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If there are problems when replacing the lead-acid batteries in a sedan car with lithium batteries, does it necessarily mean that the problem lies with the lithium batteries?
2025-08-13
Everyone knows that many sedan cars still use the original factory-configured lead-acid batteries (such as EFB or AGM start-stop batteries). Therefore, the vehicles are equipped with generators of corresponding power and output voltage based on the characteristics of lead-acid batteries, such as the full charge voltage of about 12.6V (float charge voltage of 13.8 - 14.4V). For the nominal output voltage: 12V system (the actual working voltage is 13.5 - 14.5V, to be compatible with the charging requirements of lead-acid batteries). Output power: For ordinary family cars: approximately 1kW - 2kW (such as 80A × 14V = 1120W). For high-performance cars or high-specification models: up to 3kW (to meet the requirements of high-power audio systems, seat heating, etc.). Common output current range: 50A - 150A (matched with the electrical load of the vehicle model, such as compact cars about 60A, luxury cars may exceed 100A).
The working principle of the sedan alternator is as follows:
The engine drives the rotor (excitation coil) of the generator to rotate through a belt, generating three-phase alternating current in the stator windings.
This alternating current is converted into direct current by a diode rectifier bridge for car use.
Voltage regulation:
The built-in voltage regulator adjusts the output voltage by controlling the rotor excitation current based on the battery charge level and the load requirements, avoiding overcharging or undercharging.
Collaboration with the battery:
Startup phase: The battery supplies power alone (the starting motor requires hundreds of amperes of current, and the Alternator does not participate).
Operation phase: The alternator supplies power first, and the excess energy is charged to the battery.
Idle/low load: The alternator output is reduced to lower the engine burden.
Load management:
Modern vehicles may be equipped with an intelligent power management system, which turns off non-essential loads (such as seat heating) when the battery is fully charged, prioritizing the protection of critical equipment.
The relationship between the alternator and other car equipment is as follows:
Battery: Acts as an energy buffer, providing power when the alternator is not working (such as in the engine-off state).
Stabilizes the system voltage and absorbs instantaneous large currents (such as when the starting motor is operating).
Electrical equipment: All equipment is connected in parallel to the output of the generator/battery, protected by fuses/relays.
High-power equipment (such as air conditioning compressors) may be directly driven by the engine, but the control circuit relies on power supplied by the alternator.
Fault impact: When the alternator fails, the vehicle is powered solely by the battery, and it may stop running after traveling for several kilometers due to depleted battery.
Excessive voltage may damage electronic equipment, while too low voltage will cause the battery to lose power.
If using lithium iron phosphate batteries replace lead-acid batteries, car owners can enjoy the advantages of lithium battery’s lighter weight, longer lifespan and faster charging and discharging. However, they may encounter the following problems:
1. Voltage mismatch leads to abnormal charging.
Problem: The full charge voltage of a lead-acid battery is approximately 12.6V (float charging voltage is 13.8 - 14.4V), while that of LiFePO₄ is approximately 13.6V (recommended charging voltage is 14.2 - 14.6V)..
2. Compatibility issue with start-stop system
Problem: The charging & discharging characteristics of lead-acid batteries (such as instantaneous large current discharge) are different from those of LiFePO₄. The current detection logic of the original vehicle's start-stop system may misjudge. For example: The internal resistance of LiFePO₄ is low, and the voltage drop during discharge is small. The vehicle may mistakenly think the battery is fully charged, while in fact it is almost depleted.
Phenomenon: The start-stop function frequently fails or is wrongly triggered.
3. Performance difference in low-temperature conditions
Problem: The charging efficiency of LiFePO₄ significantly decreases at low temperatures (such as <0°C), and the BMS may prohibit charging to protect the battery. Although lead-acid batteries also perform poorly at low temperatures, they can still charge slowly. Phenomenon: The vehicle cannot start or the charging alarm occurs in cold weather.
4. Conflict between BMS and vehicle systems
Problem: The LiFePO₄ battery requires an external or internal BMS for charging and discharging management, but the original vehicle's generator and circuits may not be able to recognize the BMS signals. For example, when the BMS actively cuts off charging, the vehicle may mistakenly identify it as a battery fault and trigger a fault code (such as "Charging System Fault").
Symptom: The dashboard warning lights come on (such as the battery light, ESP fault light).
5. Incompatible charging strategy
Problem: Lead-acid batteries allow "float charging" (maintaining a voltage of 13.8 - 14.4V for a long time), but LiFePO₄ will age more rapidly under long-term high-voltage float charging.
When the vehicle is traveling at high speed, the alternator may continuously output high voltage, causing the lithium battery to overcharge.
So, when replacing lead-acid batteries with lithium iron phosphate batteries, we need to consider:
1. Select LiFePO₄ batteries specifically designed for automobiles
Only choose LiFePO₄ start-stop batteries with vehicle-grade BMS. The BMS should support： overcharge/overdischarge protection，low-temperature charging restriction，and communication with the vehicle's CAN bus (some high-end models may require matching)
2. Adjust the charging voltage (requires professional operation)
Use OBD tools or flash the ECU to modify the output range of the vehicle's voltage regulator (target： 14.2 - 14.6V).
Install an external voltage regulator (such as a DC-DC charger)， and adapt the generator output to the LiFePO₄ requirements.
3. Compatibility testing and system reset
After replacement，perform：
Use a diagnostic tool to clear the original battery memory data (to prevent the system from using the charging strategy of lead-acid batteries).
Test the start-stop function with switch on high beam load， air conditioning etc. to ensure they are normal.
Monitor voltage fluctuations (keep within the range of 13.8 - 14.6V).
4. Cold environment response
In cold regions：
Choose LiFePO₄ batteries with low-temperature self-heating function (such as Yesa software version battery support starting at -30°C).
Try to connect a charger when parking (lithium batteries have a low self-discharge rate， but it is necessary to avoid completely discharging).
5. Install protection modules
Install battery isolators or intelligent relays to prevent the vehicle's sleep circuit from discharging the lithium battery with a small current (LiFePO₄ is more sensitive to overdischarge).
Even if all the above matters have been taken into consideration, problems may still occur with
lithium iron phosphate batteries. At this point, the possibility of a fault in the vehicle's generator must be considered. The automotive start-stop lithium batteries from Yesa Company have a complete protection board to ensure that the lithium battery remains powered throughout the vehicle's usage. They also have overcharge protection. Moreover, our batteries have a 500F super capacity, which can effectively absorb the instantaneous large current from the vehicle's generator and prevent overcharging.
Therefore, before replacing the lead-acid battery of the car, it is necessary to measure the voltage of the car's generator under idle condition and when the headlights, air conditioning, audio system and accelerator pedal are activated. This is to determine whether the vehicle's generator continues to output high voltage and large current due to the failure of the voltage stabilizing system. Only when it is confirmed that the vehicle's generator is in good condition can the lithium battery be replaced, effectively protecting the safety of people, vehicles and the battery, and ensuring its normal operation.