Section 4 - Charging and Discharging

Section 4 · Charging & Discharging Protocols

Charging and Discharging Protocols

A lithium-ion cell will not tolerate the open-loop charging that lead-acid accepts. It demands an algorithm that enforces a voltage ceiling, tapers current against it, and terminates rather than floats.

Push past that ceiling and the cell plates lithium or sheds cathode oxygen; hold a finished cell at voltage and it gasses. At pack scale the problem compounds, because cells in series drift apart and the weakest one sets the limit for all of them — which makes the battery management system, not the charger, the component that actually keeps the pack inside its safe operating area.

Fig. 4.0 — The CC-CV charge. In constant current, a fixed current drives the voltage up to the 4.2 V ceiling — this restores ~60–70% of capacity. In constant voltage, the voltage is held and the current tapers naturally as the cell fills. Charge terminates when current falls to ~C/10; it is never held at voltage.

The operative rule

A lithium-ion cell is charged to a finish and then disconnected — never maintained at voltage. The current taper is the safety mechanism: rather than forcing more lithium in, the charger lets the cell take progressively less, then stops.

What this section covers

The topics build from the single cell to the pack. C-rates and temperature constraints set how fast a cell can safely take charge. The CC-CV algorithm and its fast-charge variants define the charge profile and why trickle charging is incompatible with the chemistry. BMS architecture closes the section with the topologies, balancing strategies, and state-estimation algorithms that manage a multi-cell pack.

Topics in this section

• 4.12  Standard charging protocols and C-rates
• 4.13  The CC/CV algorithm and charge modalities
• 4.14  BMS architecture, balancing, and state estimation

Battery Shortcut · Li-ion Cell EssentialsSection 4 of 7