About this replacement
HT Instruments THT45 / THT46 / PQA824 — 3.7V Li-ion Replacement Battery (BAT45)
This is a 3.7V, 2600mAh Li-ion replacement battery for the HT Instruments THT45, THT46, and PQA824 portable test and measurement instruments. It replaces OEM part numbers BAT45, BAT45N, and YABA0003HT1. Voltage and cell format match the original pack exactly.
- THT45, THT46, and PQA824 compatibility: All three instruments share the same 3.7V single-cell architecture, connector pinout, and BMS handshake protocol — which is why one pack covers the entire group. Swapping between models carries no electrical risk as long as the OEM part number matches.
- Bench tested on actual hardware: We ran this pack through the THT45 power-on sequence and sustained sensor load cycles. The BMS held stable under the current spike at probe initialisation and did not trigger a false cutoff during a continuous logging session.
- Calibration cycle before first field deployment: After fitting this battery, run a full calibration cycle through the instrument menu before heading to site. The THT45 and THT46 map battery state during that routine — skip it and the instrument will flag premature low-battery warnings on the first measurement session.
BMS lockout after the instrument sat unused in a carry case for months
Li-ion cells left uncharged for extended periods drop below the BMS recovery threshold — typically around 2.5V per cell. When that happens, the protection circuit latches off and the instrument appears completely dead, even on a charger. The fix is to apply a slow pre-charge using a compatible Li-ion charger that supports recovery mode, which trickle-feeds current until the cell climbs back above the 2.8V re-enable threshold. Once the BMS re-initialises, a normal charge cycle will complete and the pack will function correctly.
Readings resetting or dropping out mid-logging session
This is not a firmware fault — it is a voltage dropout under sustained sensor load. Degraded or partially charged cells sag below the instrument's minimum operating voltage when the measurement circuit draws continuous current, causing the processor to reset mid-session. A fully charged new pack resolves the sag because the internal resistance is significantly lower than a worn original cell. Charge the replacement to 4.2V before the session and confirm the instrument shows a full charge indicator before starting a long logging run.