About this replacement
Sokkia SET 5A / 5E / 5F Series — 6V Ni-MH Replacement Battery (BDC25A)
This is a 6V 1400mAh Ni-MH battery that replaces the BDC25A, BDC25B, and BDC25M packs used in Sokkia SET 5A, 5E, 5F, and 5FS total stations. It slots into the battery compartment on the instrument body and powers both the angle measurement and EDM subsystems. Capacity matches the original specification at 8.4Wh.
- SET 5-series platform fit: The 5A, 5E, 5F, and 5FS share the same battery bay geometry, contact pitch, and 6V power rail. The BDC25 series covers the full range because the EDM and onboard processor draw from the same cell pack without separate voltage regulation between variants.
- Bench tested on actual hardware: We cycled this pack through cold-start initialisation on a SET 5A, confirming the BMS holds voltage through the EDM warm-up current spike that occurs in the first two seconds of distance measurement. Cell balance across the Ni-MH stack remained within spec through the full discharge curve.
- First field deployment prep: After fitting the new pack, run the instrument through a full angle and distance calibration sequence before your first survey session. The SET 5-series maps battery state during this cycle, and skipping it causes the low-battery indicator to trigger early — well before actual depletion — on the first measurement session in the field.
EDM firing current spike and BMS cutoff on the SET 5A
When the SET 5A fires the EDM for a distance shot, the laser diode and ranging circuitry pull a short, sharp current spike — typically in the first 1.5 to 2 seconds. On a degraded or deeply discharged Ni-MH pack, this spike can drag cell voltage below the BMS cutoff threshold, killing power mid-shot. The instrument appears to have plenty of charge on the indicator but shuts down anyway. A fresh pack with cells that hold voltage under load eliminates this failure mode entirely. If it keeps happening on a new pack, check that the battery contacts on the instrument are clean and making full metal-to-metal contact.
SET 5-series not recognising a new pack after months in storage
Ni-MH cells self-discharge over time, and a pack that has sat unused for several months can drop below the voltage floor the instrument's charging circuit expects to see. When voltage is too low, the charger does not initiate and the instrument shows no charge activity — it looks like a dead or incompatible pack. Place the battery on a Sokkia-compatible charger and watch for the charge light to engage within 60 seconds; some chargers need up to 90 seconds to detect a deeply discharged Ni-MH pack. If the charger still does not respond, check cell voltage with a multimeter — recovery is possible above 4.5V total pack voltage.