A battery bank that cannot carry the load when AC supply fails has one of these causes: it lost capacity through age or temperature, it has a failed cell or a high-resistance intercell connection, it was not charged because the charger failed without an alarm, or the load grew beyond what was sized. Per-cell voltage during discharge and the capacity test separate one cause from another.
Symptom
AC supply to the substation is lost, through a fault or in a test, and the battery bank falls short. The DC voltage drops fast and relays and controllers reset or shut down. Breakers lack enough voltage to open and communications are lost. Sometimes it is discovered during a real fault, which is the worst way to find out.
The battery bank is the last energy source for the substation protection and control. It runs on float, in parallel with the battery charger/rectifier, and must deliver the load for the backup time defined in its sizing when the charger stops supplying. While the charger works, the bank voltage looks normal even if the battery is deteriorated; the problem appears only on discharge.
Possible causes, from most to least likely
| Cause | Why it happens | What gives it away |
|---|---|---|
| Capacity lost to age or temperature | Normal aging, accelerated by high room temperature | All cells fall evenly and ahead of time |
| Failed cell | Open, shorted, dried-out (VRLA) or sulfated cell | One cell falls much faster than the others; high internal resistance |
| High-resistance intercell connection | Loose bolts, corrosion on the terminals | Voltage drop and heating at the connection during discharge |
| Battery not charged | Charger out of service, float badly set or charger alarm not attended | Low per-cell voltage before discharge; low specific gravity in vented lead-acid |
| Load greater than sized | Bays, communications equipment or heaters added after the design | Measured discharge current higher than the calculated one |
| Low temperature | Available capacity drops in the cold | Cold room; less backup in cold seasons or sites |
What to check first
- Charger status before the event: alarms, float voltage and current. If the battery was not charged, the problem lies with the charger or its supervision.
- Voltage of each cell during discharge, if there is a record or it can be repeated in a controlled way. How each cell falls shows whether the problem is general or localized.
- Intercell connections: tightness, corrosion and heating. Thermography during discharge locates them quickly.
- Actual DC load against the sizing. You can check it with our battery sizing tool.
- Age and room temperature: temperature is the factor that shortens battery life the most. See lead-acid and nickel-cadmium batteries.
- Was the charger in service and the battery on float before the event?
- No: Discharged battery: check the charger and its alarm before blaming the battery
- Yes: Does the voltage fall evenly across all cells?
- No: Is the drop concentrated at an intercell connection?
- Yes: Loose or corroded connection: clean and tighten per the manufacturer
- No: Failed cell: measure it and replace it
- Yes: Does the present load exceed the sized one?
- Yes: Bank undersized for the present load: resize
- No: Capacity lost to age or temperature: run the capacity test
- No: Is the drop concentrated at an intercell connection?
How to confirm
- Capacity test: controlled constant-current discharge, recording each cell’s voltage, down to the defined end voltage; the result is expressed as a percentage of nameplate capacity. See battery capacity test.
- Ohmic measurements (internal resistance, impedance or conductance) per cell, compared with the manufacturer’s reference and with the other cells in the bank.
- Resistance of intercell connections with a micro-ohmmeter, compared with each other.
- Measurement of the DC load and review of the sizing per IEEE 485 (lead-acid) or IEEE 1115 (nickel-cadmium).
How to keep it from coming back
- Supervise the charger and the battery from the SCADA: charger failure, high and low voltage, DC ground. See DC ground alarm.
- Run periodic ohmic tests and capacity tests at the frequency given by IEEE 450, IEEE 1188 or IEEE 1106 depending on the technology.
- Control the temperature of the battery room and adjust the float with temperature compensation when the manufacturer asks for it.
- Review the sizing every time loads are added to the DC system.
- Tighten the connections to the manufacturer’s torque and protect the terminals against corrosion.
What not to do
- Do not rely on float voltage as proof that the battery is fine.
- Do not replace one old cell with a new one without evaluating the bank: the new cell does not compensate for the others and may end up overcharged or undercharged.
- Do not run a test discharge without backup for the substation’s critical loads.
- Do not work in the battery room without ventilation and protection: vented batteries release hydrogen during charging.
