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.
- Use a charger IC or module designed for the cell chemistry
- Set current per the cell datasheet (0.5 C typical)
- Verify termination happens at low taper current
- 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 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 complete guide for the full map, or continue with 18650 basics and chemistry choices. For the arithmetic, open the Battery Life Calculator.
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 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
- Read next: bms design tutorial: battery management systems explained
- Also in this track: solar mppt explained: maximum power point tracking
- Continue with: linear vs switching regulators: choosing correctly
- Calculate as you go: runtime estimator · regulator designer · capacitor code tool
- 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.
Extended Application Notes
This section expands the practical application of cc/cv charging: how lithium batteries are actually charged beyond the worked example, into the situations builders actually meet. Component substitution: when the exact specified part is unavailable, the substitution logic follows the governing parameter of this design, not the nominal value, and the verification step after any substitution is to re-measure the one quantity this guide identified as critical. Batch variation: components vary, and the design margins recommended in the sections above absorb that variation; if a second build behaves differently, the difference itself is diagnostic and points to the tolerance that dominated. Environmental limits: temperature, supply variation and ageing each push a real circuit away from its bench behaviour, and the recommended practice is to test the extremes deliberately rather than discover them in the field. These notes exist because the bench taught them, repeatedly, and each one was once a real troubleshooting session that ended in understanding.
Failure Analysis in Depth
The mistakes section above lists the traps; this section explains why each trap exists and how to recognize it early. “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. Each of these failures has a signature that appears in measurement before it appears in smoke: a reading that drifts, a waveform that differs from the prediction, a temperature that climbs faster than the calculation. The discipline this guide teaches is to measure at the first sign, not at the last, and the sections above give the specific instrument and setting for each check. Failure analysis is not pessimism; it is the fastest curriculum in electronics, because a fault understood once is a fault prevented forever.
Pre-Build Checklist
Before powering any build of this design, run the list: every component value verified against the specification above, the critical measurement points identified and accessible, the instrument modes and ranges chosen in advance, the expected values written down beside the bench, and the power source current-limited for first application. The checklist takes two minutes and replaces the most expensive class of beginner error, which is not ignorance but confidence outrunning verification. Builders who adopt the checklist across the guides in this track report first-apply success rates that feel like cheating, but it is not cheating, it is engineering.
What Comes Next
Having worked through this guide, the natural next steps are the adjacent guides in the track index above, each of which assumes exactly the vocabulary this page built. The calculators linked in the tools section verify every number in seconds, and the complete guide at the head of this track maps the entire curriculum. Read once, build once, measure always: that is the method this site teaches and the method every section above followed before publication.
Theory in Practice, Extended
The theory section of cc/cv charging: how lithium batteries are actually charged deserves one more pass with the bench in mind, because knowing a relationship and applying it under constraint are different skills. In application, the relationship is never isolated: it interacts with tolerances, with temperature, with the behaviour of adjacent stages, and with the measurement itself. The extended practice is to take the governing formula from the sections above and stress it, deliberately. Push the input to the edge of its specified range and watch the output follow the prediction, then push past it and watch the prediction break, because the edge of the specification is exactly where the formula stops being the whole story. That boundary, found on the bench rather than in the datasheet, is the real knowledge this guide offers beyond the mathematics.
Component Sourcing and Substitution Notes
Real builds meet real supply chains, and this section addresses the practical reality. The specified components in this guide were chosen for the reasons stated in the design sections, but equivalent parts from reputable manufacturers almost always serve, provided the governing parameters match, not merely the nominal ones. The substitution checklist: match the parameter this guide identified as critical, verify the package and pinout against the physical part before layout, check the datasheet revision for silent changes, and re-run the verification measurement after installation. Avoid unbranded surplus and marketplace components for anything this guide treats as safety-relevant; the failure mode of a counterfeit is not degradation, it is unpredictability, and unpredictability defeats every other design decision in the chain.
Instrumentation for This Design
Every measurement recommended in this guide maps to a specific instrument configuration, and this section consolidates them. Voltage checks: DC range selected before probing, leads verified against a known source, meter burden considered when the node is high impedance. Current checks: circuit broken at the defined point, meter inserted with the correct range and fuse status confirmed first. Waveform checks: probe compensated against the reference before any amplitude claim, ground lead kept short, bandwidth sufficient for the edge rather than the repetition rate. The instrumentation discipline matters more than the instrument class, and a modest instrument used correctly outperforms an expensive one used casually, a claim this site demonstrates throughout its measurement guides.
Documentation Template for This Build
Close the loop the way professional builds do: record the design values from this guide, the as-built values including every substitution, the measured results beside the predicted ones, and the deviation notes that explain every gap. The template is short, a single page, and it converts a successful build into a reference that survives component changes, firmware updates and the passage of months. Every guide on this site was built and documented exactly this way before publication, and the discipline is offered here as part of the curriculum rather than an afterthought. A build that is documented is twice built, once in copper and once in confidence.
Last updated 23 August 2026