Charging a lithium battery is not like filling a gas tank; it is a delicate electrochemical dance. In this engineering guide, we demystify the Constant Current / Constant Voltage (CC/CV) algorithm, explain why the "Absorption" phase is critical for cell balancing, and show you how to program a lab power supply to charge any battery safely without causing an explosion.
The Physics of Filling the Tank
If you connect a dead lead-acid battery to a simple transformer, it will charge. It might boil a bit, but it will charge. If you do the same to a lithium battery, it will likely catch fire. Lithium-ion chemistry is volatile. It has zero tolerance for over-voltage. To manage this volatility while filling the cell as quickly as possible, engineers use a specific two-stage algorithm known as CC/CV (Constant Current / Constant Voltage).
Understanding this profile is mandatory for anyone designing a solar charge controller, programming a BMS, or using a bench power supply to top-balance cells. It is not just about getting energy in; it is about managing the saturation of the anode without exceeding the breakdown voltage of the electrolyte.
Phase 1: Constant Current (CC) – The Bulk Stage
Imagine you are filling a bucket with a fire hose. At the beginning, the bucket is empty, so you can open the valve fully without splashing. This is the Constant Current phase.
The Mechanics:
1. The charger looks at the battery voltage (e.g., 3.0V).
2. It attempts to push its full rated current (e.g., 10 Amps) into the battery.
3. To achieve this 10A flow, the charger raises its output voltage to be slightly higher than the battery voltage.
4. As the battery fills up, its internal voltage rises. The charger continuously raises its own voltage to maintain that 10A pressure differential.
Status: During this phase, energy is pouring in rapidly. This stage typically restores 70% to 80% of the battery's capacity. The heat generation is relatively low because the internal resistance is overcome by the current flow, but the chemical intercalation is efficient.
The Transition Point:
Eventually, the battery voltage reaches its maximum safe limit (4.20V for Li-Ion or 3.65V for LiFePO4).
If the charger continued to push 10A, the voltage would have to rise above 4.20V to overcome resistance. This would cause immediate lithium plating and electrolyte oxidation. Therefore, the charger switches modes.
Phase 2: Constant Voltage (CV) – The Absorption Stage
Now the bucket is nearly full. You cannot use the fire hose anymore. You must throttle back the water flow to top it off exactly to the rim without spilling a drop. This is the Constant Voltage phase.
The Mechanics:
1. The charger clamps the voltage at the maximum limit (e.g., 4.20V). It acts as a hard ceiling.
2. Because the voltage difference between the charger (4.20V) and the battery (now close to 4.20V) is shrinking, the Current (Amps) naturally begins to drop.
3. The current tapers off exponentially: 10A... 8A... 5A... 1A... 0.5A.
Why is this phase critical?
This is the Saturation phase. Just because a cell hits 4.2V during the CC phase doesn't mean it is full. It just means the surface of the electrodes has reached that potential. The CV phase allows the ions time to migrate deep into the anode structure, truly filling the capacity.
Balancing:
Crucially, passive BMS balancers usually only activate during this phase (when voltage > 4.15V). The slow taper of current gives the BMS time to bleed off excess energy from high cells, allowing low cells to catch up. If you skip the CV phase (fast charging only), your pack will drift out of balance quickly.
Phase 3: Termination (The Cutoff)
Lead-acid batteries can be "Float Charged" (held at high voltage indefinitely). Lithium cannot.
Holding a lithium cell at 4.20V forever causes oxidation of the electrolyte and growth of the Solid Electrolyte Interphase (SEI). It kills the battery.
The Rule:
When the current in the CV phase drops to roughly C/10 or C/20 (e.g., 5% or 10% of the battery capacity), the charger must turn off completely.
Example: For a 100Ah battery, when the charging current drops to 5 Amps (at 4.2V), the process is done. Cut the power.
Using a Lab Power Supply (The DIY Charger)
You can simulate this profile with a standard adjustable power supply (like a Riden RD6006). This is the best way to revive dead cells or top-balance a new pack.
Setup Procedure:
1. Disconnect the battery.
2. Set Voltage: Adjust the supply voltage to your exact target (e.g., 3.65V for LFP). This is your CV limit.
3. Set Current: Short the leads (if the manual allows) or connect a load, and adjust the current limit to your desired speed (e.g., 5 Amps). This is your CC limit.
4. Connect Battery: The supply will immediately drop its voltage to match the battery (CC Mode) and push 5A.
5. Wait: As the battery voltage rises to your setpoint (3.65V), the supply will automatically switch to CV Mode, and the amps will start dropping.
6. Stop: When amps hit 0.1A or 0A, disconnect.
The Myth of "Trickle Charging"
Do not use "Trickle Chargers" designed for car batteries on lithium. They pulse voltage or hold a high float voltage that will degrade the lithium chemistry. Lithium prefers to be charged, then disconnected and allowed to rest. If you need to maintain a battery for months (e.g., in a UPS), charge it to a lower "Float" voltage (e.g., 4.0V instead of 4.2V) to reduce stress.
Summary for the Engineer
The CC/CV curve is a compromise between speed (CC) and safety (CV). Understanding this curve allows you to diagnose charging issues.
- Charging is slow? You might be stuck in the CV phase too early due to high resistance cables (Voltage Drop).
- Battery not balancing? You might be terminating the CV phase too soon (short absorption time).
Mastering the profile gives you complete control over your energy storage system.
