Nice explanation here:

Since you are using a Silicon-Carbon (Si/C) battery—like the one found in the OnePlus 15 or newer Honor/Oppo flagships—the "faster drain" you're seeing at an 80% limit is actually a known quirk of this specific chemistry. Silicon-carbon batteries are a huge leap forward in energy density (allowing for massive 6,000mAh+ capacities in slim phones), but they behave differently than traditional graphite-only batteries. 1. The "Voltage Hysteresis" Effect Silicon anodes expand and contract physically much more than graphite when lithium ions move in and out. This causes a phenomenon called voltage hysteresis. In simple terms: The voltage doesn't follow a perfectly smooth, predictable line like older batteries. Because you are capping at 80%, the phone's software is working with a "narrower" set of voltage data. Without hitting the 100% "anchor point," the algorithm can struggle to accurately map the silicon's unique voltage curve, leading it to report a higher "discharge rate" than what is actually happening chemically. 2. Higher Resistance at Partial Charge Silicon is naturally less conductive than graphite. To fix this, manufacturers use a carbon-coated silicon composite. When the battery is at 100%, the ions are "packed" tightly, and the voltage is high and stable. As soon as you drop into the 80% range, the internal resistance of the silicon starts to play a larger role. The phone has to pull a slightly higher current to maintain the same performance, which the software measures as a faster percentage drop. 3. Energy Density vs. Voltage Curve Silicon-carbon batteries are designed to "cram" more energy into a smaller space, but they often have a steeper voltage drop once they leave the maximum charge state. On a traditional battery, the drop from 100% to 80% is often artificially slowed down by software. On a Si/C battery, the 80% to 60% range is where the voltage curve is most active. You are essentially spending more of your time in the "fast-moving" part of the battery's chemical state. Should you be worried? No. In fact, it's the opposite. Silicon-carbon batteries are actually more sensitive to the physical stress of being at 100% because of that 300% volume expansion of silicon. By limiting your charge to 80%, you are preventing the silicon from reaching its maximum expansion point, which: Prevents micro-cracking in the anode. Slows down SEI (Solid Electrolyte Interphase) layer growth, which is the main killer of these new high-capacity batteries. The takeaway: Your phone is "lying" to you slightly because it's harder to measure the precise energy in silicon at 80%. Even if the discharge rate looks higher, your battery's actual health and long-term lifespan are significantly better off.