Telecom backup has a clarity other battery applications envy: the load is known, the bus voltage is standardised at 48V, and the requirement is written in an SLA as hours of autonomy. What remains is engineering — choosing the format, sizing the string, and respecting the two slow enemies, heat and age. This guide works the method against the three formats a 48V plant chooses between: front-terminal 12V strings, OPzV 2V plants, and 48V rack-mounted storage modules of 5.12 kWh — the OPzV and rack formats are in XD’s documented telecom range; the front-terminal format is covered as a selection guide.
Step 1: load and autonomy, honestly
Measure the site load in amps at the bus — radios, transmission, cooling controllers, the lot — at its real duty, not its nameplate sum. Multiply by the autonomy hours your SLA demands at that site class (a metro macro site and a rural relay do not carry the same hours, and pretending otherwise buys batteries for the wrong sites). The product is the amp-hours the string must deliver at the discharge rate the autonomy implies — and at short autonomies the rate penalty is severe, so size from discharge behaviour, not from the C20 label. Add the ageing margin your replacement policy assumes: a string is sized to still meet autonomy at end of life, not on commissioning day.
Step 2: the format decision
- Cabinet sites, aisle service, standard autonomy: front-terminal 12V strings — four blocks per 48V string, every terminal reachable from the front. See the front-terminal page.
- Plant rooms, decade briefs, large autonomy: OPzV 2V strings — 24 cells, per-cell monitoring, no parallel strings.
- Modernised sites, lithium-standard operators: 48V-class rack modules — supervised, compact, modular; the telecom page’s rack route.
- Flooded OPzS survives in staffed legacy plants; new unmanned builds rarely choose it, for the staffing reason the OPzV vs OPzS guide details.
Step 3: temperature, the site survey item
Outdoor cabinets in hot climates are where telecom battery budgets go to die early: every sustained 10 °C above the design basis roughly halves lead acid life, and a sun-baked cabinet can sit 15 °C over ambient at noon. The format decision and the thermal decision are the same decision — gel disciplines tolerate heat best within lead acid; lithium modules supervise and report temperature but still age by it. Shade, ventilation and cabinet colour are battery-life equipment. Put the site’s real temperature in the sizing document; a string sized at 25 °C and installed at 40 °C is a two-year string wearing a five-year plan.
Step 4: replacement policy, written at birth
Telecom strings fail by divergence: one cell or block ages out of line and the string’s autonomy follows it down. The defences are per-unit monitoring where the format allows it (the 2V argument again), scheduled discharge testing, and matched-set replacement — never one new block shining among the tired. Record commissioning data per string at install; the operator who inherits the site in year six will bless or curse that record. Our role in the policy is supply-side: documented models with stable designations, so the replacement ordered in year six is the battery the string was born with.
Lithium or lead acid for a new macro site?
Both are live options in this range. Decide on autonomy hours, cabinet thermal reality, your organisation’s battery standards and budget rhythm; we quote the same load both ways.
How is autonomy verified after install?
By a controlled discharge test at commissioning, logged against the SLA figure — the only autonomy number that has ever meant anything.
A miniature worked example
A macro site draws 40 A at 48V; the SLA demands 4 hours of autonomy at end of battery life. Delivered capacity required: 160 Ah at the 4-hour discharge rate — which, after the rate penalty and the ageing margin are honestly applied, specifies a nameplate class in the region of 200Ah and upward per string, or two slimmer strings in parallel where the format allows. In front-terminal blocks that is a four-block string of the appropriate capacity tier; in OPzV it is 24 cells of the 200Ah-class and beyond; in rack modules it is a module count against the 5.12 kWh unit. Three formats, one arithmetic — the example is deliberately rough, because the last step belongs to the discharge tables of the actual models, which is the quotation’s job. Send the site load and SLA hours through the contact page and the rough becomes exact.
How often should telecom strings be discharge-tested?
At commissioning, then on the operator’s standard cycle — annually is the common discipline for critical sites, with impedance trending between tests where the format supports it.
Retrofit patterns in live networks
Telecom backup rarely starts from a blank room; it starts from an incumbent bank aging inside a live rectifier system. The patterns repeat. The like-for-like swap: same format, new bank, minimal works — correct when the SLA and load are unchanged and the format still fits the site class. The format migration: OPzV cells out, front-terminal strings in, buying back floor space and maintenance hours — correct when the site count has outgrown the visiting discipline. The lithium overlay: rack modules added beside the lead acid plant to carry new load or extend autonomy without retiring a healthy incumbent — correct where the rectifier and the BMS conversation can be made to work, and increasingly common as 5G raises site loads. Each pattern has its own commissioning discipline, but all three share one: the autonomy figure is re-proven by discharge test at handover, on the record, whatever the format. A backup system whose hours are believed rather than measured is a hope with a part number.
The discipline, compressed: derive the hours from the load and the SLA, never from tradition; choose the format by the site class, not by the catalogue order; commission with a measured discharge; and keep the per-string record that turns a fleet of batteries into a managed asset. Telecom backup is unglamorous exactly until the night it is the only thing standing between a network and silence — and on that night, the arithmetic in this guide is the whole difference.
A closing note on documentation: keep the per-site battery record with the site file — format, string arithmetic, commissioning discharge result, charger settings. Networks change hands, contractors rotate, and the record is the only memory the site keeps. The operators with the calmest outage nights are, without exception, the ones whose site files can answer the battery question before anyone drives anywhere.
Backup power is a promise an operator makes to people it will never meet, on the worst night of the year. The arithmetic is how the promise is kept.