CC/CV Charging: How Lithium Batteries Are Actually Charged

Lithium charging is constant current until the cell reaches 4. 2 V, then constant voltage while current tapers terminate near 3–5 % of rated current. Simple to state, unforgiving to violate.
> At a glance: 6 minute guide · part 8 of 10 in the power supplies and batteries complete guide track · includes a worked example and a quick-reference table.

## Phase one: constant current

The charger acts as a current source (typically 0. 5 C). Cell voltage climbs steadily. This phase restores most capacity. The CC limit exists because lithium cells accept current, not voltage, while below target.

| Phase | Condition | Behaviour |
| — | — | — |
| CC | Vcell < 4.2 V | Constant current, voltage rising | | CV | Vcell = 4.2 V | Voltage held, current tapers | | Termination | I < ~0.03–0.05 C | Charge complete | | Balancing | During CV | Highest cells bled | | Fault | Out of range | BMS disconnects | ## Phase two and termination What this means at the bench: At 4. 2 V the charger holds voltage and watches current fall as the cell saturates. Terminating too early leaves capacity. Never terminating stresses the cell dedicated charger ICs make the decision automatically and safely. ## Balancing and the BMS Multi-cell packs add balancing: bleeding the highest cells slightly during CV so all reach full together. The BMS enforces per-cell limits it is protection, not a charger, a distinction that saves packs. ## How to apply this in your build Work through the sequence below each step assumes the previous one passed. For numbers that need calculating, the linked tools at the end of this guide do the arithmetic instantly. 1. Use a charger IC or module designed for the cell chemistry 2. Set current per the cell datasheet (0.5 C typical) 3. Verify termination happens at low taper current 4. For packs, confirm balancing activates during CV ### Worked example Charging a 2500 mAh cell at 1. 25 A: CC phase reaches 4. 2 V in ~90 minutes with ~70 % restored. The CV tail needs another hour for the rest. Impatience here costs capacity, not safety. Run the numbers yourself with the Battery Life Calculator and the result should agree to within rounding.

> Practical note from the bench. Charger selection gets the same scrutiny as cell selection in our builds the IC datasheet is a short, worthwhile read.

## Field mistakes we see again and again

– “Trickle topping” a full lithium cell there is no float, only stress
– Setting charge current above the cell datasheet rating
– Treating the BMS protection trip as a charging strategy

## Key takeaways

– Phase one: constant current the foundation of this guide; revisit it if any measurement here surprises you.
– Phase two and termination the foundation of this guide; revisit it if any measurement here surprises you.
– Balancing and the BMS the foundation of this guide; revisit it if any measurement here surprises you.

## Prerequisites and preparation

Before starting: use a charger ic or module designed for the cell chemistry and set current per the cell datasheet (0. 5 c typical). Keep the [Battery Life Calculator](/tools/battery-life) open every number in the worked example is reproducible. Total time including the bench steps: about 6–6 minutes.

## Who benefits most

Hobbyists meeting this topic for the first time, students who want the version with real numbers instead of abstract symbols. Returning engineers refreshing a corner of the craft. The mistake list alone justifies the visit every entry in it was learned the expensive way.

### Quick reference card

| Aspect | Where to find it in this guide |
| — | — |
| Core theory | Phase one: constant current |
| Application steps | How to apply this in your build |
| Worked numbers | Worked example |
| Failure modes | Field mistakes we see again and again |

## How this fits the power supplies and batteries complete guide track

This guide is one stop in the structured learning path. Start from the [power supplies and batteries complete guide](/tutorial/power-batteries-complete-guide) pillar page for the full map, or continue with [18650 basics](/tutorial/18650-battery-guide) and [chemistry choices](/tutorial/lithium-ion-vs-lipo). For the arithmetic, open the [Battery Life Calculator](/tools/battery-life).

## Frequently asked questions

Can I fast-charge lithium?
Modern cells often accept 1–2 C with approved chargers; heat and cycle life pay for speed.

Why does charging slow near full?
The CV phase saturates the cell current decays exponentially as the last ions find seats.

Is there a calculator for this?
Yes the [Battery Life Calculator](/tools/battery-life) tool runs the formulas from this guide instantly, client-side, with no signup.

## Keep going with this track

– The complete power & batteries guide: [Power & Batteries complete guide](/tutorial/power-batteries-complete-guide)
– Read next: [bms design tutorial: battery management systems explained](/tutorial/bms-design-tutorial)
– Also in this track: [solar mppt explained: maximum power point tracking](/tutorial/solar-mppt-explained)
– Continue with: [linear vs switching regulators: choosing correctly](/tutorial/linear-vs-switching-regulators)
– Calculate as you go: [runtime estimator](/tools/battery-life) · [regulator designer](/tools/lm317-regulator) · [capacitor code tool](/tools/capacitor-code)
– Bookmark this page against the day a measurement surprises you. Most readers return to the table and the mistake list first, and that is the correct order.

## Working method notes

Keep a lab notebook entry for every build in this track. The measured values, the deviations from the guide and the reason for each. Six months from now, those notes are worth more than any tutorial. They describe your bench and your components rather than a general case.

When a result here disagrees with your expectation, write down both numbers before changing anything. The gap between predicted and measured is where the real engineering lives. It is usually a tolerance, a parasitic or an assumption that was never checked.
## Formulas and checks from this guide

Verification checklist for this track. Verify regulation under load, not just open-circuit, measure inrush where it matters. Treat every lithium cell as energetic chemistry that has earned its protection chain.

Bookmark this page against your next build in the track. The checklist above is the same one used across 18 guides in this series.

## What the bench taught us

No. Oversizing hammers the diodes with inrush and buys ripple you no longer need once regulation follows.

Any lithium pack of 2S or more, without exception.

## How to revisit this guide

Second readings work best with a purpose. Pick one section from Phase one constant current,Phase two and termination,Balancing and the BMS and rebuild only that part at the bench, predicting each value before measuring. Prediction errors mark exactly which concept needs the next pass, and the linked power calculators resolve any arithmetic doubt in seconds. Keep the marked sections in your notebook: after a month of builds, that list becomes your personal Power syllabus.