Telecom towers

Battery bank

A battery bank fails during the outage it was bought for, unless you watch its temperature and its float.

Lead-acid life halves for every 10 °C above 25 °C. Backup time falls faster than capacity because of Peukert’s law. The twin tracks both, so you know how long each site will really hold the radios, not just what the nameplate says.

48 V, 24 × 2 V VRLA cells, 600 Ah at the 10-hour rate, 10-year design life at 25 °C

Built and run to IEC 60896-21 and -22 (VRLA)

Start with one cluster of sitesSee how we prove results
Sketch of a 48 V battery bank: 24 two-volt VRLA cells on a two-tier rack joined in series, with the string shunt, a temperature probe, cell voltage leads to a monitor and a conductance clip, and beside it a lithium-ion cabinet of four 48 V modules with a battery management system, with numbered sensor points.12345624 × 2 V VRLA, 600 Ahlithium, 4 × 48 Vto −48 V busblock monitor
Backup at 50 A, new
12.3 h
Life at 25 °C
10 years
Life at 35 °C
5.0 years
Stored energy
28.8 kWh

Anatomy and nameplate

Sketch of a 48 V battery bank: 24 two-volt VRLA cells on a two-tier rack joined in series, with the string shunt, a temperature probe, cell voltage leads to a monitor and a conductance clip, and beside it a lithium-ion cabinet of four 48 V modules with a battery management system, with numbered sensor points.12345624 × 2 V VRLA, 600 Ahlithium, 4 × 48 Vto −48 V busblock monitor
  1. 1String voltage. Float, discharge and recharge.
  2. 2Charge and discharge current. From the string shunt. Gives the float current too.
  3. 3Battery temperature. The number that sets the aging rate.
  4. 4Cell or block voltages. If monitored. One block below the rest is a weak cell.
  5. 5Internal resistance or conductance. If monitored. Rises as the cell ages.
  6. 6BMS state of charge and health. Lithium strings report their own. Reference: 4 × 100 Ah 51.2 V modules.
Battery bank nameplate
VRLA string
Cells24 × 2 V, valve-regulated lead-acid
Capacity600 Ah at the 10-hour rate to 1.80 V per cell
Energy28.8 kWh nominal
Design life10 years at 25 °C, end of life at 80% capacity
Peukert exponent1.15
Float54.0 V at 25 °C, 0.48 A float current
Round trip82% watt-hour
Lithium-ion reference
Modules4 × 100 Ah, 51.2 V lithium iron phosphate
Energy20.5 kWh
Charge limit0.5C (200 A)
Round trip93% watt-hour
MonitoringBMS state of charge, health and cell balance

Telecom site power chain

The grid or the generator feeds the site through the transfer switch. The rectifier makes −48 V DC for the radios and keeps the battery charged. The air conditioner keeps the battery cool.

Grid: Mains supply, 400 VGridMains, 400 VFuel tank: 500 L diesel tankFuel tank500 LGenerator: 20 kVA diesel generator setGenerator20 kVA dieselTransfer switch: Switches the site between mains and generatorTransferswitchMains or DGRectifier: Turns AC into −48 V DCRectifierAC to −48 V DCShelter cooling: Draws from the AC board and keeps the battery coolSheltercoolingAir conditionerRadios: Radio equipment on the −48 V DC busRadios−48 V DC loadBattery: Carries the radios in an outageBatteryBackup in outage
  • Air
  • AC power
  • −48 V DC
  • Diesel
  • Has its own twin page

The physics

The equations behind every curve on this page.

Peukert

t = H · (C / (I · H))^k

k ≈ 1.15 for VRLA. Higher draw, less time than linear.

Arrhenius life

L(T) = L_25 · 2^(−(T − 25) / 10)

Life halves for every 10 °C above 25 °C.

Life from history

L = L_25 / mean(2^((T − 25)/10))

Added up hour by hour, so a hot afternoon counts for more than a cool night.

Autonomy

t = H · (C·SOH·f_T / (I·H))^k

Usable amp-hours after aging and temperature, divided by the load, corrected by Peukert.

Float current

I_f = I_f0 · 2^((T − 25)/10)

Warmer cells draw more float current. More than the temperature explains is a warning.

Backup time and life

How long the radios stay up depends on the load, the age of the string and the heat it has lived through.

Backup hours against load current, new and aged

600 Ah VRLA string, discharged at 25 °C

Backup hours against load current, new and aged0510152025303520406080100120140Site load 50 A12.3 h new10.1 h agedLoad current (A)Backup (hours)
  • New
  • 4 years at 25 °C
  • 4 years at 35 °C
  • New, if it were linear

Draw twice the current and you get less than half the time. Heat takes capacity away year by year, so an old string in a hot shelter holds the radios for hours less than its nameplate says.

t = H · (C / (I · H))^k, k = 1.15, H = 10 h

Battery life against average battery temperature

VRLA, 10-year design life at 25 °C

Battery life against average battery temperature0246810121420253035404510 years at 25 °C7.1 at 30 °C5.0 at 35 °C3.5 at 40 °CAverage battery temperature (°C)Expected life (years)
  • Expected life

Every 10 °C above 25 °C halves the life. The shelter temperature is the battery’s clock.

