The Physics of Lithium Battery Charging: CC, CV, and Float
Volume I · May 2026 · 923 words
The charge time specification on a portable power station — "0–80% in 45 minutes" — describes only the first of two charging phases. Understanding why charging slows after 80%, and why the final 20% takes as long as the first 80%, requires understanding the two-phase CC-CV charging protocol used by every lithium battery charger.
The CC-CV Protocol
Lithium batteries are charged in two sequential phases:
Phase 1: Constant Current (CC)
The charger delivers a fixed current — typically 0.2C to 1C, where C is the battery's capacity in amp-hours divided by 1 hour. For a 20 Ah LiFePO₄ battery, 1C = 20 A. During CC, the battery voltage rises as lithium ions move from the cathode to the anode. The charger maintains constant current by increasing its output voltage as the battery voltage rises. This phase ends when the battery reaches its terminal voltage — 3.65 V per cell for LiFePO₄, 4.20 V for NMC. At this point, the battery is approximately 80% charged.
Phase 2: Constant Voltage (CV)
The charger holds the terminal voltage constant. As the battery approaches full charge, the voltage difference between the charger and the battery decreases, and the current naturally tapers — following Ohm's law. The CV phase ends when the current drops below a termination threshold, typically 0.05C (1 A for a 20 Ah battery). At this point, the battery is fully charged and the charger disconnects.
The CV phase takes as long as the CC phase because current tapers exponentially. The last 5% of charge (95–100% SOC) may take 30–50% of the total charge time, depending on the termination threshold. This is why fast-charge specifications quote 0–80% time: the last 20% is slow regardless of charger power.
Why Not Charge Faster?
Charging above 1C accelerates degradation through two mechanisms: lithium plating at the anode (metallic lithium deposits instead of intercalating, permanently reducing capacity) and increased cell temperature (Arrhenius acceleration of side reactions). The 1C limit is a compromise between charge speed and cycle life. Some cells are rated for 2C or 3C charging (primarily LTO chemistry), but the cycle life penalty is 20–40% per doubling of charge rate. For the 0.2–0.5C charge rates typical of portable power stations, the cycle life impact is negligible — the manufacturer has already derated the charger to a conservative level.
Float Charging and Lithium Batteries
Lead-acid batteries are float-charged: held at a constant voltage indefinitely to compensate for self-discharge. Lithium batteries should not be float-charged. Holding a lithium cell at its terminal voltage after full charge drives electrolyte decomposition and accelerates calendar aging. A properly designed lithium charger terminates charge completely when the current drops below the threshold. If the battery self-discharges to a restart threshold (typically 95% SOC), the charger initiates a new charge cycle rather than maintaining a continuous float voltage.
This is why keeping a power station plugged in indefinitely is not recommended: the charger cycles between full charge and restart, repeatedly exposing the battery to its terminal voltage. Each micro-cycle causes a small amount of calendar aging. For long-term storage, charge to 50–70% and disconnect. See our maintenance guide.