LiFePO4 (Lithium Iron Phosphate) batteries are widely used in energy storage, electric vehicles, and solar systems due to their high safety, long cycle life, and stable performance. In practical applications, multiple batteries are often connected in series or parallel to meet different voltage and capacity requirements. However, incorrect charging methods can lead to battery damage, reduced performance, or even safety hazards.

This article will explain in detail:

• Voltage changes after series and parallel connections

• Correct charging voltage and current selection

• The role of BMS (Battery Management System)

• Common mistakes and solutions

2. Basic Parameters of LiFePO4 Batteries

Before discussing charging methods, let’s review the key parameters of a single LiFePO4 battery:

• Nominal voltage: 3.2V (fully charged ~3.65V, fully discharged ~2.5V)

• 12V LiFePO4 battery: Typically consists of 4 cells in series (4S), nominal voltage 12.8V, charging voltage 14.2V~14.6V.

• 24V LiFePO4 battery: 8S (25.6V), charging voltage 29.2V.

• 48V LiFePO4 battery: 16S (51.2V), charging voltage 58.4V.

3. Charging After Series Connection

3.1 Voltage Change in Series Connection

• Voltage adds up, capacity remains the same.

• Example:

  • 4 × 12V batteries in series = 12V × 4 = 48V (capacity remains the same as a single battery, e.g., 100Ah).

3.2 Charging Voltage Calculation

• Each 12V LiFePO4 battery has a charging voltage of 14.2V~14.6V.

• 4 in series: 14.6V × 4 = 58.4V (max charging voltage).

• Therefore, a 48V LiFePO4-specific charger with an output range of 56V~58.4V must be used.

3.3 Charging Current Selection

• In series, current flows through all batteries, so the charging current must not exceed the maximum allowed charging current of a single battery.

• Example: If a single battery supports 50A charging, the max charging current for the entire 48V battery pack remains 50A (not additive).

3.4 Importance of BMS (Battery Management System)

• In series, voltage imbalance may occur, leading to overcharging or undercharging of some cells.

• Functions of BMS:

  • Balances cell voltages to prevent overcharge (>3.65V/cell) or over-discharge (<2.5V/cell).

  • Provides overcurrent, short-circuit, and temperature protection.

4. Charging After Parallel Connection

4.1 Voltage Change in Parallel Connection

• Voltage stays the same, capacity adds up.

• Example:

  • 4 × 12V 100Ah batteries in parallel = 12V (voltage unchanged), capacity = 100Ah × 4 = 400Ah.

4.2 Charging Voltage Calculation

• After parallel connection, the charging voltage remains the standard 12V LiFePO4 charging voltage (14.2V~14.6V).

• A 12V LiFePO4-specific charger must be used.

4.3 Charging Current Selection

• After parallel connection, the total charging current can increase, but it is recommended not to exceed 0.5C (where C = total capacity).

  • Example: 4 × 100Ah in parallel (400Ah), max charging current = 400Ah × 0.5 = 200A.

  • In practice, lower currents (e.g., 0.2C~0.3C) are safer and extend battery life.

4.4 Is BMS Necessary for Parallel Connection?

• If all batteries have the same voltage, internal resistance, and health, a single BMS can be used.

• If batteries are mismatched (e.g., old and new mixed), current distribution may be uneven. Recommended solutions:

  • Use individual BMS for each battery or employ balanced parallel wiring.

5. Charging After Series-Parallel Hybrid Connection

5.1 Common Configurations (e.g., 2S2P = 24V System)

• Connection method:

  1. First, connect two batteries in series (12V × 2 = 24V).

  2. Then, connect the two 24V groups in parallel (capacity adds up).

• Example:

  • 4 × 12V 100Ah batteries → 2S2P → 24V 200Ah.

5.2 Charging Voltage Calculation

• The standard charging voltage for a 24V LiFePO4 battery is 29.2V (14.6V × 2).

• Use a 24V LiFePO4-specific charger (output voltage 28V~29.2V).

5.3 Charging Current Selection

• Current depends on the parallel-connected capacity:

  • Example: 2 groups in parallel (200Ah), recommended charging current ≤ 0.5C = 100A.

5.4 BMS Configuration

• Each series-connected group should have its own BMS before parallel connection.

6. Common Mistakes and Solutions

• Series connection → Voltage adds up; use a charger matching the total voltage (e.g., 58.4V for a 48V pack).

• Parallel connection → Capacity adds up, voltage remains the same; charging current can increase (but ≤0.5C recommended).

• Series-parallel hybrid → Calculate total voltage and capacity first, then select the appropriate charger.

• Always use a LiFePO4-specific charger + BMS to prevent overcharging, over-discharging, and uneven current distribution.

Proper charging significantly extends LiFePO4 battery life and ensures safety and performance. For further doubts, consult a professional battery supplier or engineer.