Sumitomo T-400S Replacement Battery 10.8V 3400mAh BU-15

Sumitomo T-400S — 10.8V Li-ion Replacement Battery (BU-15)

This is a 10.8V 3400mAh Li-ion battery for the Sumitomo T-400S digital theodolite. It replaces OEM part BU-15 and slots into the T-400S battery compartment for continued field use. Voltage and capacity match the original pack exactly.

• T-400S and T400S compatibility: Both model designations refer to the same instrument platform. The BU-15 connector pinout, BMS handshake protocol, and 10.8V nominal rail are identical across both designations — the difference is labelling only.
• Bench tested on actual hardware: We cycled this pack through instrument power-up, angle measurement sequences, and sustained horizontal circle drive loads. The BMS held stable at load without tripping, and the pack delivered consistent draw across the measurement cycles we ran.
• Post-install calibration before field deployment: After fitting this pack, run a full calibration cycle through the T-400S instrument menu before taking it to site. The instrument maps battery state during that routine. Skip it, and the first measurement session will throw premature low-battery warnings even with a full charge.

BMS cutoff when the T-400S initialises its horizontal drive motor

At power-up, the T-400S runs a self-check that briefly drives the horizontal circle motor to find its zero reference. That initialisation draws a short current spike well above the steady measurement load. An aged or partially depleted cell can't sustain the spike, and the BMS trips before the instrument finishes booting. A fresh pack resolves this — but if it happens on a new battery, let the pack charge to full before the first power-on. Starting the instrument with a partially charged new pack can still trigger the same cutoff.

T-400S display shows incorrect battery percentage after pack replacement

The T-400S uses a voltage-threshold method to estimate charge state, not a fuel gauge IC. When a new pack goes in, the instrument's internal reference still reflects the discharge curve of the old cell. The percentage shown will read inconsistently — often jumping or displaying low — until the instrument recalibrates its threshold against the new cell's curve. Run one full charge-to-discharge-to-full-charge cycle through normal instrument use. After that cycle, the display percentage stabilises. If it still reads erratically, check that the terminal voltage at the pack connector reads 10.8V ± 0.2V under no load.