L(T) = L_25 · 2^(−(T − 25) / 10)

Float current

A charged string still draws a small current on float. It follows temperature. When it climbs on its own, something is wrong inside.

Float current over two years

Free-cooled shelter, so the battery follows the seasons

Float current over two years0.00.51.01.52.02.53.03.510152025303540450102030405060708090100Flagged in week 62WeekFloat current (A)Battery temperature (°C)
  • Expected for the temperature
  • Healthy string
  • String with a cell drying out
  • Battery temperature (right axis)

Float current rises in summer because the battery is warmer. That part is expected. Current the temperature does not explain means a cell is aging, or the string is heading for thermal runaway.

I_f = I_f0 · 2^((T − 25)/10) · 2^((V_cell − 2.25)/0.05)

Failure modes

What breaks, what it looks like, what the twin does about it.

Failure modeThe physicsSignals that show itHow the twin catches it
Heat agingGrid corrosion and water loss speed up with temperature on an Arrhenius curve.Battery temperature history. Capacity falling at autonomy tests.Adds up life used hour by hour from the temperature and reports the years left.
Undercharge and sulfationA string that never reaches full between outages grows hard sulfate crystals that do not convert back.Recharge cut short by the next outage. Float current never settles. Backup falls.Tracks charge in against charge out over each outage cycle.
Thermal runawayFloat current heats the cells, heat raises the current, and the loop feeds itself.Float current climbing with temperature. Float voltage not compensated.Compares float current with what the temperature explains and flags the gap early.
Weak cell in the stringOne cell with less capacity or more resistance limits the whole series string.One block below the others on discharge. A deeper voltage dip at each transfer.Watches block voltage spread and the dip at every outage.
Lithium BMS faults and imbalanceCells drift apart in charge. The BMS limits the string to its weakest cell.BMS alarms. Cell voltage spread. State of charge that jumps.Reads the BMS and checks its state of charge against charge in and out.

How we catch it early

A physics twin and a PINN, working on one battery bank.

The twin is a set of equations for this machine, tuned to its nameplate. Give it the flow, speed and conditions and it returns what every reading should be. It always gives the same answer for the same inputs.

A physics-informed neural network, or PINN, sits on top. It is trained on the plant’s own data and it is penalized whenever it breaks a physics rule. So it can only learn what the equations leave out: wear, fouling, friction. It cannot invent an answer that the machine could never produce.

  1. Step 1

    Measure

    Flow, pressure, current, temperature, vibration. Whatever the asset already has.

  2. Step 2

    Physics twin

    The equations of this machine, set to its nameplate. It says what the readings should be right now.

  3. Step 3

    Residual

    Measured minus expected. Demand, weather and speed are already taken out.

  4. Step 4

    PINN

    A small neural network that learns the cause of the gap, under the physics rules.

  5. Step 5

    Finding

    A named cause, a date, and the kWh or hours at stake. An engineer reviews it.

loss = (measured − predicted)² + λ · (physics residual)²

The second term is the rulebook. It is zero when the network obeys the equations.

Why this beats black-box AI

A black-box model learns patterns from history. It needs examples of each failure, and it goes quiet when the pump runs at a point it has not seen. The twin starts from physics, so it works on day one. The PINN adds what physics cannot know about this particular machine.

That is also what makes the savings countable. The baseline is a calculation, not a guess, so the gap between baseline and meter is a number a customer can check.

What the twin checks on this asset

  • Expected autonomy at today’s load: 12.3 h new at 50 A
  • Life used by the temperature history
  • Float current trend against temperature
  • Block voltage spread on discharge
Fixed alarmBlack-box AITwin + PINN
Needs failure history to learnNoYes, lotsNo
Says what is wrongNoRarelyYes, by name
Works at a flow it has never seenNoOften failsYes, the equations still hold
Puts a number on the savingNoNoYes, in kWh and dollars
Lead time on slow wearLateVariesWeeks to months
Every number can be tracedYesNoYes, to an equation

What the twin puts a number on

Energy and cost, from the equations.

Life lost at 35 °C

5.0 years

5.0 years against 10 at 25 °C. Each °C above 25 °C costs 0.67 years.

Backup left after 4 years at 35 °C

10.1 h

Against 12.3 h new, at the 50 A site load

Replacement brought forward

$550 a year per site

A $5,500 string every 5 years instead of every 10

What we do for you

We watch it. We catch it. We prove it.

Our reliability engineers run the twin on your battery bank, review every finding, and help your crew fix it. After the repair we compare the meter to the baseline and write the result in the ledger.

Start with one cluster of sitesSee the 90-day pilot

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