Replacing UPS Batteries with Lithium: Five Minutes or an Hour Changes Everything

A UPS battery is bought to do one of two quite different jobs, and the specification that suits one is wrong for the other. Either it carries the load for a few minutes while a generator starts, or it carries the load for half an hour or more because there is no generator, or because the generator sometimes does not start. Deciding which of those you are buying is the first question, and it changes almost everything after it.

Why a few minutes is the harder job

It sounds backwards, but the short-autonomy battery is under more strain. Five minutes of backup at full load means delivering the pack's entire energy in a twelfth of an hour — a very high discharge rate. Thirty minutes at the same load draws the same energy far more gently.

That rate is what the C-rate describes, and it is why our data-centre range is two series rather than one:

• The 5 to 15 minute series is built for the generator-handover job: 240V and 480V systems at 12 and 24 kWh, rated 50 A standard discharge (1C) and up to 300 A (6C).
• The 30 to 60 minute series is built to carry the load: 240V, 384V and 480V systems at 24, 38.4 and 48 kWh, rated 50 A standard discharge (0.5C) and up to 200 A (2C).

A system sized only on kilowatt hours can be perfectly adequate on paper and still be the wrong buy, because it cannot deliver them fast enough. Ask for the rated and peak discharge current, and check both against the load you actually have to hold.

The DC bus voltage has to match the UPS

This is the constraint people meet last and should meet first. A UPS runs a fixed DC bus, and the battery must be built for it — ours come at 240, 384 and 480 Vdc nominal, made up of 48V modules in series (1P15S per module, then 75, 125 or 150 cells in series across the pack). Get the bus voltage and the charging voltage window from the UPS manual before anything else is discussed. A battery that cannot sit on your bus is not a cheaper option, it is not an option.

What lithium actually changes in a data hall

Four things, in the order the facilities team will notice them:

• Floor space and weight. Ours are single cabinets: 600 × 800 mm on the floor, 1,400 or 2,000 mm tall, 290 to 590 kg depending on the model. Check the floor loading in a raised-floor room before the delivery date, not on it.
• Temperature. Rated 15 to 45°C, recommended 20 to 25°C — which a data hall already provides, so the pack lives in near-ideal conditions and the rated cycle life is realistic rather than theoretical.
• Cycle life. 3,000 cycles at 80% depth of discharge, 0.5C in and out. For a UPS that discharges a handful of times a year, cycle count is rarely the limit — but it is the number that lets the same bank be used for peak shaving later without a separate argument.
• What you can see. CAN, RS485 and dry contacts. A dry contact tells your building management system that something is wrong; the data link tells you which module, at what temperature, at what state of health. A VRLA string tells you almost nothing until it fails a load test.

Certification, for a battery inside your building

This is the section to read twice, because the battery is going into a room full of things you cannot afford to lose. Ours carry UL1642, UN38.3, IEC62619 and IEC62133. Ask for the certificates covering the exact model you are buying rather than a company-level statement, and ask what fire detection and suppression the room already has and whether the supplier's installation assumes more. What each standard actually covers takes ten minutes to read and saves an argument later.

Changing over without going off supply

The question that decides whether a replacement happens this year or next is what happens to the load during it. It can be done inside a maintenance window: at RailTel's command control centre in Pondicherry we put 180 kWh behind a 100 kW UPS, and the changeover was completed in a two-hour window with the centre never off supply. What that takes is planning the bypass, the DC isolation and the sequence in advance, and having someone on site who has done it before.

At that capacity the bank covers far more than a generator handover needs, which is the other reason to think about autonomy deliberately: a generator that fails to start becomes an inconvenience rather than an outage.

The short version

Decide the minutes first. Get the DC bus voltage from the UPS manual. Ask for discharge current as well as kilowatt hours. Check the floor loading and the room temperature. Ask what the battery reports and to what. Then compare suppliers on those answers rather than on the headline capacity.

The full range, with the specification table for each model, is under data centre batteries. For commercial and industrial storage more broadly — containerised systems, generator replacement, peak shaving — our energy storage site covers UPS replacement in more depth.

Questions

Can lithium batteries replace VRLA in an existing UPS?

Usually yes, provided the battery is built for the UPS's DC bus voltage and the UPS can charge a lithium profile. Take the bus voltage and the charging voltage window from the UPS manual first, then match the battery to it. Where the UPS cannot be set for lithium, that is a conversation about the UPS, not the battery.

How long should a data centre UPS battery last on backup?

It depends on what comes after it. Where a generator starts automatically, five to fifteen minutes is the usual design and the battery must be rated for a high discharge rate to deliver it. Where there is no generator, or where one has failed to start before, thirty to sixty minutes is the normal specification and the battery is sized for energy instead.

What is C-rate and why does it matter for a UPS?

C-rate is how fast a battery is charged or discharged relative to its capacity: 1C empties it in an hour, 2C in half an hour, 6C in ten minutes. A short-autonomy UPS battery must be rated for a high discharge rate, which is why our 5 to 15 minute series is rated to 300 A (6C) while the 30 to 60 minute series is rated to 200 A (2C). Two systems with the same kilowatt hours are not interchangeable.

How much floor space does a lithium UPS battery need?

Our data-centre cabinets are 600 × 800 mm on the floor and 1,400 or 2,000 mm tall, weighing 290 to 590 kg depending on the model. In a raised-floor room the weight is the constraint worth checking first, since it concentrates on a small footprint rather than spreading along a battery rack.