Li-ion CC/CV Charging Curve Explained: Secrets of Constant Current & Constant Voltage
I still remember staring at the display of my first hobby charger years ago, watching the voltage creep up from 3.6 V to 4.2 V while the current sat stubbornly at 1.0 amp, and then — suddenly — the voltage froze at 4.20 V and the current began a slow, hypnotic decline. At the time, I had no idea what I was looking at. I just wanted to know if my battery was fully charged and whether I could yank it out and use it. It turns out I was watching the lithium-ion CC/CV charging algorithm perform exactly as it was designed to, and understanding what those two phases mean — constant current and constant voltage — is one of the most practical pieces of knowledge you can acquire if you use any rechargeable lithium cell, from 18650s and 21700s to the giant prismatic cells in a solar storage bank.
The CC/CV curve is not an arbitrary charging protocol. It is the direct engineering consequence of lithium-ion electrochemistry and the physical limits of the cell’s internal structure. Push current too high during the wrong phase, and you plate metallic lithium onto the anode, permanently reducing capacity and creating internal short-circuit risks. Terminate the charge too early, and you leave usable capacity on the table. This article unpacks each phase, explains why the cut-off current threshold matters, and shows how to pick a charger that respects the curve rather than fights it.
Phase One: Constant Current (CC)
When you connect a discharged lithium-ion cell to a charger, the first thing that happens — assuming the cell voltage is above the over-discharge threshold, usually around 2.5 V for NMC and 2.0 V for LiFePO4 — is that the charger applies a fixed, regulated current. This is the constant current (CC) phase. The charger holds the current steady — say, at 1 A, 2 A, or whatever rate you have selected — and the cell’s voltage rises gradually as it absorbs energy.
The voltage during this phase is not constant because the cell’s open-circuit voltage depends on its state of charge. For a typical NMC 18650 or 21700, the CC phase will take the voltage from wherever it started (often 3.4–3.6 V if you are topping up, or lower if deeply discharged) up to the cell’s maximum rated voltage: 4.20 V. For LiFePO4 cells, the CC phase runs up to 3.65 V. The time spent in CC mode depends on the charge current and the cell’s capacity. Charging a 3000 mAh cell at 1.5 A (0.5C) puts roughly half the cell’s rated capacity back per hour, so the CC phase lasts a little under two hours from empty to roughly 70–80% state of charge. At 3 A (1C), you are down to under an hour.
The CC phase is where charging speed is determined, and it is also where inexperienced users sometimes make dangerous choices. A high-quality 18650 like the Samsung 25R can safely accept a 1C charge (2.5 A) without significant degradation, while an energy-optimized cell like the Panasonic NCR18650B is rated for only 0.3C (roughly 1 A). Exceeding the manufacturer’s recommended charge current does not usually cause catastrophic failure immediately — the real damage is cumulative, accelerating the growth of the solid-electrolyte interphase (SEI) layer and gradually increasing internal resistance until the cell fades long before its rated cycle life. This is precisely why a charger that lets you choose the charge rate is essential, especially if you work with multiple cell chemistries and capacities. You can browse our Chargers category to find models with adjustable current settings and automatic chemistry detection — a feature that has saved me from accidentally charging a LiFePO4 cell at 4.2 V more times than I care to admit.
Phase Two: Constant Voltage (CV)
Once the cell voltage reaches 4.20 V (or 3.65 V for LiFePO4), the charger switches modes. From this point on, voltage is held constant, and the current begins to taper downward. This is the constant voltage (CV) phase, and it is where the remaining 20–30% of the cell’s capacity is absorbed.
Why can’t the charger simply cut off the moment 4.20 V is reached? Because at that instant, the cell’s internal electrochemistry has not finished redistributing lithium ions throughout the electrode structure. If you cut the charge immediately when the voltage first touches 4.20 V, the cell might only be at 70–80% of its true full capacity. The CV phase allows the current to naturally decay as the cell reaches saturation — the lithium ions need time to diffuse into the deeper layers of the anode material. This is not a design flaw; it is a physical diffusion process that cannot be rushed without damaging the cell.
