Litech Insights

A fibre network is only as reliable as the power behind it. An OLT, a switch in a street cabinet, a microwave link or a small base station stops the moment the mains supply fails, and every customer behind it goes offline. That is why almost every telecom and network site runs on a 48 V DC power system with a battery bank as backup. This guide compares the three battery types you will meet most often, AGM, gel and lithium iron phosphate (LiFePO4), explains how heat and frequent outages change the choice, and shows step by step how to size a battery bank for your site.

Why telecom sites use 48 V DC batteries

Most telecom and network equipment, from OLTs and aggregation switches to radio units and transmission equipment, runs from a −48 V DC supply. A rectifier converts mains AC to DC and keeps the batteries on float charge: a small, steady voltage that holds them full. When the mains fails, the batteries take over instantly, with no switching gap, until the generator starts or the power returns.

A 48 V bank is built from cells in series. With lead-acid batteries that means 24 cells of 2 V or four 12 V blocks. Smaller sites and cabinets usually use 12 V blocks; large exchanges and core sites use 2 V cells, which offer higher capacity and a longer life. Lithium systems usually come as a ready-made 48 V module with its own battery management system (BMS).

AGM, gel and LiFePO4 at a glance

AGM and gel are both VRLA (valve-regulated lead-acid) batteries: sealed, maintenance-free and safe to install in cabinets and equipment rooms. The difference is how the acid is held. In AGM batteries it is soaked into glass-fibre mats; in gel batteries it is mixed with silica into a gel. LiFePO4 is a lithium chemistry known for being far more stable than the lithium types used in phones.

Design life figures are the manufacturer's rating at the stated temperature, not a promise of service life. Real life depends mostly on temperature and on how often and how deeply the battery is discharged.

Heat: the biggest battery killer

Lead-acid batteries are rated at 20 to 25 °C. Above that, the chemical reactions that wear out the plates speed up. A widely used rule of thumb is that VRLA battery life roughly halves for every 8 to 10 °C above 25 °C. A 10-year AGM battery that sits in a cabinet at 35 °C all summer may last only about five years.

In practice this means:

• Keep battery rooms and cabinets cool where you can, with ventilation, shade, a heat exchanger or air conditioning.
• Use temperature-compensated charging. Most telecom rectifiers lower the float voltage as the temperature rises, which reduces wear. Check that the battery temperature sensor is fitted and working.
• Choose a chemistry that handles heat better for outdoor cabinets and rooftop sites: gel and tubular gel batteries, or LiFePO4 with a BMS, usually cope better than standard AGM.

Frequent outages change the choice

Standby batteries were designed for networks where the mains rarely fails. Where there are scheduled outages every day, a battery may go through a deep discharge and recharge hundreds of times a year. That is cycling, and it wears lead-acid batteries much faster than float service.

• Rare, short outages: a standard AGM battery such as the Wolong 6-GFM series or, for 19 inch and 23 inch cabinets, the front-terminal 6-GFMF series is usually the most economical choice.
• Regular outages of a few hours: a gel battery such as the 6-GFMF-XJ hybrid gel or the GFM-XJ series tolerates repeated discharges and heat better.
• Long daily outages, or solar and hybrid sites: a 2 V tubular gel OPzV battery (GFMJ series) or a 48 V LiFePO4 system is built for heavy cycling. Lithium also recharges much faster, which matters when the mains is only back for a few hours between outages.

For UPS loads that need a lot of power for a short time, such as a server room UPS, a high-rate battery like the 6-GFM-XU series delivers more power in the first few minutes than a standard standby battery of the same size.

How to size a battery bank: step by step

Battery sizing starts from three numbers: the load, the backup time you need and the system voltage. Then you add margins for ageing, temperature and the battery's usable capacity.

1. Add up the DC load in watts. Use the measured current on the rectifier if you can, or the typical power from the equipment datasheets. Include everything on the battery: OLT, switches, routers, radios and fans.
2. Convert to amps. Load current (A) = load (W) ÷ nominal voltage (48 V).
3. Multiply by the backup time. Required amp-hours = load current × hours of backup.
4. Add an ageing margin. A battery is usually considered at the end of its life when it can only deliver 80% of its rated capacity, so multiply by 1.25 to make sure an old battery still meets the target.
5. Allow for the discharge rate. Lead-acid capacity is usually quoted at the 10-hour rate (C10). Discharged faster, a battery delivers less. Use the manufacturer's discharge table to find how many amp-hours it actually gives over your backup time.
6. Allow for temperature and depth of discharge. Lead-acid batteries give less capacity in the cold and wear faster when discharged deeply; lithium batteries are usually sized for 80 to 90% depth of discharge.
7. Round up to the next standard battery size, and leave space for future growth.

