"I drove my car down to 20% battery on the motorway, and suddenly the petrol engine started revving loudly at 4,000+ RPM even at 60 km/h! Is my gearbox slipping?"

Short answer: Your car isn't broken — it doesn't even have a multi-speed gearbox! Here is how BYD's DM-i (Dual Mode Intelligent) system actually works and the exact settings to keep it whisper-quiet on long trips.

Why It Revs Loudly When Battery Drops Below 25%

Unlike a traditional car where engine RPM is locked to wheel speed, the Sealion 6 / Seal U DM-i operates primarily as a Series Hybrid below ~70 km/h (and under heavy load). The 1.5L engine spins a dedicated generator to feed electricity to the drive motor and Blade Battery.

• If you run the battery all the way down to 18%–25% SOC in EV Mode and then hit a steep hill or accelerate onto a 110 km/h motorway, the BMS enters Forced Charge Protection. It commands the engine to run at peak power RPM (~4,200 RPM) to rapidly rebuild a safety buffer in the battery.

The 3-Step Fix for Silent Highway Cruising

1. Swipe down on the DiLink screen → Energy Management.
2. Change SOC Save Mode from Auto to Custom and slide the target to 50% (or 60% if hilly) before your battery drops below 50%.
3. Switch the centre console toggle from EV to HEV as soon as you enter the motorway (80–110 km/h).
4. Once you are within 35–40 km of home (or your destination charger), flick it back to EV Mode so you arrive home at ~15–20% SOC and fill up on cheap solar/night-rate electricity!