What Is The Expected Intensity Formula In CCV?
The expected intensity formula in CC-CV (Constant Current-Constant Voltage) charging calculates current (I) during the CC phase as I = C-rate × Capacity (Ah), switching to CV when voltage peaks. Termination occurs when current tapers to 3–5% of initial CC current. For lithium-ion, CC phase applies 0.2–1C until ~80% SOC, then CV holds voltage (e.g., 4.2V/cell) while current decays exponentially.
What defines the CC-CV charging phases?
CC-CV involves two-stage charging: first constant current (rapid bulk charge), then constant voltage (topping charge). Transition triggers at ~80% SOC when cell voltage nears its upper limit (e.g., 4.2V for Li-ion). Pro Tip: Use temperature-compensated voltage thresholds—heat lowers required CV by 3mV/°C to prevent overcharging.
In CC mode, a 100Ah battery charged at 0.5C receives 50A until voltage reaches 72V (for a 20S LiFePO4 pack). The BMS then switches to CV, reducing current to avoid gassing. For example, a 72V30Ah e-scooter battery charging at 10A (0.33C) takes 2.5 hours in CC, then 1 hour in CV. But why does CV take longer? Because ionic diffusion slows as cells near full capacity, requiring lower currents. Transitional phrases like “Practically speaking” help link concepts.
How is current intensity calculated during the CC phase?
CC current depends on C-rate and pack capacity. Formula: I_CC = C-rate × Capacity (Ah). A 0.5C charge for a 50Ah battery = 25A. Pro Tip: Higher C-rates (≥1C) reduce cycle life—NMC cells lose 15% capacity after 500 cycles at 1C vs 8% at 0.5C.
| C-rate | Current (50Ah) | Time (CC Phase) |
|---|---|---|
| 0.2C | 10A | 5h |
| 0.5C | 25A | 2h |
| 1C | 50A | 1h |
Transitioning to CV, consider a 72V LiFePO4 pack: 24 cells × 3.6V = 86.4V fully charged. CC phase applies 25A until 86.4V, then CV holds while current drops to 2–3A. But what if capacity degrades? Aged cells hit CV faster due to increased internal resistance, shortening CC duration. Use adaptive C-rates for aged packs—reduce by 0.1C per 20% capacity loss.
What determines the CC-to-CV transition voltage?
The cell chemistry and pack configuration set the transition voltage. For 72V LiFePO4 (24S), CV starts at 86.4V (24 × 3.6V). NMC packs (20S) trigger CV at 84V (20 × 4.2V). Pro Tip: Balance cells before charging—≥50mV imbalance delays CV phase, causing overcharge in weak cells.
Take a 24S LiFePO4 golf cart battery: each cell’s 3.65V CV limit sums to 87.6V. However, manufacturers often set CV 2–3% lower (85V) to accommodate cell variances. Why the buffer? To prevent the BMS from tripping due to minor imbalances. For instance, a 72V30Ah pack with 10mV cell imbalance might hit CV at 84.5V instead of 85V, extending CC time by 15 minutes. Transitional phrases like “Beyond voltage thresholds” help structure explanations.
How does temperature affect CC-CV intensity?
Temperature alters internal resistance—cold increases it, requiring higher voltage to maintain CC current. At -20°C, a 72V pack may need 15% higher voltage to sustain 0.5C. Pro Tip: Use chargers with temperature compensation (-3mV/°C/cell) to adjust CV thresholds.
| Temp (°C) | CC Voltage Adjustment | CV Voltage Adjustment |
|---|---|---|
| 25 | 0% | 0% |
| 0 | +5% | -2% |
| 45 | -4% | -5% |
For example, a 72V pack at 0°C might charge at 75.6V CC (vs 72V nominal) to overcome resistance. Conversely, high temps (45°C) lower CV to 81V (from 84V) to prevent electrolyte breakdown. But how critical is this? A 5% overvoltage at 45°C can accelerate SEI layer growth, reducing cycle life by 30%.
Why is termination current crucial in CV phase?
Termination current (I_term) ends charging when CV current drops to 3–5% of initial CC current. For a 30A CC charge, I_term = 0.9–1.5A. Lower I_term increases capacity but prolongs charge time. Pro Tip: Set I_term at 0.03C for precision—1.5A for a 50Ah pack.
Consider a 72V100Ah LiFePO4 pack: 100A CC charge (1C) transitions to CV at 87.6V. Charging terminates at 3A (0.03C), adding 2–3% capacity versus 5A termination. But is the extra time worth it? For grid storage, yes—each cycle matters. For EVs, faster termination (5A) balances time and capacity. Transitional phrases like “In practical terms” aid flow.
How do battery chemistries affect CC-CV parameters?
Chemistry dictates voltage limits and C-rates. LiFePO4 uses 3.65V/cell CV vs NMC’s 4.2V. Lead-acid needs 2.45V/cell CV with tapering currents. Pro Tip: NiMH requires -ΔV detection for termination—unsuitable for pure CC-CV.
For example, a 72V LiFePO4 (24S) charges at 87.6V CV, while a 72V NMC (17S) uses 71.4V CV. But why the difference? LiFePO4’s flat voltage curve needs precise CV to avoid overcharge, whereas NMC’s sloping curve allows broader CV ranges. Transitional phrases like “Considering these factors” improve readability.
Battery Expert Insight
FAQs
Yes, but CV voltage is lower (2.4–2.5V/cell) and termination current higher (0.05C). Avoid equalization phases common in Li-ion protocols.
What happens if CC current is too high?
Excessive current (≥2C) causes voltage overshoot, premature CV transition, and lithium plating in Li-ion cells, reducing capacity by 40% in 100 cycles.
How does cell balancing affect CV phase?
Imbalanced packs hit CV earlier, leaving weak cells undercharged. Active balancing during CC ensures all cells reach CV simultaneously, improving capacity by 10–15%.