Quick answer: When a BESS humidity sensor gets wet, its reading usually sticks near 100 % RH and recovers slowly once it dries. Contamination left behind can bias it for good. That raises two separate questions: is the sensor still reliable? and did a real moisture event happen in the enclosure? Both can be true at once. Preserve the alarm record, compare with a second independent measurement and with the suspected cold surface, and verify the probe using the manufacturer's procedure before trusting it again. To prevent repeats, place sensors away from cold surfaces and airflow from cooling outlets, and keep the enclosure dew point below its surfaces.
Quick answer
- Wet probe → reads high, recovers slowly; contamination → lasting offset.
- Separate "sensor fault" from "real moisture event".
- Check against a reference; fix the location and the moisture cause.
First identify the failed point
Record the point name, physical location, measured variable, unit, sampling interval, alarm thresholds and controller state. Note whether the point controls equipment (for example a dehumidifier) or only alarms.
Preserve the event record
- Raw readings and timestamps before and after the alarm.
- Alarm codes and acknowledgements.
- Other sensors in the enclosure, HVAC and battery state, door events.
- Photos of the probe and nearby surfaces.
Keep safety and BMS fault decisions with the BESS manufacturer and qualified staff.
Three plausible fault paths
| Path | Signs | What it means |
|---|---|---|
| Probe wetted by condensation | Reads about 100 %, slow recovery; droplets on or near probe | Real moisture at the probe; check other surfaces too |
| Probe contamination or damage | Lasting offset after drying; erratic readings | Sensor needs cleaning, recalibration or replacement |
| Wiring, power or data mapping | Step changes, impossible values, frozen value | Controls or wiring fault, not moisture |
A temperature mismatch also misleads: an RH sensor at a different temperature from the air it is meant to represent reports a different RH for the same moisture content.
Verify without erasing the incident
- Compare with a calibrated handheld reference at the probe location.
- Measure dew point and the coldest nearby surface temperature.
- Follow the sensor maker's recovery and verification instructions.
- A reading that returns after drying is not proof of calibration; check against the reference.
Prevent repeat faults
| Cause | Prevention |
|---|---|
| Probe near a cold surface or AC discharge | Relocate to representative air |
| Condensation in the enclosure | Dehumidification on dew-point control |
| Dust or chemical contamination | Protective filter cap; maintenance interval |
| No redundancy | Second sensor in critical zones; plausibility checks in the controller |
See dew-point sensor placement, BESS condensation with AC and our energy storage dehumidifiers.
FAQ
Frequently asked questions
Why does my humidity sensor read 100 % after condensation?
The sensing element is wet; it reads saturation until it dries, and may then carry an offset.
Is a 100 % reading always a sensor fault?
No; the air near the probe may truly be saturated. Check with a second measurement and nearby surfaces.
How do I stop humidity sensors failing in BESS?
Place them in representative air away from cold surfaces and discharges, and control the enclosure dew point.
Sources
- ASHRAE Handbook—Fundamentals, Measurement and instruments; Psychrometrics.
