The battery is the part of a UPS most likely to let you down. Runtime depends on the load, the battery type and size, its age and the temperature of the room, and it falls steadily as batteries age. Sizing with realistic margins, keeping batteries cool and testing them on a schedule are what make the runtime on the label the runtime you actually get.
What runtime depends on
Runtime is how long the UPS can carry the load on battery alone. It is set by how much energy the batteries can deliver before their voltage drops to the point where the UPS shuts down, and by how fast the load draws that energy. Several things change it:
- Load. The heavier the load, the shorter the runtime, and not in a straight line. Batteries deliver less of their total energy when discharged quickly, so halving the load usually more than doubles the runtime.
- Battery type and size. The number of strings and the capacity of each block set the total energy available.
- Age. Capacity declines over the battery's life. Designers usually size for the end of life, not the day of installation.
- Temperature. Cold batteries deliver less capacity; hot batteries deliver normally for a while but age much faster.
- Cut-off voltage. The UPS stops discharging at a set voltage to protect the batteries. That setting is part of the runtime calculation.
Battery types
Valve-regulated lead-acid (VRLA)
Sealed VRLA batteries are the most common type in UPS systems. They need no watering, can sit in cabinets next to the UPS and have a long track record. Their service life is shorter than many people expect and is very sensitive to heat. They need regular inspection and testing because they can fail with little outward sign.
Vented (flooded) lead-acid
Flooded batteries are used in some large UPS installations and utility and telecom sites. They can last longer than VRLA and are easier to assess, but they need a dedicated room with ventilation for the hydrogen they release during charging, spill containment, regular watering and safety equipment for handling electrolyte.
Lithium-ion
Lithium-ion UPS batteries are smaller and lighter for the same energy, tolerate higher temperatures better and usually last longer than VRLA. They come with a battery management system that monitors each cell. They cost more up front and bring different fire safety considerations, and building and fire codes and local authorities may set conditions on where and how much can be installed. Our guide to battery energy storage covers some of the same issues at larger scale.
| Type | Advantages | Watch for |
|---|---|---|
| VRLA | Low cost, sealed, widely available | Heat-sensitive, shorter life, failures hard to see |
| Vented lead-acid | Long life, condition easy to assess | Dedicated room, ventilation, spill control, maintenance |
| Lithium-ion | Compact, light, longer life, cell monitoring | Higher cost, fire code requirements, manufacturer support |
How batteries are arranged
A UPS battery is a string of blocks connected in series to reach the DC voltage the UPS needs. Larger systems use two or more strings in parallel for more runtime and some resilience. The series arrangement has a consequence that surprises many owners: the whole string can only perform as well as its weakest block.
This is why testing individual blocks matters. A string can look fine on the UPS display until a real outage reveals one block that cannot deliver current, and the UPS shuts down early.
Sizing runtime properly
A sound runtime calculation starts from the real load in watts, not the UPS rating, and includes:
- The required runtime, set by generator start and transfer time plus a margin, or by the time needed for an orderly shutdown.
- Expected load growth over the battery's life.
- An aging factor, so the battery still meets the requirement near the end of its service life.
- A temperature factor if the battery will sit in a space colder than its rated temperature.
- The UPS's efficiency and cut-off voltage, taken from the manufacturer's data.
Manufacturers publish discharge tables for their batteries that are used for this calculation. A design that simply takes the runtime figure from a sales brochure at a single load point often falls short in service.
Keeping batteries healthy
- Keep the battery space at or near the temperature the manufacturer rates the battery for. Heat is the single biggest cause of short battery life.
- Inspect regularly for swelling, leaks, corrosion, loose connections and signs of overheating.
- Measure block voltages and internal resistance or conductance on the schedule the manufacturer recommends, and trend the results so a weakening block stands out.
- Check connection torque during maintenance, following the manufacturer's values.
- Replace whole strings at the end of their service life rather than waiting for a failure during an outage.
- Keep records of installation date, test results and replacements for every string.
Battery work is hazardous. UPS batteries store a lot of energy at DC voltages that can cause serious arc flash and shock injuries, and they cannot be switched off like a circuit. Battery testing and replacement should only be done by trained technicians using the right protective equipment and procedures. See arc flash basics.
Battery rooms and cabinets
Small VRLA strings usually sit in cabinets beside the UPS. Larger installations may need a dedicated battery room. Depending on the type and amount of battery, requirements can include ventilation, spill containment, seismic racking, signage, eyewash facilities, working clearances and fire separation. The Canadian Electrical Code, the building and fire codes in force and the manufacturer's instructions all apply, and the authority having jurisdiction may set additional conditions, particularly for lithium-ion.
How ehZee can help
We calculate runtime from real loads, select battery types and arrangements, and specify the space, ventilation and monitoring they need as part of our UPS system design service. For how batteries fit into the different UPS designs, see UPS topologies.
Common questions
How long do UPS batteries last?
It depends on type, temperature and how often they discharge. Manufacturers give a design life under ideal conditions, but real service life is often shorter, especially for VRLA in warm rooms. Testing and trending tell you where your batteries actually are.
Why did our UPS shut down sooner than its rated runtime?
Common causes are aged batteries, a single weak block, a load larger than assumed, a cold battery space or a rated runtime quoted at a lighter load than yours. A battery test and a load measurement normally identify which.
Can we replace only the failed blocks?
On a young string, replacing a single failed block can be reasonable. On an older string, a new block among aged ones tends to be stressed unevenly, and the next failure usually follows soon. The manufacturer's guidance and the test history help decide.
Can we switch an existing UPS to lithium-ion batteries?
Only if the UPS manufacturer supports it for that model. The charger settings, battery management communication and cabinet design all have to match the battery. The change may also bring new fire code conditions for the room, so check with the manufacturer and the authority having jurisdiction before ordering.
Does running the UPS on battery during tests wear the batteries out?
Each discharge uses a little of the battery's life, but periodic tests are worth it because they reveal weak strings before a real outage does. Most maintenance programs combine frequent non-invasive measurements with occasional discharge tests.