Worked example: a fibre aggregation site

A small fibre site has an OLT, an aggregation switch and a transmission link drawing a total of 480 W at 48 V. The operator wants 4 hours of backup.

For lead-acid, the next standard size up is a 65 Ah 12 V block, so the site needs four 12 V 65 Ah AGM or gel batteries in series (for example the Wolong 6-GFM-65). Many designers would choose 100 Ah blocks instead to leave room for growth and hot summers. For lithium, a 48 V 70 Ah LiFePO4 module such as the WL48-70FT01 covers the same load with margin, at a fraction of the weight.

The 80% and 90% figures above are example assumptions for this calculation. Always take the real discharge figures from the datasheet of the battery you plan to buy.

Installation and maintenance tips

• Do not mix old and new batteries, or batteries of different makes or capacities, in the same string. The weakest block limits the whole bank and gets overcharged or overdischarged.
• Set the rectifier to the battery's float and boost voltages from the datasheet. For example, Wolong's GFMJ tubular gel cells specify a float of 2.27 V per cell at 20 °C. Wrong settings are a common cause of early failure.
• Check torque on the terminals at installation and at each service visit. A loose connection gets hot and can fail under load.
• Record float voltages and do a discharge test once a year. A falling backup time warns you before a real outage does.
• Set a low-voltage disconnect (often around 42 to 44 V for a 48 V lead-acid bank) so the battery is never discharged to damage. Lithium systems handle this in the BMS.
• Keep the fibre side healthy too. Backup power is wasted if a dirty connector or a bad splice takes the link down. See our guides to inspecting and cleaning connectors and calculating a loss budget.

In short: AGM is the economical choice for standby with rare outages, gel and tubular gel handle heat and regular outages better, and LiFePO4 is best for heavy daily cycling, fast recharge and light weight. Size the bank from load × backup time, then add 25% for ageing and allow for the discharge rate and depth of discharge.

Frequently asked questions

How many 12 V batteries do I need for a 48 V system?

Four 12 V batteries in series make a 48 V string. With 2 V cells you need 24 cells in series. To increase backup time you can add a second string in parallel, using identical batteries.

Is gel better than AGM for telecom?

Gel batteries usually last longer in hot conditions and with frequent deep discharges, so they are often the better choice for outdoor cabinets and sites with regular outages. AGM is cheaper and handles high-rate discharges well, so it suits UPS systems and sites where outages are rare.

Can I replace lead-acid batteries with lithium?

Often yes, but check that the rectifier's charge voltage and current limits suit the lithium system (Wolong's 48 V LiFePO4 modules, for example, specify a charge voltage of 54.0 ± 0.5 V) and that the BMS can talk to your monitoring if you need alarms.

What does C10 mean on a battery?

C10 is the capacity the battery delivers when discharged evenly over 10 hours at 25 °C. A 100 Ah C10 battery gives about 10 A for 10 hours, but less than 100 Ah if it is discharged in 2 or 4 hours.

How long do telecom batteries last?

Design life ranges from about 8 years for high-rate UPS batteries to 20 years for 2 V tubular gel cells at 20 °C. In hot cabinets or with daily outages, real life can be much shorter, which is why temperature control and the right chemistry matter.

Need batteries for a network site?

Litech is an authorised Wolong dealer in Pakistan and supplies the full Wolong battery range: 12 V AGM, front-access AGM, front-access hybrid gel, 2 V telecom AGM, 2 V tubular gel OPzV, high-rate UPS and 48 V LiFePO4 batteries, alongside optical fibre cables and EXFO test equipment. Message us on WhatsApp +92 313 7776766 with your load and backup time for a quotation. Delivery is free across Pakistan.

Sources: Wolong product datasheets (6-GFM, 6-GFMF, 6-GFMF-XJ, GFM, GFM-XJ, GFMJ, 6-GFM-XU and Lithium series); IEEE 485 and IEEE 1188 (lead-acid battery sizing and VRLA maintenance practice); IEC 60896 (stationary lead-acid batteries).