The Toyota Prius — the car that made THS synonymous with 'hybrid.' (Image: EV & Auto Club)
I. The THS Era: When the Battery Was Weak, the Engine Stayed the Protagonist
The Toyota Hybrid System (THS) is the grandfather of mass-produced hybrid powertrains. Since the first Prius arrived in 1997, the system has evolved through five generations. Its core is a planetary gear set that couples the engine and the electric motors, and that mechanical design reflects a deep engineering compromise forced by the battery technology of its time.
Just how weak were those early batteries? The Toyota Camry Hybrid, a staple of the THS lineup, carried a physical battery pack of only 1.6 kWh. But the system was programmed to use only about 40% of that capacity — roughly 650 Wh — keeping the state of charge (SOC) tightly constrained between 38% and 75%. The fourth-generation THS used a nickel-metal hydride (NiMH) pack rated at 201.6 V and just 6.5 Ah. Even when the fifth generation finally switched to lithium-ion, the Camry Hybrid's battery shrank further to 1.12 kWh.
The "shallow charge, shallow discharge" strategy was born of necessity. By limiting the usable window, Toyota could push NiMH cycle life from a few hundred cycles to well over a thousand. But it also meant the battery was never treated as a serious power source. Real-world tests showed the Camry Hybrid could manage only about 4 km of pure-electric driving at an average speed of 37 km/h, consuming roughly 500 Wh. Recharging that small buffer by idling the engine was equally unimpressive: 26 seconds of idle charging added only about 25 Wh.
In the THS architecture, the engine remained the continuous mechanical driver of the wheels. The motors and the tiny battery were there to "trim" the engine — shifting its operating point toward the most efficient island. With such small capacity and low discharge power, the battery could not independently carry the full speed range. This was not because Toyota loved combustion engines; it was because the battery ceiling of that era left no alternative.
II. BYD DM-i: A Role Reversal Made Possible by Battery Leap
The BYD Qin plug-in hybrid, a high-volume DM-i model. (Image: Wikimedia Commons, CC BY 2.0, Rutger van der Maar)
BYD's DM-i family marked the arrival of the "electric-first" plug-in hybrid era.
The battery changed fundamentally. BYD's fifth-generation DM system uses a dedicated plug-in hybrid Blade Battery with an energy density of 115 Wh/kg, an improvement of 15.9% over the previous generation. Its continuous discharge rate reaches 16 C, and its regenerative braking feedback rate is 5 C. The Sealion 05 DM-i 220-km version, for example, carries a 26.628 kWh pack, supports 57 kW DC fast charging, and needs only 16.2 minutes to go from 30% to 80% SOC. Compared with the THS's 1.12 kWh sealed unit with no plug-in fast charging, this is a generational gap.
That leap flipped the roles inside the powertrain:
- Electric drive became the primary source. The P3 traction motor is the main propulsion unit.
- The engine retreated to a supporting role. It spends most of its operating time running in its efficiency sweet spot and generating electricity through the P1 motor; direct mechanical drive is engaged only when the road speed happens to coincide with that sweet spot.
- The battery enables nearly all daily driving in EV mode. With large capacity and high discharge power, combustion intervention drops dramatically.
The architectural difference is telling. THS uses a planetary gear for mechanical power blending; DM-i relies on power electronics for energy distribution. The former is a clever mechanical workaround for limited electric capability; the latter replaces complex mechanics with software and silicon once the battery became strong enough.
III. Why Japanese Makers Fell Behind: A Slow Bleed of Battery Competence
Japan's battery lag was structural. Toyota stuck with NiMH cells for more than two decades and did not move the THS to lithium-ion until the fifth generation around 2022. During that same window, China's power-cell industry moved from lithium iron phosphate (LFP) to nickel-cobalt-manganese (NCM) and on to semi-solid-state concepts. By the time Toyota finally adopted lithium-ion, BYD was already mass-producing Blade Batteries capable of 16 C discharge.
There was also a path dependency on the internal combustion engine. Japanese automakers had spent decades perfecting engines, transmissions, and mechanical drivetrains. That hard-won expertise became a form of inertia. Toyota kept refining the planetary gear and engine-centric controls; BYD started from the assumption that the battery and motor should carry the load.
The result is visible in the fifth-generation product cycle. BYD's fifth-generation DM, unveiled in May 2024, opened what the company calls the "2-liter fuel-consumption era" — sub-3 L/100 km in real-world use. Toyota's fifth-generation THS upgrade, by contrast, focused on higher power density and a 17% weight reduction for the power-control unit. Useful improvements, but not on the same axis as the battery-driven transformation happening in China.
IV. Conclusion: The Battery as the Anchor
The evolution from THS to DM-i has a single clear logic.
When the battery is weak, the hybrid system must be engine-first and motor-second. When battery capacity and discharge rate improve by orders of magnitude, the roles can invert, making electric drive primary and the engine an auxiliary range extender.
Toyota did not lose because its planetary gear was poorly engineered; the gear is still a mechanical masterpiece. It lost because the battery industry moved faster in China, and because Japan's dominance in engine engineering made it harder to abandon the combustion-first mindset. As solid-state and higher-rate batteries mature, the electrification of hybrids will only deepen. And the leadership in that transition now sits with the companies — and the supply chains — that own the battery technology.
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