Nickel-Cadmium Emergency Battery: One Hot Cell and a Mis-set Float Voltage
A 20-cell nickel-cadmium battery rated 25 Ah supplies an emergency communications installation. A capacity check is run at the 5 A rate to a cut-off of 20 V. After 4 h 20 min one cell falls to 0.3 V and its case reaches 48 °C while the remaining 19 cells sit at about 1.03 V each; the check is stopped. The battery has been on continuous float charge at 30.0 V since its last check two years ago. The maintenance manual gives a float voltage of 28.0 V ± 0.5 V and a serviceability limit of 80% of rated capacity. Records show no cell replacement since new. The operator wants the battery returned to service because the delivered capacity, read from the discharge log alone, looks close to the limit. A replacement battery is available within 24 hours.
1. Interpret the capacity check: what capacity was delivered, and does the figure show a serviceable battery?
Read the worked answer
Delivered capacity = 5 A × 4 h 20 min = 5 × 4.333 = 21.7 Ah, which is 21.7 / 25 = 87% of rated - apparently above the 80% serviceability limit.
That reading is misleading. The discharge ended because one cell had collapsed to 0.3 V, not because the string had uniformly reached its end voltage; the other 19 cells were still at about 1.03 V each (≈19.6 V), so the 19.9 V measured at the terminals is almost entirely the healthy cells, with the weak cell contributing nothing useful and in fact subtracting. The 87% figure describes the weakest cell's limit, not the battery's usable capability at the emergency load, and a C/5 test does not demonstrate behaviour at whatever the real emergency current is.
Conclusion: the battery is not serviceable. The result must be reported as a failed check with a cell defect, not as 87% and a pass. The operator's argument that the number is close to the limit ignores why the discharge terminated.
2. Explain the mechanism that drove one cell to 0.3 V and 48 °C, and why the elevated case temperature is a safety issue rather than just a capacity issue.
Read the worked answer
All cells in a series string carry the same current. A cell with reduced capacity - from age, electrolyte loss, separator degradation or a soft internal short - reaches full discharge before the rest. As the discharge continues, the string current is forced through it and it is driven into reverse: its terminal voltage goes negative, and the 0.3 V reading is the net result of a reversed cell inside the string, meaning it is being electrolysed rather than delivering energy.
Reverse current decomposes the electrolyte, generates gas and makes the cell behave as a resistor dissipating I²R, which is the source of the 48 °C case temperature. For a NiCd the electrolyte is alkaline, so a venting cell releases a corrosive aerosol and hydrogen.
On subsequent charging, a cell that has been reversed and heated may not accept charge normally and can progress into thermal runaway, particularly on constant-potential float where the healthy cells clamp the string voltage and the faulty cell can be driven further. The battery must therefore be removed from the aircraft as a fire and burn hazard, not given a boost charge and returned to service.
3. Evaluate the 30.0 V float charge against the manual, explain the likely consequence, and state the actions required before any battery returns to service.
Read the worked answer
30.0 V / 20 cells = 1.50 V per cell. The manual value, 28.0 V ± 0.5 V, corresponds to 1.40 V ± 0.025 V per cell. The charger has been running roughly 0.10 V per cell high - 2 V high across the string - for two years.
Consequence: sustained overcharge of a NiCd drives continuous gassing, water/electrolyte consumption, raised internal pressure, separator degradation and heating, all of which accelerate capacity loss and can dry out a cell. This is consistent with the defect found: one cell has aged faster than the rest, and the stack has been operating hot. The charger's temperature sensor and thermal protection should be suspected too, since the condition went unnoticed.
Actions:
- Do not return the existing battery to service on the strength of the 87% figure; remove it and fit the replacement after confirming its capacity check and cell voltage spread.
- Adjust the charger to 28.0 V ± 0.5 V measured with a calibrated meter at the battery terminals, and verify the current limit, temperature compensation and thermal sensor function.
- Inspect the battery compartment for corrosion from venting and confirm ventilation.
- Record the charger setting and the capacity result in the technical log, and treat a reversed, hot cell as non-recoverable - deep-cycle reconditioning does not restore a cell driven negative and overheated, and the manual may prohibit it in any case.