.8 to 0.9 for an online unit, divided again by 0.8 to leave 20% headroom. 740W ÷ 0.85 ÷ 0.8 ≈ 1,090VA, so a 2 kVA model.
- Work out the runtime. The battery pack decides it. A 2 kVA standard unit with internal batteries holds 740W for 12 to 18 minutes; an hour requires an external battery cabinet.
- Decide how long you actually need. For a safe shutdown, 15 minutes is enough. To wait for a generator or for mains to return, you need one to four hours and an external cabinet. For zero business interruption, you need dual mains, a generator and a long-runtime UPS together.
Two common mistakes. Buying a 3 kVA long-runtime model and connecting only its small internal pack, when long-runtime units often ship with five minutes of internal battery. And treating the UPS as a voltage regulator for air conditioners or laser printers, whose inrush current will collapse it instantly.
Online or standby?
| Type | How it works | Transfer time | Where it fits | Price, 1 kVA |
|---|---|---|---|---|
| Standby (offline) | Mains passes through; the battery takes over on failure | 4-10 ms | Home PC, a single POS terminal | RMB 300-500 |
| Line-interactive | Adds voltage regulation; the battery takes over on failure | 2-6 ms | Small office, NAS | RMB 600-1,000 |
| Online (double conversion) | Mains to DC to inverter, no transfer at all | 0 ms | Servers, server rooms, medical equipment | RMB 1,500-3,000 |
Servers and NAS always get online units. Zero transfer time means the load never notices the outage, and an online unit brings full filtering and regulation, which counts in a city where summer voltage swings are severe..
The quarterly self-check, ten minutes
- Read the panel. Input and output voltage, load percentage, battery voltage inside its range, and the alarm history.
- Listen and smell. A noisy fan or a bulging capacitor that leaks a sour smell needs attention. Any odd smell from the battery compartment means you should cut the power and inspect.
- Run a real discharge. Disconnect the mains and watch how long it holds under the real load. Below 60% of the rated runtime, replace the battery. This is the only test the green light cannot fake.
- Log the room temperature. Lead-acid life halves for every 10°C above 25°C. Three months with a broken air conditioner may already have halved it.
- Check the battery date. Past four years, it belongs in the replacement budget. Do not wait for a failure.
ACCPC covers the full UPS service: battery testing with an internal resistance report at no charge, battery replacement including environmental recycling, UPS repair and rental, and design and installation of power protection. We also wholesale servers in Guangzhou and can introduce you to licensed IDC providers whose facilities run dual mains, online UPS, diesel generators and 24/7 staffing, with deployment and operations handled by us. Call 020-39029800.