UPS Battery Guide
How Long Will Your UPS Battery Last? A Practical Guide to Lead-Acid UPS Runtime
A practical guide for server rooms and service companies on lead-acid UPS runtime: what to collect before estimating runtime, and how to check the answer.
When a customer asks how long a UPS battery will last, the number on the nameplate cannot answer it. Runtime is not printed on the UPS. It is the result of the real load, the UPS, the charger and the battery string working together.
Why kVA, Voltage and Amp-Hours Cannot Promise a Runtime
A UPS rating in kVA, a battery voltage and an amp-hour figure each describe only one part of the system. None of them tells you how long the battery will support your load.
A server room load is not the same as the UPS nameplate. A UPS at a small share of its rated capacity and the same UPS close to full load give the same battery string a very different runtime. Battery capacity is also not fixed: what a lead-acid battery can deliver changes with the discharge rate, the temperature and the battery age.
What You Need Before Anyone Can Estimate Runtime
The Real Load in kW
Start with the real load the UPS will carry, in kW, not the UPS capacity in kVA. The real load is best measured, or taken from the load the customer confirms. For a closer look at keeping that load healthy, see UPS Maintenance Planning for Server Rooms and IT Rooms.
The UPS Model and Topology
The UPS model tells you its topology and its DC voltage. An offline UPS is normally used for small loads and short runtime. A single phase online UPS covers small server rooms. For larger sites, a three phase high frequency online UPS or a three phase low frequency UPS may be used.
Charger Specification and Voltage
The charger decides whether the battery can be recharged in time after an outage. Check the charger voltage and charging current, and the time available for recharge. A string that is too large for the charger may still give a good runtime on paper, but it will take much longer to recover.
The Battery Cabinet Configuration
Record the battery cabinet configuration: how many batteries are in series, how many strings are in parallel, the capacity of each battery, and the cabinet layout.
The Required Runtime
State the backup time the site actually needs. A runtime requirement is a decision, not a measurement. It comes from what the site must keep running: the time needed to shut down safely, or the time to start a generator.
How to Estimate and Check Runtime, Step by Step
Work through the estimate in this order. First, confirm the real load in kW. Then confirm the UPS model and topology, and the DC voltage window the battery string must meet. Next, list the battery cabinet configuration. Then check that the end-of-discharge voltage stays inside the UPS window, and that the charger can recharge the string within the available time.
When the estimate is done, check it against the site. Does the string voltage match the UPS DC window? Is the assumed battery temperature the temperature the room actually holds? If any answer is no, revise the estimate before it is promised to the customer. For the broader planning steps around capacity and backup time, see UPS Backup Power Planning for Small Data Centers.
Common Mistakes That Make Runtime Figures Unreliable
The most common mistake is to ignore power factor. UPS capacity is often given in kVA and load in kW, and the two are not the same. A kW load must be converted through the power factor before it is compared with a kVA rating, or the load will be understated and the runtime overestimated.
The second mistake is to ignore the discharge curve and efficiency. A lead-acid battery delivers less energy at a high discharge rate than at a low one, and the UPS itself has losses. The third is to ignore the voltage window: the UPS stops when the battery voltage falls below the level its inverter needs. The fourth is to look only at the nominal capacity, measured under one test condition, often at a low discharge rate.
When an External Battery Cabinet Makes Sense
An external battery cabinet makes sense when the load needs more runtime than the internal UPS battery can provide and the UPS itself is still suitable. The cabinet adds lead-acid battery capacity alongside the existing UPS, so the site can run longer without replacing the UPS.
What to Send for a Quotation
To get a reliable answer, send the UPS model and topology, the real load in kW, the required backup time, the charger specification and voltage, the battery cabinet configuration, and photos of the installation space.