During the CV phase, the charger monitors the tapering current. When the current drops below a preset cut-off threshold — typically 0.05C to 0.1C of the cell’s rated capacity — the charger declares the cell fully charged and terminates. For a 3000 mAh cell, 0.05C is 150 mA. Some chargers use a fixed cut-off, like 100 mA, regardless of cell capacity. Others let you adjust it. The choice of cut-off current matters: a high cut-off (say, 0.1C or 300 mA) leaves a small amount of capacity unused but reduces the time the cell spends at high voltage, which can slightly extend cycle life. A low cut-off (0.02C) squeezes every last milliamp-hour into the cell but subjects it to more time at the stress point — and the capacity gained in the last few percent is marginal compared to the total.
My personal practice for 18650 and 21700 cells that I intend to keep in long-term service: I charge to 4.15–4.18 V with a 0.05C cut-off when using a programmable charger, and I never leave cells sitting at 100% state of charge for more than a few hours if I can avoid it. That tiny voltage reduction has a disproportionate effect on calendar life, especially in warm environments.
Temperature: The Invisible Variable
No discussion of the CC/CV curve is complete without accounting for temperature. Lithium-ion charging is exothermic during the CV phase and slightly endothermic during the CC phase, but the real thermal risk comes from the environment. Charging a cell below 0 °C without a low-temperature charging protocol can cause immediate and irreversible lithium plating, as the anode’s ability to intercalate lithium ions is severely impaired. Even at moderate temperatures above 25 °C, the combination of CC heating and ambient warmth can push the cell into territory where the SEI layer grows faster, permanently increasing internal resistance.
Most quality chargers come with temperature sensors, and some (like the Xtar series) have external probes that attach to the cell body. I have written a dedicated deep-dive on this topic that covers safe temperature ranges, cold-weather charging strategies, and what to do if your charger lacks thermal monitoring: Battery Charging Temperature Guide – Optimal Charging Practices. If you charge batteries in a garage, shed, or vehicle where temperatures swing, read that guide before your next charge cycle.
Choosing a Charger That Respects the Curve
A charger that simply pushes current until a voltage is reached and then stops is not doing CC/CV properly — it is doing CC/CV poorly, or not at all. The ideal charger displays real-time voltage and current, lets you select the charge rate, terminates reliably at the correct cut-off current, and supports the specific chemistries you use.
The charger I keep on my bench for everyday use with 18650s, 21700s, and even NiMH cells is the Xtar VC4SL. It is a four-bay USB-C powered charger that automatically detects cell chemistry, displays voltage and current for each bay independently, and lets you manually override the charge rate from 0.25 A to 3 A. The display shows the CC-to-CV transition in real time, so you can actually watch the current taper begin. It includes temperature monitoring, overheat protection, and a gentle “wake-up” mode for over-discharged cells — the kind that have been sitting in a drawer at 1.5 V and need a slow trickle before they can safely accept a normal CC charge. You can find the Xtar VC4SL USB-C Charger in our product catalog, along with detailed specifications and compatibility notes.
Putting It All Together
If you take away nothing else from this article, remember three things. First, the CC phase delivers most of the energy, and the charge rate you select directly affects cell longevity — faster is not always better. Second, the CV phase is non-negotiable for full saturation, and skipping it by pulling cells off the charger the moment the light turns green (which often happens before the CV phase is truly complete) means you are leaving capacity unused. Third, the cut-off current threshold is a tuning knob between cycle life and usable capacity — and the manufacturer’s default is usually the right middle ground.
Understanding the CC/CV curve transforms battery charging from a blind waiting game into an observable, predictable process. Once you know what the voltage and current readings on your charger actually mean, you can diagnose weak cells, spot chargers with poor termination algorithms, and make informed decisions about when fast charging is acceptable and when patience is the better part of cell longevity.
When you are ready to upgrade your charging setup, Explore All Smart Chargers in our store. Filter by bay count, charge rate, chemistry support, and power source to find the charger that fits your workflow.