Charging is a controlled process: lithium uses CC/CV (constant current then constant voltage, no float), while lead-acid adds absorption, float, and optional equalization stages. Using the wrong profile — especially float-charging lithium — damages the battery.
The Short Answer
Battery chargers follow a voltage-and-current recipe matched to the chemistry. Lithium (LiFePO4, NMC) charges in two stages: Constant Current (CC) at a set current until the cell reaches its voltage ceiling, then Constant Voltage (CV) while current tapers to near zero — then charging stops. Lead-acid charges in more stages: bulk (CC), absorption (CV), float (maintenance voltage), and occasional equalization to balance cells. The critical rule: never use a lead-acid float profile on lithium — lithium does not tolerate being held at full voltage. Matching charger to chemistry is the single most important charging decision.
The Charging Stages
| Stage | Lithium (LiFePO4) | Lead-Acid |
|---|---|---|
| Bulk (CC) | Yes — constant current | Yes |
| Absorption (CV) | Yes — constant voltage | Yes |
| Float | No — stop at full | Yes — maintenance |
| Equalization | No (BMS balances) | Occasional, flooded only |
The table shows the core difference: lead-acid is designed to sit at float voltage indefinitely; lithium is designed to be disconnected at full charge. This is why lithium batteries shipped with “lead-acid chargers” fail early.
CC/CV Explained
CC/CV is the universal lithium charging algorithm. Phase 1 (CC): the charger holds a fixed current (e.g., 0.5C or 1C) while the voltage rises — most of the energy goes in here. Phase 2 (CV): once the cell hits the voltage ceiling (e.g., 3.65V for LiFePO4), the charger holds that voltage while the current tapers down to a cutoff (typically 3-5% of the rated current) — the last few percent of capacity. Charging ends when the current reaches the cutoff, not when a timer expires. CC/CV protects cells from overcurrent (CC phase) and overvoltage (CV phase). Battery charger design implements these stages automatically.
Bulk & Absorption
Lead-acid charging uses the same physics with different names: Bulk is the constant-current stage (charger delivers its max current, voltage climbs), and Absorption is the constant-voltage stage (voltage held at the absorption setpoint — e.g., 14.4V for a 12V flooded battery — while current tapers). The absorption time is critical: too short undercharges the battery (sulfation later), too long overcharges it (gassing, water loss). Smart chargers terminate absorption by current threshold or time. For lithium, bulk/absorption are the same as CC/CV — just named differently.
Float Charging
Float is a low-voltage maintenance stage for lead-acid batteries sitting on standby — typically 13.2-13.8V for a 12V battery — that tops up self-discharge losses without overcharging. Float is essential for lead-acid systems (inverters, vehicles, backup) and harmful for lithium. The related concept of trickle charging is also lead-acid specific. A lithium pack held at float stays at high SOC indefinitely, which accelerates aging and risks overcharge if the float voltage exceeds the cell ceiling. Modern chargers detect lithium and stop at full; if you must use a multi-profile charger, select the lithium profile explicitly. For long-term lithium storage, the guidance is 30-50% SOC, not float.
Equalization
Equalization is a controlled overcharge for flooded lead-acid batteries only: a deliberate voltage bump (e.g., 15.5-16.5V for 12V) for a limited time to stir the electrolyte, break up sulfation, and equalize cell voltages. It is not for AGM, gel, or any lithium chemistry — equalizing those destroys them. Schedule equalization every 1-3 months for flooded batteries with regular cycling, and only when the electrolyte level is correct. The BMS in a lithium pack performs the same balancing job internally, at the cell level, without any user action.
Lithium vs Lead-Acid Charging
- Voltage: lithium charges to a higher, flatter voltage per cell (3.2-3.65V for LiFePO4); lead-acid needs higher voltages to overcome internal resistance.
- Float: lead-acid wants float; lithium must not be floated.
- Temperature: lithium must not charge below ~0°C (plating risk); lead-acid can charge at lower temperatures with derating.
- Efficiency: lithium accepts ~95-99% of input; lead-acid ~75-85%.
- BMS: lithium packs supervise themselves via BMS; lead-acid depends on the charger.
LiFePO4 battery packs should be paired with a charger that has a LiFePO4 profile (or a smart charger that detects the chemistry). Using the wrong profile is the fastest way to reduce lithium cycle life.
Charging Mistakes to Avoid
- Wrong profile: always use the chemistry-specific profile.
- Cold charging: block charging below 0°C for lithium.
- Oversized charger current: follow the manufacturer’s max charge rate (C-rate).
- Overcharging lead-acid: too-long absorption boils the battery.
- Ignoring the BMS: never bypass BMS protections to “force” a charge.
FAQ
What does CC/CV mean in battery charging?
Constant Current / Constant Voltage: the charger delivers a fixed current while voltage rises, then holds a fixed voltage while current tapers. It is the standard lithium charging algorithm.
Do lithium batteries need float charging?
No — lithium must not be held at float voltage. Charging stops at full, and long-term storage is best at 30-50% SOC. Float is a lead-acid maintenance stage.
What is battery equalization?
A controlled overcharge for flooded lead-acid batteries that stirs electrolyte and equalizes cells. It must not be applied to AGM, gel, or lithium batteries.
What is the absorption stage?
For lead-acid, absorption is the constant-voltage stage after bulk, where the battery finishes charging while current tapers. Its duration must be controlled to avoid under- or overcharging.
Can I charge a LiFePO4 battery with a lead-acid charger?
Not safely with the default profile — the voltage setpoints and float stage are wrong for lithium. Use a charger with a LiFePO4 profile or a smart charger that detects chemistry.
Batteries & Chargers Matched by Jetray
Sources: Wikipedia – Battery Charger.
