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Can We Connect Lead Acid and LiFePO4 Batteries in Parallel?

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Connecting lead acid and LiFePO4 (Lithium Iron Phosphate) batteries in parallel is generally not recommended due to significant inherent differences, but can be technically feasible with careful management.

Key Challenges & Risks:

Voltage Discrepancy:

  • Nominal Voltage: Lead acid batteries (e.g., 12.6V fully charged for a 6-cell) have a slightly different nominal voltage than LiFePO4 batteries (e.g., 12.8V for a 4-cell). This minor difference can cause current flow between batteries and imbalance during charging and discharging, leading to one battery constantly trying to charge or discharge the other.
  • Voltage Sag: During high discharge, lead acid batteries experience a more pronounced voltage sag, further complicating parallel operation.

Charging Requirements:

  • Charge Voltage Profiles: Lead acid batteries require a multi-stage charging process (bulk, absorption, float), while LiFePO4 batteries need a precise Constant Current/Constant Voltage (CC/CV) charge profile.
  • Overcharging/Undercharging: A charger optimized for one type will likely damage or undercharge the other when connected in parallel, leading to reduced lifespan and safety hazards.

Discharge Characteristics & Capacity:

  • Discharge Rates: LiFePO4 batteries can sustain higher discharge rates with less voltage drop. When paralleled, the LiFePO4 often carries a disproportionately higher load, potentially leading to over-discharging of the lead acid or premature shutdown by the LiFePO4’s internal BMS.
  • Usable Capacity: LiFePO4 batteries offer nearly 100% usable capacity, whereas lead acid typically offers only 50-70%. This disparity complicates energy management in a mixed system.

Internal Resistance & Health:

  • Internal Resistance: Different chemistries have varying internal resistance. This affects how current is shared and can lead to uneven loading and heating.
  • Battery Degradation: Mixing batteries of different ages, health, or types accelerates battery degradation for all connected cells due to the constant stress of imbalance.

 

 

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Risks Involved:

  • Safety Hazards: Imbalances can cause overheating, thermal runaway (especially for LiFePO4 if improperly charged), or battery swelling, leading to damage or fire.

  • Reduced Efficiency: Energy is lost managing the inherent imbalances, decreasing overall system efficiency.

  • Shortened Lifespan: Both battery types operate outside their optimal parameters, significantly reducing their expected cycle life.

Best Practices and Alternatives:

  • Separate Systems: The safest and most efficient approach is to maintain separate battery banks for each chemistry.

  • Advanced BMS: For unavoidable mixed setups, a highly sophisticated, custom Battery Management System (BMS) is essential. This advanced BMS must be capable of independent monitoring, cell balancing, and charge/discharge control for each battery type simultaneously.

How Himax Electronics Can Help:

Himax Electronics specializes in custom BMS solutions that can manage complex, mixed-chemistry energy storage systems. Our expertise helps design configurations that prioritize safety and efficiency, mitigating the inherent risks of parallel connections between disparate battery chemistries.

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