Which standard answers which question
The IEEE standards for stationary batteries are split by technology and by task. Knowing which one applies avoids measuring against the wrong criterion.
| Task | Vented lead-acid | Valve-regulated lead-acid (VRLA) | Vented nickel-cadmium |
|---|---|---|---|
| Maintenance, testing and replacement | IEEE 450 | IEEE 1188 | IEEE 1106 |
| Sizing | IEEE 485 | IEEE 485 | IEEE 1115 |
| Installation | IEEE 484 | IEEE 1187 | IEEE 1106 |
IEEE 1375 is cross-cutting: it gives guidance on physical and electrical protection of the battery system, including DC protective devices and the behavior of the bank and charger under a short circuit. Its scope reaches the first protective device downstream of the battery terminals; selective coordination of the rest of the panel is outside it.
Three different discharges
A frequent confusion is calling any discharge a “capacity test”. IEEE 450 distinguishes different purposes:
- Acceptance test: done on receiving or installing the bank, comparing its performance with what the manufacturer guarantees.
- Performance test: controlled discharge that measures actual capacity against nameplate. It is the one that decides replacement when capacity falls to 80 %.
- Service test: discharge with the load and the time of the design duty cycle. It does not measure capacity; it proves the bank does its job in that substation, and that is why it is the one most like the real event.
The technical references we consult agree that the periodic performance test is repeated at intervals not exceeding 25 % of the bank’s expected life. They also agree that it becomes annual when the bank approaches 85 % of its expected life or shows degradation. The exact frequency depends on the edition of the standard and the technology; confirm it in the one your company adopts.
Illustrative example
The bank looks healthy: float voltage is correct, there are no alarms and the visual inspection is clean. When AC fails, the voltage drops in the first minute, just when the spring-charging motors, the trip coils and the start-up of the communications equipment coincide. The duty cycle in the sizing per IEEE 485 includes those momentary loads; if equipment was added after the design, the bank can remain healthy and still fall short in that first instant. That is why the service test and the review of the load cycle are done together.
VRLA batteries behave differently. They have no electrolyte that can be measured or topped up, so their condition is tracked with ohmic measurements, temperature and discharge tests. Charging them without temperature compensation in a hot room speeds up their aging and can lead to thermal runaway, a risk that the manufacturer and the VRLA guides warn about.
When to call us
If your substation’s battery bank did not hold the load and you need to know whether it is the battery, the charger or the sizing, we handle it with our testing and commissioning in Colombia or elsewhere on request. We run capacity or discharge tests, impedance or conductance tests and per-cell voltage checks, plus inspection. We also review the charger and its supervision. If the DC system needs resizing, we do it from secondary engineering, remotely and worldwide. Write to us.
Related guides
- Battery capacity test
- Lead-acid and nickel-cadmium batteries
- Battery charger/rectifier
- Substation DC system
- DC distribution panel
- DC ground alarm
- Trip circuit supervision alarm
References
- IEEE 450, Recommended Practice for Maintenance, Testing, and Replacement of Vented Lead-Acid Batteries for Stationary Applications.
- IEEE 1188, Recommended Practice for Maintenance, Testing, and Replacement of Valve-Regulated Lead-Acid (VRLA) Batteries for Stationary Applications.
- IEEE 1106, Recommended Practice for Installation, Maintenance, Testing, and Replacement of Vented Nickel-Cadmium Batteries for Stationary Applications.
- IEEE 485, Recommended Practice for Sizing Lead-Acid Batteries for Stationary Applications.
- IEEE 1115, Recommended Practice for Sizing Nickel-Cadmium Batteries for Stationary Applications.
Frequently asked questions
When should a substation battery bank be replaced?
IEEE 450 recommends replacing a vented lead-acid bank when its actual capacity falls to 80 % of nameplate, because from there deterioration accelerates. IEEE 1188 covers valve-regulated (VRLA) batteries and IEEE 1106 nickel-cadmium ones, each with its own criteria.
Why does the battery show good voltage on float and collapse when the load is connected?
Because float voltage is imposed by the charger and says nothing about how much energy is stored. A damaged cell, a loose connection or lost capacity only show up when the battery has to deliver current. That is why periodic ohmic tests and capacity tests are done.
Can a single bad cell leave the whole substation without backup?
Yes. The cells are in series: an open cell interrupts the current of the whole bank, and a cell with low capacity runs out first and pulls the bank voltage below the minimum of the equipment. In discharge, that cell can even reverse polarity.