Every telecom site has the same two questions about its backup battery:
- How long will it run during an outage? — backup time
- How many years will it last before it needs replacing? — service life
Both answers look simple on paper — “100Ah gives you X hours, and it lasts Y years” — and both are wrong in practice without understanding how the numbers are actually calculated. This guide covers both questions properly: how to calculate backup time for a 48V site, what really determines battery life, and how to know when a battery is ready for replacement.
Calculating backup time: the formula that matters
Backup time is simple arithmetic once you have two numbers: the site’s average DC load in kilowatts, and the battery’s usable energy in kilowatt-hours.
Backup time (hours) = Battery energy (kWh) × efficiency factor ÷ Site load (kW)
For a 48V system, battery energy is easy to derive from capacity:
- 48V × 100Ah = 4.8 kWh
- 48V × 150Ah = 7.2 kWh
- 48V × 200Ah = 9.6 kWh
All three are available from the same 48V lithium battery range, in standard and smart versions.
The efficiency factor accounts for conversion losses and the fact that a battery should not be discharged to zero. A practical factor is roughly 0.8–0.85 for a healthy LiFePO4 site battery.
A worked example: a site drawing 1.2 kW of average load with a 48V 150Ah battery (7.2 kWh):
7.2 kWh × 0.8 ÷ 1.2 kW = 4.8 hours of backup
The same battery behind a 2.4 kW site gives 2.4 hours. The same site with a 200Ah battery gives 6.4 hours. This is why the sizing conversation always starts with the site load — two sites with identical batteries can have very different backup times.
Site loads vary widely: rural sites with a single small radio can draw a few hundred watts; urban or high-capacity sites with multiple carriers can draw several kilowatts. Measure your actual load rather than guessing — most sites have monitoring or a power audit from the rectifier vendor.
How 48V backup systems are built
Telecom backup batteries are rack-mount units that sit beside the site’s DC power system:
- Nominal voltage is 48V DC, matching the site power plant, so multiple battery strings can be paralleled onto the same DC bus.
- Capacity is sized in Ah (100/150/200Ah in LINEX’s telecom range), and parallel strings add capacity — two 100Ah strings give 200Ah total, extending backup time without changing the system architecture.
- The battery pairs with the rectifier/charger system: the rectifier powers the load and recharges the battery; the battery carries the load when grid power fails.
LINEX 48V telecom battery solutions are built on rack-mount, 19-inch form factor units designed for base station cabinets, with a BMS that reports voltage, current and state of charge — the basics for any serious site operation.
What actually determines battery life
Battery life is quoted in cycles, but real service life depends on how the battery is used:
Cycle life. A LiFePO4 battery rated at 6,000+ cycles is the headline number. In telecom duty — where a site may see daily grid outages in weak-grid regions — one cycle per day means the rated chemistry life stretches to roughly 16 years. That is theoretical: real conditions shorten it, and the next two factors are why.
Depth of discharge (DoD). Every discharge is not equal. Deep discharges to the low-voltage cut-off stress the battery more than shallow ones. A site that regularly drains batteries to 20% remaining capacity will see shorter life than one that cycles gently between 60% and 90%. Sizing with 20–30% headroom is not just about backup time — it is also a life-extension strategy.
Temperature. Heat is the silent killer of every battery chemistry. Telecom cabinets bake in the sun in many markets; every sustained temperature increase above comfortable operating range accelerates aging. LiFePO4 tolerates heat far better than lead-acid, but an indoor cabinet with poor ventilation can still shorten life measurably. Keep batteries out of direct heat sources and allow airflow.
Charge management. Consistent, correctly-profiled charging (proper absorption voltage, not over-charging, not leaving the battery in a chronically under-charged state) keeps capacity loss slow. A good BMS handles the hard parts automatically — low-voltage disconnect, over-voltage protection, cell balancing.
When to replace a battery
The industry-standard practical threshold is 80% of rated capacity: when a battery’s measured capacity drops below 80%, it is considered end-of-life, because its degradation accelerates from there and backup time starts falling unpredictably.
Three signals that a battery is approaching replacement:
- Measured capacity below 80%. A capacity test (discharge test through the site’s monitoring or a test load) is the only honest measurement. Run one periodically — quarterly for older units.
- Voltage sag under load. A healthy 48V LiFePO4 string holds ~48V during discharge until near the end. If the string voltage drops sharply early in an outage, capacity is gone or cells are unbalanced.
- Frequent BMS alarms. Cell over/under-voltage warnings, repeated disconnects, or temperature alarms are the BMS telling you the string is stressed. One alarm is a warning; recurring alarms are a replacement signal.
LiFePO4 vs lead-acid in telecom duty
Telecom sites impose a specific duty cycle: long periods of float/standby interrupted by frequent short discharges in weak-grid regions. That pattern is exactly where chemistries diverge.
- Lead-acid degrades in this duty: repeated partial discharges cause sulfation, and deep discharges damage the plates. Many lead-acid site batteries in high-outage regions are replaced within 2–3 years.
- LiFePO4 is built for the pattern: thousands of cycles, no memory effect, no sulfation, full depth of discharge capability, and flat voltage under load — which also means the radio gear sees stable power for the whole outage.
For the full cost comparison, see our earlier guide: LiFePO4 vs Lead-Acid: The Real Cost Difference Over 10 Years.
Three practical recommendations for site operators
- Size with 20–30% headroom. More headroom means both longer backup time and longer battery life — the cheapest insurance in the system.
- Manage temperature. Ventilated cabinets, shade, and keeping batteries off hot surfaces. In hot markets, this one habit extends life more than any other.
- Monitor, don’t guess. Use the BMS reports, set voltage/current alarms, and run capacity tests on a schedule. The first sign of trouble is always visible in the data before it becomes an outage.
The bottom line
Backup time is arithmetic: battery energy times efficiency, divided by site load. Battery life is physics: cycle count, depth of discharge, temperature and charging discipline. Both are manageable — and both are why a battery with 6,000 cycles and a real BMS is worth more than a cheaper unit with a shorter real life. For the full selection walkthrough, see our guide on how to choose a 48V telecom battery.
LINEX manufactures 48V rack-mount telecom lithium batteries (100/150/200Ah) with intelligent BMS, 19-inch form factor, and 6,000+ cycle ratings — factory-direct with OEM/ODM support for operators, TowerCos and EPC partners.